Intro

Hello, World!

It's time to start writing some real, working Python code. It'll be very simple for the time being.
As we're going to show you some fundamental concepts and terms, these snippets of code won't be serious or complex. Run the code in the editor window on the right. If everything goes okay here, you'll see the lines of text in the console window.
Alternatively, launch IDLE, create a new Python source file, fill with this code, name the file and save it. Now run it. If everything goes okay, you'll see the rhyme's line in the IDLE consol windows. The code you have run should look familiar. You saq something very similar when we led you through the setting up of the IDLE environment.

the print() function

Look at the line of code below:

• Cause some effect (e.g. send text to the terminal, create a file, fraw an image, play a sound, etc.) this is something completely unheard of in the world of mathematics.
• evaluate a values or some values (e.g. the square root of a value or the length of a given text), this is what makes Python's functions the relatives of mathematical concepts.

• They may come from Python itself; the print function is one of this kind; such a function is an added value received together with Python and its environment (it is built-in ), you don't have to do anything special (e.g. ask anyone for anything) if you want to make use of it.
• They may come from one or more of Python's add-ons named modules, some of the modules come with Python, others may require separate installation - whatever the case, they all need to be explicitly connected with your code (we'll show you how to do that soon).
• You can write them yourself, placing as many functions as you want and need inside your program to make it simpler, clearer and more elegant.

• an effect
• a result

• First, Python checks if the name specified is legal (it browses its internal data in order to find an existing function of the name; if this search fails, Python aborts the code).
• Second, Python checks if the function's requirements for the number of arguments allows you to invoke the function in this way (e.g., if a specific function demands exactly two arguments, any invocation delivering only one argument will be considered erroneous, and will abort the code's execution).
• Third, Python leaves your code for a moment and jumps into the function you want to invoke; of course, it takes your argument(s) too and passes it/them to the function.
• Fourth, the function executes its code, causes the desired effect (if any), evaluates the desired result(s) (if any) and finishes its task.
• Finally, Python returns to your code (to the place just after the invocation) and resumes its execution.

LAB: The print

Estimated time
5 minutes
Level of difficulty
Very easy
Objectives
becoming familiar with the print() function and its formatting capabilities, experimenting with Python code.
Scenario
The print() command, which is one of the easiest directives in Python, simply prints out a line to the screen.
In your first lab:

use the print() function to print the line Hello, Python! to the screen. Use double quotes around the string.
having done that, use the print() function again, but this time print your first name.
remove the double quotes and run your code. Watch Python's reaction. What kind of error is thrown?
then, remove the parentheses, put back the double quotes, and run your code again. What kind of error is thrown this time?
experiment as much as you can. Change double quotes to single quotes, use multiple print() functions on the same line, and then on different lines. See what happens.

The print() function

Three important questions have to be answered as soon as possible:

1. What is the effect the print() function causes?

The effect is very useful and very spectacular. The function:

takes its arguments (it may accept more than one argument and may also accept less than one argument) converts them into human-readable form if needed (as you may suspect, strings don't require this action, as the string is already readable) and sends the resulting data to the output device (usually the console); in other words, anything you put into the print() function will appear on your screen.

2. What arguments does print() expect?

Any. We'll show you soon that print() is able to operate with virtually all types of data offered by Python. Strings, numbers, characters, logical values, objects - any of these may be successfully passed to print().

3. What value does the print() function evaluate?

None. Its effect is enough - print() does not evaluate anything.

The print() function - instructions

You already know that this program contains one function invocation. In turn, the function invocation is one of the possible kinds of Python instruction. Ergo, this program consists of just one instruction, of course, any complex program usually contains many more instructions than one. The question is: how do you couple more than one instruction into the Python code?
Python's syntax is quite specific in this area. Unlike most programming languages, Python requires that there cannot be more than one instruction in a line. A line can be empty (i.e., it may contain no instruction at all) but it must not contain two, three or more instructions. This is strictly prohibited.
Note: Python makes one exception to this rule - it allows one instruction to spread across more than one line (which may be helpful when your code contains complex constructions). Let's expand the code a bit, you can see it in the editor. Run it and note what you see in the console. Your Python console should now look like this:
The itsy bitsy spider climbed up the waterspout. Down came the rain and washed the spider out.
This is a good opportunity to make some observations:

• the program invokes the print() function twice, and you can see two separate lines in the console - this means that print() begins its output from a new line each time it starts its execution; you can change this behavior, but you can also use it to your advantage.
• each print() invocation contains a different string, as its argument and the console content reflects it - this means that the instructions in the code are executed in the same order in which they have been placed in the source file; no next instruction is executed until the previous one is completed (there are some exceptions to this rule, but you can ignore them for now).

The escape and newline characters

We've changed the example a bit - we've added one empty print() function invocation. We call it empty because we haven't delivered any arguments to the function, you can see it in the editor window. Run the code.
What happens?
If everything goes right, you should see something like this:
The itsy bitsy spider climbed up the waterspout.
Down came the rain and washed the spider out.
As you can see, the empty print() invocation is not as empty as you may have expected - it does output an empty line, or (this interpretation is also correct) its output is just a newline. This is not the only way to produce a newline in the output console. We're now going to show you another way.

We've modified the code again. Look at it carefully. There are two very subtle changes - we've inserted a strange pair of characters inside the rhyme. They look like this: \n. Interestingly, while you can see two characters, Python sees one.
The backslash (\) has a very special meaning when used inside strings - this is called the escape character.
The word escape should be understood specifically - it means that the series of characters in the string escapes for the moment (a very short moment) to introduce a special inclusion. In other words, the backslash doesn't mean anything in itself, but is only a kind of announcement, that the next character after the backslash has a different meaning too, the letter n placed after the backslash comes from the word newline.
Both the backslash and the n form a special symbol named a newline character, which urges the console to start a new output line, run the code. Your console should now look like this: The itsy bitsy spider climbed up the waterspout.
Down came the rain and washed the spider out. As you can see, two newlines appear in the nursery rhyme, in the places where the \n have been used.

This convention has two important consequences:

1. If you want to put just one backslash inside a string, don't forget its escaping nature - you have to double it, e.g., such an invocation will cause an error:
print("\")
while this one won't:
print("\\")

2. Not all escape pairs (the backslash coupled with another character) mean something. Experiment with your code in the editor, run it, and see what happens.

This convention has two important consequences:

1. If you want to put just one backslash inside a string, don't forget its escaping nature - you have to double it, e.g., such an invocation will cause an error:
print("\")
while this one won't:
print("\\")
2. Not all escape pairs (the backslash coupled with another character) mean something.

