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Note

Recap

  1. What is the defining property of an expression?
    ↳ An expression always evaluates to a single value
  2. What kind of expression can't be split into further expressions?
    ↳ Literal expressions
  3. Solve the following expression. 
        s = "hello world"
    (10 + 5 // 3 - 3) / 4 + len("7" * s.index("w")) + s[2:9].count("l")
    ↳ 10.0 (of float data type)

Control Flow 1

The "Boolean" data type (bool)

While the integer, floating-point, and string data types have an unlimited number of possible values, the Boolean data type has only two values: True and False.

Tip

The boolean literals True and False have to be capitalized. Also not to be confused with the string literals "True" and "False" (marked by the double quotes).

The Boolean data type is a subclass of the integer data type, meaning every Boolean is also an integer (but not vice versa!)

False is equivalent to 0 and True is equivalent to 1.

As a result, any arithmetic expression (like x + y) can also use Booleans instead of the numeric data types. By extension, wherever an integer is valid, you can always use a Boolean.

>>> True + True
2

Comparison Expressions

Let x and y be any expression, and s be an expression of a sequence data type (e.g. string). We define the following comparison expressions:

Expression Operation Example Result
x == y Equal to 5 == 5 True
x != y Not equal to 5 != 3 True
x < y Less than 10 < 15 True
x > y Greater than 20 > 18 True
x <= y Less than or equal to 8 <= 8 True
x >= y Greater than or equal to 25 >= 30 False
x in s Membership (x in a sequence s) 'a' in 'hello' False
x not in s Negated membership (x not in a sequence s) 'z' not in 'hello' True

Tip

Values of different types, except different numeric types, never compare equal. e.g. 5 == "5", evaluates to False. However, 5 == 5.0 evaluates to True.

Caution

Do not confuse x == y (comparison expression) with = (assignment statement)

>>> 5 == "5"
false

5 (integer) and "5" (string) are different data types

Logical Expressions

Let x and y be expressions of the boolean data type. We define the following logical expressions:

Expression Operation Example Result
x and y Logical AND True and False False
x or y Logical OR True or False True
not x Logical NOT not True False

Interacting with the Terminal

Input: The input() Expression

Until now we have only used fixed values, that we defined as a literal expression at some point. Chances are, you want to receive inputs from the user.

To do that, we can use the input() Expression. It pauses the program and waits for the user to input some text. Once the user confirms their input by pressing Enter, the expression evaluates to that text.

>>> input()
Hello # <-- type here, then press Enter
'Hello'
>>> input("Please enter your name: ")
Please enter your name: John # <-- type here, then press Enter
'John'

We can optionally provide a prompt, by inserting a string expression inside the parantheses

Output: The print() Statement

Inside the Interactive Shell, we always conveniently get the result of an expression displayed. However, this is not the case when executing a Python-Script/.py-file

The print() statement takes any string expression inside the parantheses, and outputs it to the terminal.

>>> print("Hello")
Hello

Tip

print() is a statement, not an expression. The Hello you see in the interactive shell is not the result of an expression. It just immediatly executes the print() statement, which displays text without representing a value.

1

Programming a Python-Script

Up until now we only used the Interactive Shell to evaluate expressions and create variables using assignment statements. With the basics covered, we are ready to leave the interactive shell and start writing proper programs.

A Python-Script is a file with a .py suffix, e.g. my_first_script.py. Avoid using spaces in your filename. Use underscores (_) instead. A Python-Script contains a series of instructions (i.e. statements and expressions) to execute.

You can open the file in any text editor you desire. It's recommended to use either PyCharm or Visual Studio Code, which we installed in the first course.

my_first_script.py

name = input("Please enter your name: ")
print("Your name is " + name)

Execute it by running the following command

python3.12 my_frst_script.py

Tip

Depending on the Operating System and/or Python version you have installed the exact command may differ slightly. If the command above does not work, you can also try one of the following:

python my_frst_script.py
python3 my_frst_script.py

The program will display Please enter your name: and wait for the user to input some text. Once confirmed by pressing Enter the resulting string is then stored in the name variable, which then again is being displayed in the terminal with the text Your name is <xyz>, where <xyz> is the name you typed into the terminal before.

