Python Control Flow

Comparison Operators

These operators evaluate to True or False depending on the values you give them.

Examples:

>>> 42 == 42
True

>>> 40 == 42
False

>>> 'hello' == 'hello'
True

>>> 'hello' == 'Hello'
False

>>> 'dog' != 'cat'
True

>>> 42 == 42.0
True

>>> 42 == '42'
False

Boolean Operators

There are three Boolean operators: and, or, and not. In the order of precedence, highest to lowest they are not, and and or.

The and Operator’s Truth Table:

The or Operator’s Truth Table:

The not Operator’s Truth Table:

Mixing Operators

You can mix boolean and comparison operators:

>>> (4 < 5) and (5 < 6)
True

>>> (4 < 5) and (9 < 6)
False

>>> (1 == 2) or (2 == 2)
True

Also, you can mix use multiple Boolean operators in an expression, along with the comparison operators:

>>> 2 + 2 == 4 and not 2 + 2 == 5 and 2 * 2 == 2 + 2
True
>>> # In the statement below 3 < 4 and 5 > 5 gets executed first evaluating to False
>>> # Then 5 > 4 returns True so the results after True or False is True
>>> 5 > 4 or 3 < 4 and 5 > 5
True
>>> # Now the statement within parentheses gets executed first so True and False returns False.
>>> (5 > 4 or 3 < 4) and 5 > 5
False

if Statements

The if statement evaluates an expression, and if that expression is True, it then executes the following indented code:

>>> name = 'Debora'

>>> if name == 'Debora':
...    print('Hi, Debora')
...
# Hi, Debora

>>> if name != 'George':
...    print('You are not George')
...
# You are not George

The else statement executes only if the evaluation of the if and all the elif expressions are False:

>>> name = 'Debora'

>>> if name == 'George':
...    print('Hi, George.')
... else:
...    print('You are not George')
...
# You are not George

Only after the if statement expression is False, the elif statement is evaluated and executed:

>>> name = 'George'

>>> if name == 'Debora':
...    print('Hi Debora!')
... elif name == 'George':
...    print('Hi George!')
...
# Hi George!

the elif and else parts are optional.

>>> name = 'Antony'

>>> if name == 'Debora':
...    print('Hi Debora!')
... elif name == 'George':
...    print('Hi George!')
... else:
...    print('Who are you?')
...
# Who are you?

Ternary Conditional Operator

Many programming languages have a ternary operator, which define a conditional expression. The most common usage is to make a terse, simple conditional assignment statement. In other words, it offers one-line code to evaluate the first expression if the condition is true, and otherwise it evaluates the second expression.

<expression1> if <condition> else <expression2>

Example:

>>> age = 15

>>> # this if statement:
>>> if age < 18:
...    print('kid')
... else:
...    print('adult')
...
# output: kid

>>> # is equivalent to this ternary operator:
>>> print('kid' if age < 18 else 'adult')
# output: kid

Ternary operators can be chained:

>>> age = 15

>>> # this ternary operator:
>>> print('kid' if age < 13 else 'teen' if age < 18 else 'adult')

>>> # is equivalent to this if statement:
>>> if age < 18:
...     if age < 13:
...         print('kid')
...     else:
...         print('teen')
... else:
...     print('adult')
...
# output: teen

Switch-Case Statement

The Switch-Case statements, or Structural Pattern Matching, was firstly introduced in 2020 via PEP 622, and then officially released with Python 3.10 in September 2022.

Matching single values

>>> response_code = 201
>>> match response_code:
...     case 200:
...         print("OK")
...     case 201:
...         print("Created")
...     case 300:
...         print("Multiple Choices")
...     case 307:
...         print("Temporary Redirect")
...     case 404:
...         print("404 Not Found")
...     case 500:
...         print("Internal Server Error")
...     case 502:
...         print("502 Bad Gateway")
...
# Created

Matching with the or Pattern

In this example, the pipe character (| or or) allows python to return the same response for two or more cases.