Using multiple arguments

So far we have tested the print() function behavior with no arguments, and with one argument. It's also worth trying to feed the print() function with more than one argument. Look at the editor window. This is what we're going to test now:
print("The itsy bitsy spider" , "climbed up" , "the waterspout.")
There is one print() function invocation, but it contains three arguments. All of them are strings.
The arguments are separated by commas. We've surrounded them with spaces to make them more visible, but it's not really necessary, and we won't be doing it anymore, in this case, the commas separating the arguments play a completely different role than the comma inside the string. The former is a part of Python's syntax, the latter is intended to be shown in the console, if you look at the code again, you'll see that there are no spaces inside the strings.
Run the code and see what happens, the console should now be showing the following text:
The itsy bitsy spider climbed up the waterspout.
The spaces, removed from the strings, have appeared again. Can you explain why? Two conclusions emerge from this example:

• A print() function invoked with more than one argument outputs them all on one line.
• The print() function puts a space between the outputted arguments on its own initiative.

The positional way of passing the arguments

Now that you know a bit about print() function customs, we're going to show you how to change them, you should be able to predict the output without running the code in the editor.
The way in which we are passing the arguments into the print() function is the most common in Python, and is called the positional way (this name comes from the fact that the meaning of the argument is dictated by its position, e.g., the second argument will be outputted after the first, not the other way round).
Run the code and check if the output matches your predictions.

The keyword arguments

Python offers another mechanism for the passing of arguments, which can be helpful when you want to convince the print() function to change its behavior a bit. We aren't going to explain it in depth right now. We plan to do this when we talk about functions. For now, we simply want to show you how it works. Feel free to use it in your own programs.
The mechanism is called keyword arguments. The name stems from the fact that the meaning of these arguments is taken not from its location (position) but from the special word (keyword) used to identify them.
The print() function has two keyword arguments that you can use for your purposes. The first of them is named end, in the editor window you can see a very simple example of using a keyword argument. In order to use it, it is necessary to know some rules:

• a keyword argument consists of three elements: a keyword identifying the argument (end here); an equal sign (=); and a value assigned to that argument.
• any keyword arguments have to be put after the last positional argument (this is very important), in our example, we have made use of the end keyword argument, and set it to a string containing one space.

And now it's time to try something more difficult, if you look carefully, you'll see that we've used the end argument, but the string assigned to it is empty (it contains no characters at all).
What will happen now? Run the program in the editor to find out, as the end argument has been set to nothing, the print() function outputs nothing too, once its positional arguments have been exhausted, the console should now be showing the following text:
My name is Monty Python.
Note: no newlines have been sent to the output.
The string assigned to the end keyword argument can be of any length. Experiment with it if you want.

We've said previously that the print() function separates its outputted arguments with spaces. This behavior can be changed, too, the keyword argument that can do this is named sep (like separator).
Look at the code in the editor, and run it, the sep argument delivers the following results:
My-name-is-Monty-Python.
The print() function now uses a dash, instead of a space, to separate the outputted arguments.
Note: the sep argument's value may be an empty string, too. Try it for yourself.

Both keyword arguments may be mixed in one invocation, just like here in the editor window. The example doesn't make much sense, but it visibly presents the interactions between end and sep.
Can you predict the output?
Run the code and see if it matches your predictions, now that you understand the print() function, you're ready to consider how to store and process data in Python. Without print(), you wouldn't be able to see any results.

Lab The print() function

Estimated time

Objectives

Expected output

Lab 2 Formatting the output

Estimated time

5-10 minutes

Level of difficulty

Easy

Objectives

experimenting with existing Python code,discovering and fixing basic syntax errors,becoming familiar with the print() function and its formatting capabilities.

Scenario

We strongly encourage you to play with the code we've written for you, and make some (maybe even destructive) amendments. Feel free to modify any part of the code, but there is one condition - learn from your mistakes and draw your own conclusions.

Try to:

• Minimize the number of print() function invocations by inserting the \n sequence into the strings
• make the arrow twice as large (but keep the proportions)
• duplicate the arrow, placing both arrows side by side; note: a string may be multiplied by using the following trick: "string" * 2 will produce "stringstring" (we'll tell you more about it soon)
• remove any of the quotes, and look carefully at Python's response; pay attention to where Python sees an error - is this the place where the error really exists?
• do the same with some of the parentheses.
• change any of the print words into something else, differing only in case (e.g., Print) - what happens now?
• replace some of the quotes with apostrophes; watch what happens carefully.

Key takeways

1. The print() function is a built-in function. It prints/outputs a specified message to the screen/consol window.
2. Built-in functions, contrary to user-defined functions, are always available and don't have to be imported. Python 3.7.1 comes with 69 built-in functions. You can find their full list provided in alphabetical order in the Python Standard Library.
3. To call a function ( function invocation ), you need to use the function name followed by parentheses. You can pass arguments into a function by placing them inside the parentheses. You must separate arguments with a comma, e.g., print("Hello,", "world!"). An "empty" print() function outputs an empty line to the screen.
4. Python strings are delimited with quotes, e.g., "I am a string", or 'I am a string, too'.
5. Computer programs are collections of instructions. An instruction is a command to perform a specific task when executed, e.g., to print a certain message to the screen.
6. In Python strings the backslash (\) is a special character which announces that the next character has a different meaning, e.g., \n (the newline character ) starts a new output line.
7. Positional arguments are the ones whose meaning is dictated by their position, e.g., the second argument is outputted after the first, the third is outputted after the second, etc.
8. Keyword arguments are the ones whose meaning is not dictated by their location, but by a special word (keyword) used to identify them.
9. The end and sep parameters can be used for formatting the output of the print() function. The sep parameter specifies the separator between the outputted arguments (e.g., print("H", "E", "L", "L", "O", sep="-"), whereas the end parameter specifies what to print at the end of the print statement.

Literals - the data in itself

Now that you have a little knowledge of some of the powerful features offered by the print() function, it's time to learn about some new issues, and one important new term - the literal.
A literal is data whose values are determined by the literal itself.
As this is a difficult concept to understand, a good example may be helpful. Take a look at the following set of digits:
123
Can you guess what value it represents? Of course you can - it's one hundred twenty three, but what about this:
c
Does it represent any value? Maybe. It can be the symbol of the speed of light, for example. It also can be the constant of integration. Or even the length of a hypotenuse in the sense of a Pythagorean theorem. There are many possibilities.
You cannot choose the right one without some additional knowledge and this is the clue: 123 is a literal, and c is not, you use literals to encode data and to put them into your code. We're now going to show you some conventions you have to obey when using Python.

Literals- the data in itself

Let's start with a simple experiment - take a look at the snippet in the editor. The first line looks familiar. The second seems to be erroneous due to the visible lack of quotes.

Try to run it.

• a string, wich you alredy know
• and an integer number, something completely new.

Integers

You may already know a little about how computers perfom calculations on numbers. Perhaps ypu're heard of the binary system, and know that it's the system computers use for storing numbers, and that they can perfom any operation upon them.
We won't explore the intricacies of positional numeral system here, but we'll say that the numbers handled by modern computers are of two types:

• Integers, that is, those which are devoid of the fractional part.
• and floating-point numbers (or simply floats ), that contain (or are able to contain) the fractional part.