Conditional Code Execution (if, else and elif)

Simple Statements vs. Compound Statements

Expanding upon the definition of a Statement, we introduce two subclasses of statements:

  1. Simple Statements
  2. Compound Statements

A simple statement is comprised within a single logical line. For example the assignment statement is a simple statement2.

Unlike simple statements, compound statements contain (groups of) other statements; they affect or control the execution of those other statements in some way. In general, compound statements span multiple lines, although in simple cases a whole compound statement may be contained in one line3.

Tip

A block of code or code block are one or more lines of code, that share the same indentation. Python uses the indentation to determine, whether a line of code belongs to a block or not.

if statement

An if statement’s clause (that is, the block following the if statement) will execute if the statement’s condition is True. The clause is skipped if the condition is False.

In plain English, an if statement could be read as, “If this condition is true, execute the code in the clause.” In Python, an if statement consists of the following:

  • The if keyword
  • A condition (that is, an expression that evaluates to True or False)
  • A colon (:)
  • Starting on the next line, an indented block of code

Tip

Indentation is used to create a code block, that is executed or skipped depending on the if statement's condition.

name = input()

if name == "Alice":
    print("You are Alice")

else clause

An if statement can optionally be followed by an else clause, which is only executed when the if statement's condition is False. In plain English, an else clause could be read as, “If this condition is true, execute this code. Or else, execute that code.”

name = input()

if name == "Alice":
    print("You are Alice")
else:
    print("You aren't Alice")

elif clause

elif is a shorthand for having an else, immediatly followed by an if.

name = input()
age = int(input())

if name == "Alice":
    print("You are Alice")
elif age >= 21:
    print("You aren't Alice, but you are allowed to drink")
else:
    print("You are neither Alice nor drinking age")

This creates a distinction between 3 cases, only one of which is ever executed.

  1. name == Alice (The name is Alice)
  2. name != Alice and age >= 21 (The name isn't Alice and the age is >= 21)
  3. name != Alice and age < 21 (The name isn't Alice and the age is < 21)

if and elif are not equivalent

If you are wondering whether elif and if are equivalent, consider the following code:

name = input()
age = int(input())

if name == "Alice":
    print("You are Alice")
if age >= 21:
    print("You aren't Alice, but you are allowed to drink")
else:
    print("You are neither Alice nor drinking age")

This code does indeed behave differently to the one before. If you set the name to Alice and age to something above or equal to 21, e.g. 23, you will get two outputs:

You are Alice
You aren't Alice, but you are allowed to drink

Let's add an empty line between the two if statements to see what's happening here:

name = input()
age = int(input())

if name == "Alice":
    print("You are Alice")

if age >= 21:
    print("You aren't Alice, but you are allowed to drink")
else:
    print("You are neither Alice nor drinking age")

Unlike before, where we used elif, the age >= 21 condition is checked regardless if name was Alice or not. Likewise, the else clause is attached to the age check and completely independent of the Alice name check.

We no longer have three distinct cases, but rather two sequential statements with two distinct cases each.

The first statement has the following two cases:

  1. name == Alice (The name is Alice), so print You are Alice
  2. name != Alice (The name isn't Alice), so do nothing (since there is no else clause)

The second statement differentiates between these:

  1. age >= 21 (The age is greater than or equal to 21), so print You aren't Alice, but you are allowed to drink
  2. age < 21 (The age is below 21), so print You are neither Alice nor drinking age

Changing the print statements to reflect the functionality of the code, it should look something like this:

name = input()
age = int(input())

if name == "Alice":
    print("You are Alice")

if age >= 21:
    print("You are allowed to drink")
else:
    print("You are not allowed to drink")

And if we add an else clause to the first if statement, we could do the following:

name = input()
age = int(input())

if name == "Alice":
    print("You are Alice")
else:
    print("You aren't Alice")

if age >= 21:
    print("You are allowed to drink")
else:
    print("You are not allowed to drink")

Footnotes

  1. The Python Standard Library: Built-in Functions ↩

  2. The Python Language Reference: Simple Statements ↩

  3. The Python Language Reference: Compound Statements ↩