>>> response_code = 502
>>> match response_code:
...     case 200 | 201:
...         print("OK")
...     case 300 | 307:
...         print("Redirect")
...     case 400 | 401:
...         print("Bad Request")
...     case 500 | 502:
...         print("Internal Server Error")
...
# Internal Server Error

Matching by the length of an Iterable

>>> today_responses = [200, 300, 404, 500]
>>> match today_responses:
...     case [a]:
...             print(f"One response today: {a}")
...     case [a, b]:
...             print(f"Two responses today: {a} and {b}")
...     case [a, b, *rest]:
...             print(f"All responses: {a}, {b}, {rest}")
...
# All responses: 200, 300, [404, 500]

Default value

The underscore symbol (_) is used to define a default case:

>>> response_code = 800
>>> match response_code:
...     case 200 | 201:
...         print("OK")
...     case 300 | 307:
...         print("Redirect")
...     case 400 | 401:
...         print("Bad Request")
...     case 500 | 502:
...         print("Internal Server Error")
...     case _:
...         print("Invalid Code")
...
# Invalid Code

Matching Builtin Classes

>>> response_code = "300"
>>> match response_code:
...     case int():
...             print('Code is a number')
...     case str():
...             print('Code is a string')
...     case _:
...             print('Code is neither a string nor a number')
...
# Code is a string

Guarding Match-Case Statements

>>> response_code = 300
>>> match response_code:
...     case int():
...             if response_code > 99 and response_code < 500:
...                 print('Code is a valid number')
...     case _:
...             print('Code is an invalid number')
...
# Code is a valid number

while Loop Statements

The while statement is used for repeated execution as long as an expression is True:

>>> spam = 0
>>> while spam < 5:
...     print('Hello, world.')
...     spam = spam + 1
...
# Hello, world.
# Hello, world.
# Hello, world.
# Hello, world.
# Hello, world.

break Statements

If the execution reaches a break statement, it immediately exits the while loop’s clause:

>>> while True:
...     name = input('Please type your name: ')
...     if name == 'your name':
...         break
...
>>> print('Thank you!')
# Please type your name: your name
# Thank you!

continue Statements

When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop.

>>> while True:
...     name = input('Who are you? ')
...     if name != 'Joe':
...         continue
...     password = input('Password? (It is a fish.): ')
...     if password == 'swordfish':
...         break
...
>>> print('Access granted.')
# Who are you? Charles
# Who are you? Debora
# Who are you? Joe
# Password? (It is a fish.): swordfish
# Access granted.

For loop

The for loop iterates over a list, tuple, dictionary, set or string:

>>> pets = ['Bella', 'Milo', 'Loki']
>>> for pet in pets:
...     print(pet)
...
# Bella
# Milo
# Loki

The range() function

The range() function returns a sequence of numbers. It starts from 0, increments by 1, and stops before a specified number:

>>> for i in range(5):
...     print(f'Will stop at 5! or 4? ({i})')
...
# Will stop at 5! or 4? (0)
# Will stop at 5! or 4? (1)
# Will stop at 5! or 4? (2)
# Will stop at 5! or 4? (3)
# Will stop at 5! or 4? (4)

The range() function can also modify its 3 defaults arguments. The first two will be the start and stop values, and the third will be the step argument. The step is the amount that the variable is increased by after each iteration.

# range(start, stop, step)
>>> for i in range(0, 10, 2):
...    print(i)
...
# 0
# 2
# 4
# 6
# 8

You can even use a negative number for the step argument to make the for loop count down instead of up.

>>> for i in range(5, -1, -1):
...     print(i)
...
# 5
# 4
# 3
# 2
# 1
# 0

For else statement

This allows to specify a statement to execute in case of the full loop has been executed. Only useful when a break condition can occur in the loop:

>>> for i in [1, 2, 3, 4, 5]:
...    if i == 3:
...        break
... else:
...    print("only executed when no item is equal to 3")

Ending a Program with sys.exit()

exit() function allows exiting Python.

>>> import sys

>>> while True:
...     feedback = input('Type exit to exit: ')
...     if feedback == 'exit':
...         print(f'You typed {feedback}.')
...         sys.exit()
...
# Type exit to exit: open
# Type exit to exit: close
# Type exit to exit: exit
# You typed exit