Integers: Octal and hexadecimal numbers

There are two additional conventions in Python that are unknown to the world of mathematics. The first allows us to use numbers in an octal representation. If an integer number is preceded by an 0O or 0o prefix (zero-o), it will be treated as an octal value. This means that the number must contain digits taken from the [0..7] range only. 0o123 is an octal number with a (decimal) value equal to 83. The print() function does the conversion automatically. Try this:
print(0o123)
The second convention allows us to use hexadecimal numbers. Such numbers should be preceded by the prefix 0x or 0X (zero-x). 0x123 is a hexadecimal number with a (decimal) value equal to 291. The print() function can manage these values too. Try this:
print(0x123)

Floats

Now it's time to talk about another type, which is designed to represent and to store the numbers that (as a mathematician would say) have a non-empty decimal fraction. They are the numbers that have (or may have) a fractional part after the decimal point, and although such a definition is very poor, it's certainly sufficient for what we wish to discuss. Whenever we use a term like two and a half or minus zero point four, we think of numbers which the computer considers floating-point numbers:

2.5
-0.4

Ints vs floats

The decimal point is essentially important in recognizing floating-point numbers in Python. Look at these two numbers:

4
4.0

Coding floats

Let's see how this convention is used to record numbers that are very small (in the sense of their absolute value, which is close to zero). A physical constant called Planck's constant (and denoted as h), according to the textbooks, has the value of: 6.62607 x 10^-34. If you would like to use it in a program, you should write it this way:
6.62607E-34

Lab: The print() function

LAB

Estimated time 5 minutes

Level of difficulty Very easy

• Becoming familiar with the print() function and its formatting capabilities.
• experimenting with Python code.

Scenario

• use the print() function to print the line Hello, Python! to the screen. Use double quotes around the string
• having done that, use the print() function again, but this time print your first name.
• remove the double quotes and run your code. Watch Python's reaction. What kind of error is thrown?
• then, remove the parentheses, put back the double quotes, and run your code again. What kind of error is thrown this time?
• experiment as much as you can. Change double quotes to single quotes, use multiple print() functions on the same line, and then on different lines. See what happens.

The print() function

Three important questions have to be answered as soon as possible:

• Takes its arguments (it may accept more than one argument and may also accept less than one argument)
• Converts them into human-readable form if needed (as you may suspect, strings don't require this action, as the string is already readable)
• And sends the resulting data to the output device (usually the console); in other words, anything you put into the print() function will appear on your screen.

The print() function - instructions

You already know that this program contains one function invocation. In turn, the function invocation is one of the possible kinds of Python instruction. Ergo, this program consists of just one instruction.
Of course, any complex program usually contains many more instructions than one. The question is: how do you couple more than one instruction into the Python code?
Python's syntax is quite specific in this area. Unlike most programming languages, Python requires that there cannot be more than one instruction in a line.
A line can be empty (i.e., it may contain no instruction at all) but it must not contain two, three or more instructions. This is strictly prohibited.
Note: Python makes one exception to this rule - it allows one instruction to spread across more than one line (which may be helpful when your code contains complex constructions).
Let's expand the code a bit, you can see it in the editor. Run it and note what you see in the console.

Your Python console should now look like this:

The itsy bitsy spider climbed up the waterspout.
	Down came the rain and washed the spider out.

• The program invokes the print() function twice, and you can see two separate lines in the console - this means that print() begins its output from a new line each time it starts its execution; you can change this behavior, but you can also use it to your advantage.
• Each print() invocation contains a different string, as its argument and the console content reflects it - this means that the instructions in the code are executed in the same order in which they have been placed in the source file; no next instruction is executed until the previous one is completed (there are some exceptions to this rule, but you can ignore them for now)

The print() function - the escape and newline characters

We've modified the code again. Look at it carefully.
There are two very subtle changes - we've inserted a strange pair of characters inside the rhyme. They look like this:

\n

(\)

The itsy bitsy spider
climbed up the waterspout.

Down came the rain
and washed the spider out.

This convention has two important consequences:
1. If you want to put just one backslash inside a string, don't forget its escaping nature - you have to double it, e.g., such an invocation will cause an error:
print("\")
while this one won't:
print("\\")
2. Not all escape pairs (the backslash coupled with another character) mean something.
Experiment with your code in the editor, run it, and see what happens.

The print function - using multiples argumets

So far we have tested the print() function behavior with no arguments, and with one argument. It's also worth trying to feed the print() function with more than one argument.
Look at the editor window. This is what we're going to test now:

print("The itsy bitsy spider" , "climbed up" , "the waterspout.")

• a print() function invoked with more than one argument outputs them all on one line.
• The print() function puts a space between the outputted arguments on its own initiative.

The print() function - the position way of passing the arguments

Now that you know a bit about print() function customs, we're going to show you how to change them.
You should be able to predict the output without running the code in the editor.
The way in which we are passing the arguments into the print() function is the most common in Python, and is called the positional way (this name comes from the fact that the meaning of the argument is dictated by its position, e.g., the second argument will be outputted after the first, not the other way round).
Run the code and check if the output matches your predictions.

The print() function - the keyword arguments

Python offers another mechanism for the passing of arguments, which can be helpful when you want to convince the print() function to change its behavior a bit.
We aren't going to explain it in depth right now. We plan to do this when we talk about functions. For now, we simply want to show you how it works. Feel free to use it in your own programs.
The mechanism is called keyword arguments. The name stems from the fact that the meaning of these arguments is taken not from its location (position) but from the special word (keyword) used to identify them.
The print() function has two keyword arguments that you can use for your purposes. The first of them is named end.
In the editor window you can see a very simple example of using a keyword argument.
In order to use it, it is necessary to know some rules:

• A keyword argument consists of three elements: a keyword identifying the argument (end here); an equal sign (=); and a value assigned to that argument.
• Any keyword arguments have to be put after the last positional argument (this is very important)

And now it's time to try something more difficult. If you look carefully, you'll see that we've used the end argument, but the string assigned to it is empty (it contains no characters at all). What will happen now? Run the program in the editor to find out.
As the end argument has been set to nothing, the print() function outputs nothing too, once its positional arguments have been exhausted. The console should now be showing the following text:
My name is Monty Python.

We've said previously that the print() function separates its outputted arguments with spaces. This behavior can be changed, too. The keyword argument that can do this is named sep (like separator). Look at the code in the editor, and run it. The sep argument delivers the following results:
My-name-is-Monty-Python.
The print() function now uses a dash, instead of a space, to separate the outputted arguments.

Both keyword arguments may be mixed in one invocation, just like here in the editor window.
The example doesn't make much sense, but it visibly presents the interactions between end and sep. Can you predict the output?
Run the code and see if it matches your predictions.
Now that you understand the print() function, you're ready to consider how to store and process data in Python.
Without print(), you wouldn't be able to see any results.

Lab the print() function

LAB

Estimated time

Level of difficulty

Objectives

Scenario

LAB: Formatting the output

Estimated time

Level of difficulty

Objectives

Scenario

Try to:

Key takeaways

• The print() function is a built-in function. It print/outputs a sepecified message to the screen/consol windows.
• Built-in functions, contrary to user-defined funtions, are always available and don't have to be imported. Python 3.7.1 comes with 69 built-in functions. You can their full list provides in alphabetical order in the Python Standard Library.
• To call a function( function invocation ), you need to use the function name followed by parentheses. You can pass arguments into a function by placing them inside the parentheses. You must separate argumets with a comma, e.g, print("Helli", "World!") .
  And "empty" print() function outputs an empty line to the screen.
• Python string are delimited with quotes, e.g. "I am a string" or 'I'm a string, too'.
• Computer programs are collections of intructions. An intruction is a command to perform a specific task when executed, e.g. to print a certain message to the screen.
• In Python string the backslash \ is a special character which announces that the next character has a different meaning e.g. \n the newline character start a new output line.
• Positional argumets are the ones whose meaning is dictated by their position, e.g. the second argument is outputted after the first, the outputted after the second, etc.
• Keyword arguments are the ones whose meaning isn't dictated by their location, but by a special word (keyword) used to identify them.
• The end and sep parametes can be used for formatting the output of the print() function. The sep parameter specifies the separator between the outputted arguments (e.g. print("H","E","L","L","O", sep="-"), where as the end parameter specifies what to print at the end of the print statement.

Literals - data in itself

Now that you a little knowledge of some of the powerful features offered by the print() function, it's time to learn abouy some new issues, and one important new term - the literal.
A literal is data whose values are determined by the literal itself.
As this is a difficult concept to understand, a good example may be helpful. Take a look at the following set of digits: 123
Can you guess what values it represents? Of course you can - It's one hundred twenty three
But what about this: c
Does it represent any values? Maybe. It can be the symbol of speed of light, for example. It also can be the contant of integration. Or even the length of a hypotenuse in the sense of a Pythagorean theorem. There are many possibilities
You cannot choose the roght one without some additional knowledge. And this is the clue: 123 is a literal, and c is not.
You use literals to encode data and to put them into your code. We're now going to show you some conventions you have obey when using Python.

Let's start with a simple experiment - take a look at the snippet in the editor.
The first line looks familiar. The second seems to be erroneous due to the visible lack of quotes.
Try to run it.
If everything went okay, you should now see two identical lines.
What happened? What does it mean?
Through this example, you encounter two different types of literals:

• a string, which you already know.
• and an integer number, something completely new.

Integers

You may already know a little about how computers perfom calculations on numbers. Perhaps you've heard of the binary system, and know that it's the system computers use for storing numbers, and that they can perform any operation upon them.
We won't explore the intrincacies of positional numeral systems here, but we'll say that the numbers handled by moderm computers are of two types:

• Integers that is, those which are devoid of the fractional part.
• And floating-point numbers (or simply floats ) that contain (or are able to contain) the fraction part.

Strings

Strings are used when you need to process text (like names of all kinds, address, novels, etc), not numbers.
You already known a bit about them, e.g. that string need quotes the way floats need points.
This's a very typical string: "I am a string
However, there's a catch. The catch is how to encode a quote inside a string which is already delimited by quotes. Let's assume that we want to print a very simple message saying:
I like "Monty Python"
How do we do it without generation an error? There're two possible solutions.
The first is based on the concept we already know of the escape character, which you should remember is played by the backslash. The backslash can escape quotes too. A quote preceded by a backslash changes its meaning - it's not a delimiter, but just a quote. This will work as intended:
print("I like \"Monty Python\"")

Coding strings

Now, the next question is: how do you embed an apostrophe into a string placed between apostrophes? You should already know the answer, or to be precise, two possible answers. Try to print out a string containing the following message:
I'm Monty Python.
Do you know how to do it?
print("I'm Monty Python.")
or
print('I'm Monty Python.')

As you can see, the backslash is a very powerful tool - it can escape not only quotes, but also apostrophes. We've shown it already, but we want to emphasize this phenomenon once more - a string can be empty - it may contain no characters at all. An empty string still remains a string:
'' or ""

Boolean values

To conclude with Python's literals, there are two additional ones. They're not as obvious as any of the previous ones, as they're used to represent a very abstract value - truthfulness. Each time you ask Python if one number is greater than another, the question results in the creation of some specific data - a Boolean value. The name comes from George Boole (1815-1864), the author of the fundamental work, The Laws of Thought, which contains the definition of Boolean algebra - a part of algebra which makes use of only two distinct values: True and False, denoted as 1 and 0.
A programmer writes a program, and the program asks questions. Python executes the program, and provides the answers. The program must be able to react according to the received answers.
Fortunately, computers know only two kinds of answers:

• Yes, this is true.
• No, this is false.

Lab: Python literals - strings

Estimated time

5 minutes

Level of difficulty

Easy

Objectives

• Becoming familiar with the print() function and its formatting capabilities.
• Practicing coding strings.
• Experimentint with Python code.

Scenario

Write a one-line piece of code, using the print() function, as well as the newline and escape characters, to match the expected result outputted on three lines.

Expected output

"I'm"
""learning""
"""Python"""

Key takeaways

1. Literals are notations for representing some fixed values in code. Python has various types of literals - for exameple, a literal can be a number ( numeric literals, e.g. 123 ), or string (string literals, e.g. "I am a literal. ).
2. The binary system is a system of numbers that employs 2 as the base. Therefore,a binary number is made up of 0's and 1's only, e.g. 1010 is 10.
Octal and hexadecimal numeration systems, similarly, employ 8 and 16 as their bases respectively. The hexadecimal system uses the decimal numbers and six extra letters.
3. Integers (or simply int's) are one of the numerical types supported by Python. They are numbers written without fractional component, e.g. 256, or -1 (negative integers).
4. Floating-point numbers (or simply float's ) are another one of the numerical types supported by Python. They're numbers that contain (or are able to contain) a fractional component, e.g. 1.27.
5. To encode an apostrophe or a quote inside a string you can either use the escape character, e.g. 'I\'m happy, or open and close the string using an opposite set of symbols to the ones you wish to encode, e.g. "I'm happy." to encode an apostrophe, and 'He said "Python", not "typhoon"' to encode a (double quote).
6. Boolean values are the two constant objects True and False used to represent truth values (in numeric context 1 is True, while 0 is False.)

Extra

Python as a calculator

Now, we're going to show you a completely new side of the print() function. You already know that the function is able to show you the values of the literals passed to it by arguments.
In fact, it can do something more. Take a look at the snippet:
print(2+2)
Retype the code in the editor and run it. Can you guess the output?
You should see the number four. Feel free to experiment with other operators.
Without taking this too seriously, you've just discovered that Python can be used as a calculator. Not a very handy one, and definitely not a pocket one, but a calculator nonetheless.
Taking it more seriously, we are now entering the province of operators and expressions.

Basic operators.

An operator is a symbol of the programming language, which is able to operate on the values. For example, just as in arithmetic, the + (plus) sign is the operator which is able to add two numbers, giving the result of the addition. Not all Python operators are as obvious as the plus sign, though, so let's go through some of the operators available in Python, and we'll explain which rules govern their use, and how to interpret the operations they perform. We'll begin with the operators which are associated with the most widely recognizable arithmetic operations:
+, -, *, /, //, %, **
The order of their appearance is not accidental. We'll talk more about it once we've gone through them all.
Remember: Data and operators when connected together form expressions. The simplest expression is a literal itself.

Arithmetic operators: exponentiation.

A ** (double asterisk) sign is an exponentiation (power) operator. Its left argument is the base, it's right, the exponent.
Classical mathematics prefers notation with superscripts, just like thos: 23. Pure text editors don't accept that, so Python uses ** instead, e.g. 2**3. Take a look at our examples in the editor window.
Note: We're surrounded the double asterisks with spaces in our examples. It's not compulsory, but it improves the readability of the code.
The examples show a very import feature of virtually all Python numerical operators.
Run the code and look carefully at the results it produces. Can you see any regularity here?

Remember: It's possible to formulate the following rules based on this result:

• When both ** arguments are integers, the result is an integer, too.
• When at least one ** argument is a float, the result is a float, too.

Arithmetic operators: multiplication

An * sign is a multiplication operator. Run the code below and check if our integer vs float rule is still working.

Arithmetic operators: division

A / sign is a divisional operator.
The value in front of the slash is a dividend, the value behind the slash, a divisor. run the code below and analyze the results.

You should see that there is an exception to the rule. The result produced by the division operator is always a float, regardless of whether or not the result seems to be a float at first glance: 1/2, or if it looks like a pure integer: 2/1.
Is this a problem? Yes, it is. It happens sometimes that you really need a division that provides an integer value, not a float.
Fortunately, Python can help you with that.

Arithmetic operators: integer

A // (double slash) sign is an integer division operator. It differs from the standard / operator in two details:

• It result lacks the fractional part - It's absent (for integers), or is always equal to zero (for floats). This means that the results are always rounded.
• It conforms to the integer vs float rule.

Operators: Remainder (Modulo)

The next operator is quite a peculiar one, because it has no equivalent among traditional arithmetic operators. Its graphical representation in Python is the % (percent) sign, which may look a bit confusing. Try to think of it as of a slash (division operator) accompanied by two funny little circles. The result of the operator is a remainder left after the integer division.
In other words, it's the value left over after dividing one value by another to produce an integer quotient.
Note The operator is sometimes called modulo in other programming lenguages.
Take a look at the snippet - try to predicts it's results and then run it:

As you can see, the results is two. This is why:

• 14//4 gives 3 → This is the integer quotient.
• 3*4 gives 12 → As a result of quotient and divisor multiplication.
• 14 - 12 gives 2 → This is the remainder.

Operators: How not to divide

As you probably know, division by zero doesn't work.
Do no try to:

• Perform a division by zero.
• Perform an integer division by zero.
• Find a remainder of a division by zero.

Operators: addition

The addition operator is the + (plus) sign, which is fully in line with mathematical standards. Again, take a look at the snippet of the program below: print (-4 + 4) print(-4. + 8)
The result should be nothing surprising. Run the code to check it.

The subtraction operator, unary and binary operators

The subtraction operator is obviously the - (minus) sign, although you should note that this operator also has another meaning - it can change the sign of a number.
This is a great opportunity to present a very important distinction between unary and binary operators. In subtracting applications, the minus operator expects two arguments: the left (a minuend in arithmetical terms) and right (a subtrahend ).
For this reason, the subtraction operator is considered to be one of the binary operators, just like the addition, multiplication and division operators. But the minus operator may be used in a different (unary) way - take a look at the last line of the snippet below:
By the way: there is also a unary + operator. You can use it like this: print(+2)

The operator preserves the sign of its only argument - the right one.
Although such a construction is syntactically correct, using it doesn't make much sense, and it would be hard to find a good rationale for doing so.

Operators and their priorities

So far, we've treated each operator as if it had no connection with the others. Obviously, such an ideal and simple situation is a rarity in real programming. Also, you will very often find more than one operator in one expression, and then this presumption is no longer so obvious. Consider the following expression: 2 + 3 * 5
You probably remember from school that multiplications precede additions. You surely remember that you should first multiply 3 by 5 and, keeping the 15 in your memory, then add it to 2, thus getting the result of 17. The phenomenon that causes some operators to act before others is known as the hierarchy of priorities.
Python precisely defines the priorities of all operators, and assumes that operators of a larger (higher) priority perform their operations before the operators of a lower priority.
So, if you know that * has a higher priority than +, the computation of the final result should be obvious.

Operators and their bindings

The binding of the operator determines the order of computations performed by some operators with equal priority, put side by side in one expression. Most of Python's operators have left-sided binding, which means that the calculation of the expression is conducted from left to right.
This simple example will show you how it works. Take a look: print(9 % 6 % 2)
There are two possible ways of evaluating this expression:

• From left to right: first 9 % 6 gives 3, and then 3 % 2 gives 1.
• From right to left: first 6 % 2 gives 0, and then 9 % 0 causes a fatal error.

Operators and their bindings: exponentiation

Repeat the experiment, but now with exponentiation. Use this snippet of code: print(2 ** 2 ** 3)
The two possible results are:

• 2 ** 2 → 4; 4 ** 3 → 64
• 2 ** 3 → 8; 2 ** 8 → 256

List of priorities

Since you're new Python operators, we don't want to present the complete list of operator priorities right now.
Instead, we'll show you it's truncated form, and we'll expand it consistently as we introduce new operators. Look at the table below:

1

Operators and parentheses

Of course, you're always allowed to use parentheses, which can change the natural order of a calculation. In accordance with the arithmetic rules, subexpressions in parentheses are always calculated first.
You can use as many parentheses as you need, and they're often used to improve the readability of an expression, even if they don't change the order of the operations.
An example of an expression with multiple parentheses is here:
print((5 * ((25 % 13) + 100) / (2 * 13)) // 2) Try to compute the value that's printed to the console. What's the result of the print() function?

10.0

Key takeways

1. An expression is a combination of values (or variables, operators, calls to functions - you will learn about them soon) which evaluates to a value, e.g., 1 + 2.
2. Operators are special symbols or keywords which are able to operate on the values and perform (mathematical) operations, e.g., the * operator multiplies two values: x * y.
3. Arithmetic operators in Python: + (addition), - (subtraction), * (multiplication), / (classic division - returns a float if one of the values is of float type), % (modulus - divides left operand by right operand and returns the remainder of the operation, e.g., 5 % 2 = 1), ** (exponentiation - left operand raised to the power of right operand, e.g., 2 ** 3 = 2 * 2 * 2 = 8), // (floor/integer division - returns a number resulting from division, but rounded down to the nearest whole number, e.g., 3 // 2.0 = 1.0)
4. A unary operator is an operator with only one operand, e.g., -1, or +3.
5. A binary operator is an operator with two operands, e.g., 4 + 5, or 12 % 5.
6. Some operators act before others - the hierarchy of priorities:
• unary + and - have the highest priority.
• then: **, then: *, /, and %, and then the lowest priority: binary + and -.
7. Subexpressions in parentheses are always calculated first, e.g., 15 - 1 * (5 * (1 + 2)) = 0.
8. The exponentiation operator uses right-sided binding, e.g., 2 ** 2 ** 3 = 256.

Exercise 1

What is the output od the following snippet? print((2**4), (2*4), (2*4)) → 16 8.0 8

Exercise 2

What is the output od the following snippet? print((-2/4), (2/4), (2//4), (-2//4)) → -0.5 0.5 0 -1

Exercise 3

What is the output od the following snippet? print((2%-4), (2%4), (2**3**2)) → 2 -2 512

What are variables?

It seems fairly obvious that Python should allow you to encode literals carrying number and text values.
You already know that you can do some arithmetic operations with these numbers: add, subtract, etc. You'll be doing that many times.
But it's quite a normal question to ask how to store the results of these operations, in order to use them in other operations, and so on. How do you save the intermediate results, and use them again to produce subsequent ones?
Python will help you with that. It offers special "boxes" (containers) for that purpose, and these boxes are called variables - the name itself suggests that the content of these containers can be varied in (almost) any way.
What does every Python variable have?

• A name.
• A value (the content of the container)

• The name of the variable must be composed of upper-case or lower-case letters, digits, and the character _ (underscore).
• The name of the variable must begin with a letter.
• The underscore character is a letter.
• Upper- and lower-case letters are treated as different (a little differently than in the real world - Alice and ALICE are the same first names, but in Python they are two different variable names, and consequently, two different variables).
• The name of the variable must not be any of Python's reserved words (the keywords - we'll explain more about this soon).

Correct and incorrect variable names

Note That the same restrictions apply to function names.
Python does not impose restrictions on the length of variable names, but that doesn't mean that a long variable name is always better than a short one.
Here are some correct, but not always convenient variable names:
MyVariable, i, t34, Exchange_Rate, counter, days_to_christmas, TheNameIsSoLongThatYouWillMakeMistakesWithIt, _. Moreover, Python lets you use not only Latin letters but also characters specific to languages that use other alphabets. These variable names are also correct:
Adiós_Señora, sûr_la_mer, Einbahnstraße, переменная. And now for some incorrect names:
10t (does not begin with a letter), Exchange Rate (contains a space)
The PEP 8- Style Guide for Python recommends the following naming convention for variables and functions in Python:

• Variable names should be lowercase, with words separated by underscores to improve readability (e.g var, my_variable )
• Function names follow the same convention as variable names (e.g. fun my_function )
• It's also possible to use mixed case (e.g myVariable ), but only in contexts where that's already the prevailing style, to retain backwards compatibilit with the adopted convention.

Keywords

Take a look at the list of words that play a very special role in every Python program.
['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']
They are called keywords or (more precisely) reserved keywords. They are reserved because you mustn't use them as names: neither for your variables, nor functions, nor any other named entities you want to create. The meaning of the reserved word is predefined, and mustn't be changed in any way.
Fortunately, due to the fact that Python is case-sensitive, you can modify any of these words by changing the case of any letter, thus creating a new word, which is not reserved anymore. For example - you can't name your variable like this: import
You mustn't have a variable named in such a way - it is prohibited. But you can do this instead:
Import
These words might be a mystery to you now, but you'll soon learn the meaning of them.

Creating variables

What can you put inside a variable?
Anything.
You can use a variable to store any value of any of the already presented kinds, and many more of the ones we haven't shown you yet.
The value of a variable is what you have put into it. It can vary as often as you need or want. It can be an integer one moment, and a float a moment later, eventually becoming a string.
Let's talk now about two important things - how variables are created, and how to put values inside them (or rather - how to give or pass values to them).
REMEMBER
A variable comes into existence as a result of assigning a value to it. Unlike in other languages, you don't need to declare it in any special way. If you assign any value to a nonexistent variable, the variable will be automatically created. You don't need to do anything else.
The creation (or otherwise - its syntax) is extremely simple: just use the name of the desired variable, then the equal sign (=) and the value you want to put into the variable.
The concept of 'you can put anything inside a variable'. Take a look at the snippet:

var = 1
print(var)

• The first of them creates a variable named var, and assigns a literal with an integer value equal to 1.
• The second prints the value of the newly created variable to the console.

Using variables

You're allowed to use as many variable declarations as you need to achieve your goal, like this:

var = 1
account_balance = 1000.0
client_name = 'John Doe'
print(var, account_balance, client_name)
print(var)

var = 1
print( Var)

var = "3.7.1"
print("Python version: " + var)

Assigning a new value to an already existing variable

How do you assign a new value to an already created variable? In the same way. You just need to use the equal sign. The equal sign is in fact an assignment operator. Although this may sound strange, the operator has a simple syntax and unambiguous interpretation.
It assigns the value of its right argument to the left, while the right argument may be an arbitrarily complex expression involving literals, operators a nd already defined variables.
Look at the code below:

var = 1
print(var)
var = var + 1
print(var)

1
2

var = 100
var = 200 + 300
print(var)

Solving simple mathematical problems

Now you should be able to construct a short program solving simple mathematical problems such as the Pythagorean theorem.
The square od the hypotenuse is equal to the sum of the squeares of the other two sides.
The following code evaluates the length of the hypotenuse (i.e, the longest side of a right angled triangle, the one opposite of the right angle) using the Pythagorean theorem:

a = 3.0
b = 4.0
c = (a**2 + b**2)**0.5
print("c = ", c)

Lab: variables

Estimated time
10 minutes
Level of difficulty
Easy
Objectives
Becoming familiar with the concept of storing and working with different data types in Python, experimenting with Python code.
Scenario
Here is a short story:
Once upon a time in Appleland, John had three apples, Mary had five apples, and Adam had six apples. They were all very happy and lived for a long time. End of story.
Your task is to:

• Create the variables: john, mary, and adam.
• Assign values to the variables. The values must be equal to the numbers of fruit possessed by John, Mary, and Adam respectively.
• Having stored the numbers in the variables, print the variables on one line, and separate each of them with a comma.
• Now create a new variable named totalApples equal to addition of the three former variables.
• print the value stored in totalApples to the console.

Shortcut operators

It's time for the next set of operators that make a developer's life easier.
Very often, we want to use one and the same variable both to the right and left sides of the = operator.
For example, if we need to calculate a series of successive values of powers of 2, we may use a piece like this:
x = x * 2
You may use an expression like this if you can't fall asleep and you're trying to deal with it using some good, old-fashioned methods:
sheep = sheep + 1
Python offers you a shortened way of writing operations like these, which can be coded as follows:

x *= 2
sheep += 1

Lab: Variables

Lab

Estimated time

Level of difficulty

Objectives

Scenario

Tip

There's one more interesting thing happening there. Can you see another function inside the print() function? It's the round() function. Its job is to round the outputted result to the number of decimal places specified in the parentheses, and return a float (inside the round() function you can find the variable name, a comma, and the number of decimal places we're aiming for). We're going to talk about functions very soon, so don't worry that everything may not be fully clear yet. We just want to spark your curiosity.
After completing the lab, open Sandbox, and experiment more. Try to write different converters, e.g., a USD to EUR converter, a temperature converter, etc. - let your imagination fly! Try to output the results by combining strings and variables. Try to use and experiment with the round() function to round your results to one, two, or three decimal places. Check out what happens if you don't provide any number of digits. Remember to test your programs.
Experiment, draw conclusions, and learn. Be curious.

Expect output

7.38 miles is 11.88 kilometers
12.25 kilometers is 7.61 miles

Lab: Operator and expression

LAB

Estimated time

Level difficulty

Objectives

• Becoming familiar with the concept of numbers, operators and arithmetic operations in Python.
• Perfomring basic calculations.

Scenario

Test Data

x = 0
x = 1
x = -1

y = -1.0
y = 3.0
y = -9.0

Key takeaways

1. A variable is a named location reserved to store values in the memory. A variable is created or initialized automatically when you assign a value to it for the first time. (2.1.4.1)
2. Each variable must have a unique name - an identifier. A legal identifier name must be a non-empty sequence of characters, must begin with the underscore(_), or a letter, and it cannot be a Python keyword. The first character may be followed by underscores, letters, and digits. Identifiers in Python are case-sensitive. (2.1.4.1)
3. Python is a dynamically-typed language, which means you don't need to declare variables in it. (2.1.4.3) To assign values to variables, you can use a simple assignment operator in the form of the equal (=) sign, i.e., var = 1.
4. You can also use compound assignment operators (shortcut operators) to modify values assigned to variables, e.g., var += 1, or var /= 5 * 2. (2.1.4.8)
5. You can assign new values to already existing variables using the assignment operator or one of the compound operators, e.g.: (2.1.4.5)


var = 2
print(var)
var = 3
print(var)
var += 1
print(var)


6. You can combine text and variables using the + operator, and use the print() function to output strings and variables, e.g.: (2.1.4.4)
var = "007" print("Agent " + var)

Exercise 1

var = 2
var = 3
print(var)

Exercise 2

my_var
m
101
averylongvariablename
m101
m 101
Del
del

Exercise 3

a = '1'
b = "1"
print(a + b)

Exercise 4

a = 6
b = 3
a /= 2 * b
print(a)

2 * b = 6
a = 6 → 6 / 6 = 1.0

Leaving comments in code: why, how and when.

You may want to put in a few words addressed not to Python but to humans, usually to explain to other readers of the code how the tricks used in the code work, or the meanings of the variables, and eventually, in order to keep stored information on who the author is and when the program was written.
A remark inserted into the program, which is omitted at runtime , is called a comment.
How do you leave this kind of comment in the source code? It has to be done in a way that won't force Python to interpret it as part of the code.
Whenever Python encounters a comment in your program, the comment is completely transparent to it - from Python's point of view, this is only one space (regardless of how long the real comment is).
In Python, a comment is a piece of text that begins with a # (hash) sign and extends to the end of the line.
If you want a comment that spans several lines, you have to put a hash in front of them all.
Just like here:

# This program evaluates the hypotenuse c.
# a and b are the lengths of the legs.
a = 3.0
b = 4.0
c = (a**2 + b**2) ** 0.5 # We use ** instead of squeare root
print("c =", c)

# This is a test program.
x = 1
y = 2
# y = y + x
print(x + y)

TIP

LAB: Comments

Estimated time

Level of difficulty

Objectives

• Becomming familiar with the concept of comments in Python.
• Using and not using comments.
• Replacing comments with code.
• Experimenting with Python code.

Scenario

Note

Key takeaways

1. Comments can be used to leave additional information in code. They are omitted at runtime. The information left in source code is addressed to human readers. In Python, a comment is a piece of text that begins with #. The comment extends to the end of line.
2. If you want to place a comment that spans several lines, you need to place # in front of them all. Moreover, you can use a comment to mark a piece of code that is not needed at the moment (see the last line of the snippet below), e.g.:


# This program prints
# an introduction to the screen.
print("Hello!") # Invoking the print() function
# print("I'm Python.")


3. Whenever possible and justified, you should give self-commenting names to variables, e.g., if you're using two variables to store a length and width of something, the variable names length and width may be a better choice than myvar1 and myvar2.
4. It's important to use comments to make programs easier to understand, and to use readable and meaningful variable names in code. However, it's equally important not to use variable names that are confusing, or leave comments that contain wrong or incorrect information!
5. Comments can be important when you are reading your own code after some time (trust us, developers do forget what their own code does), and when others are reading your code (can help them understand what your programs do and how they do it more quickly).

The input() function

We're now going to introduce you to a completely new function, which seems to be a mirror reflection of the good old print() function.
Why? Well, print() sends data to the console.
The new function gets data from it.
print() has no usable result. The meaning of the new function is to return a very usable result.
The function is named input(). The name of the function says everything. The input() function is able to read data entered by the user and to return the same data to the running program.
The program can manipulate the data, making the code truly interactive.
Virtually all programs read and process data. A program which doesn't get a user's input is a deaf program. Take a look at our example:

print("TRell me anything...")
anything = input()
print("Hmmm .... ", anything , "... Really?")

The input() function with an argument

The input() function can do something else: it can prompt the user without any help from print().
We've modified our example a bit, look at the code.

anything = input("Tell me anything...")
print("Hmmmm...", anything, "...Really?")

• The input() function is invoked with one argument - it's a string containing a message.
• The message will be displayed on the console before the user is given an opportunity to enter anything.
• Input() will then do its job.

The result of the input() function

We've said it already, but it must be unambiguously stated once again: the result of the input() function is a string.
A string containing all the characters the user enters from the keyboard. It is not an integer or a float.
This means that you mustn't use it as an argument of any arithmetic operation, e.g., you can't use this data to square it, divide it by anything, or divide anything by it.

anything = input("Enter a number: ")
something = anything ** 2.0
print(anything, "to the power of 2 is", something)

The input() function - prohibited operations

Look at the code in the editor. Run it, enter any number and press Enter.
What happens?
Python should have given you the following output:

Traceback (most recent call last):
File ".main.py", line 4, in <module>
something = anything ** 2.0
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'float'

Type casting

Python offers two simple functions to specify a type of data and solve this problem - here they are: int() and float(). Their names are self-commenting: - the int() function takes one argument (e.g., a string: int(string)) and tries to convert it into an integer; if it fails, the whole program will fail too (there is a workaround for this situation, but we'll show you this a little later); - the float() function takes one argument (e.g., a string: float(string))and tries to convert it into a float (the rest is the same). This is very simple and very effective. Moreover, you can invoke any of the functions by passing the input() results directly to them. There's no need to use any variable as an intermediate storage. We've implemented the idea in the editor - take a look at the code. Can you imagine how the string entered by the user flows from input() into print()? Try to run the modified code. Don't forget to enter a valid number. Check some different values, small and big, negative and positive. Zero is a good input, too.

More about input() and type casting

Having a team consisting of the trio input()- int()- float() opens up lots of new possibilities.
You'll eventually be able to write complete programs, accepting data in the form of numbers, processing them and displaying the results.
Of course, these programs will be very primitive and not very usable, as they cannot make decisions, and consequently are not able to react differently to different situations.
This is not really a problem, though; we'll show you how to overcome it soon.
Our next example refers to the earlier program to find the length of a hypotenuse. Let's rewrite it and make it able to read the lengths of the legs from the console.
Check out the editor window - this is how it looks now.
The program asks the user twice for both legs' lengths, evaluates the hypotenuse and prints the result.
Run it and try to input some negative values.
The program - unfortunately - doesn't react to this obvious error.
Let's ignore this weakness for now. We'll come back to it soon.

String operators - Introduction

It's time to return to these two arithmetic operators: + and *
We want to show you that they have a second function. They are able to do something more just add and multiply.
we've seen them in action where their arguments are numbers (floats or integers, it doesn't matter), now we're going to show you that they can handle strings, too, albeit in a very specific way.

Concatenation

The + (plus) sign, whaen applied to two strings, becomes a concatenation operator:
string + string
It simply concatenates (glues) two strings into one. Of course, like its arithmetic sibling, it can be used more that once expression, and in such a context it behaves according to left-sided binding.
In contrast to its arithmetic sibling, the concatenation operator is not commutative, i.e. "ab" + "ba" is not the same as "ba" + "ab".
Don't forget - if you want the + sign to be a concatenator, not an adder, you must ensure that both its arguments are strings.
You cannot mix types here.

fnam = input("May I have your first name, please? ")
lnam = input("May I have your last name, please? ")
print("Thank you.")
print("\nYour name is " + fnam + " " + lnam + "."

Replication

The * (asterisk) sign, when applied to a string and number (or a number and string, as it remains commutative in this position) becomes a replication operator:
string * number number * string
It replicates the string the same number of times specified by the number.
For example:

"James" * 3 gives "JamesJamesJames"
3 * "an" gives "ananan"
5 * "2" (or "2" * 5) gives "22222" (not 10!)

REMEMBER

print("+" + 10 * "-" + "+")
print(("|" + " " * 10 + "|\n") * 5, end="")
print("+" + 10 * "-" + "+")

Type conversion: str()

You already know how to use the int() and float() functions to convert a string into a number.
This type of conversion is not a one-way street. You can also convert a number into a string, which is way easier and safer - this operation is always possible.
A function capable of doing that is called str():
str(number)
To be honest, it can do much more than just transform numbers into strings, but that can wait for later.

The "right-angle triangle" again

leg_a = float(input("Input first leg length: "))
leg_b = float(input("Input second leg length: "))
print("Hypotenuse length is " + str((leg_a**2 + leg_b**2) ** .5))

LAB: Simple input and output

Lab

Estimated time

5-10 minutes

Level of difficulty

Easy

Objectives

• Becoming familiar with the inputting and outputting of data in Python.
• Evaluating simple expressions.

Scenario

Your task is to complete the code in order to evaluate the results of four basic arithmetic operations.
The results have to be printed to the console, you may not be able to protect the code from a user who wants to divide by zero. That's okay, don't worry about it for now.
Test your code - does it produce the results you expect?
We won't show you any test data - that would be too simple.

LAB: Operators and expressions

Estimated time

20 minutes

Level of difficulty

Intermediate

Objetives

• Becoming familiar with the concept of numbers, operators and arithmetic operations in Python.
• Understanding the precendence and associativity of Python operators, as well as the proper use of parentheses.

Scenario

Your task is to complete the code in order to evaluate the following expression:

The result should be assigned to y. Be careful - watch the operators and keep their priorities in mind. Don't hesitate to use as many parentheses as you need.
You can use additional variables to shorten the expression (but it's not necessary). Test your code carefully.

Test Data

Sample input 1:
Expected output:
y = 0.6000000000000001
Sample input: 10
Expected output:
y = 0.09901951266867294
Sample input: 100
Expected output:
y = 0.009999000199950014
Sample input: -5
Expected output:
y = -0.19258202567760344

LAB: Operators and expressions

Estimated time

Level of difficulty

Objectives

Scenario

Test Data

12
17
59

23
58
642

0
1
2939

Key Takeaways

1. The print() function sends data to the console, while the input() function gets data from the console.
2. The input() function comes with an optional parameter: the prompt string. It allows you to write a message before the user input, e.g.:


name = input("Enter your name: ")
print("Hello, " + name + ". Nice to meet you!")

3. When the input() function is called, the program's flow is stopped, the prompt symbol keeps blinking (it prompts the user to take action when the console is switched to input mode) until the user has entered an input and/or pressed the Enter key.

NOTE

You can test the functionality of the input() function in its full scope locally on your machine. For resource optimization reasons, we have limited the maximum program execution time in Edube to a few seconds. Go to Sandbox, copy-paste the above snippet, run the program, and do nothing - just wait a few seconds to see what happens. Your program should be stopped automatically after a short moment. Now open IDLE, and run the same program there - can you see the difference?
Tip: the above-mentioned feature of the input() function can be used to prompt the user to end a program. Look at the code below:


name = input("Enter your name: ")
print("Hello, " + name + ". Nice to meet you!")

print("\nPress Enter to end the program.")
input()
print("THE END.")


4. The result of the input() function is a string. You can add strings to each other using the concatenation (+) operator. Check out this code:


num1 = input("Enter the first number: ") # Enter 12
num2 = input("Enter the second number: ") # Enter 21

print(num1 + num2) # the program returns 1221


5. You can also multiply (* - replication) strings, e.g.:


myInput = ("Enter something: ") # Example input: hello
print(myInput * 3) # Expected output: hellohellohello

Exercise 1.

What is the output of the following snippet?

x = int(input("Enter a number: ")) # User write '2'
print(x * "5")

Exercise2

What is the expected output of the following snippet?

x = input("Enter a number: ") # the user enters 2
print(type(x))