control-flow.md

title	description
Python Control Flow - Python Cheatsheet	Control flow is the order in which individual statements, instructions or function calls are executed or evaluated. The control flow of a Python program is regulated by conditional statements, loops, and function calls.

Python Control Flow

Python control flow Control flow is the order in which individual statements, instructions, or function calls are executed or evaluated. The control flow of a Python program is regulated by conditional statements, loops, and function calls.

Comparison Operators

Operator	Meaning
==	Equal to
!=	Not equal to
<	Less than
>	Greater Than
<=	Less than or Equal to
>=	Greater than or Equal to

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.

The and Operator’s Truth Table:

Expression	Evaluates to
True and True	True
True and False	False
False and True	False
False and False	False

The or Operator’s Truth Table:

Expression	Evaluates to
True or True	True
True or False	True
False or True	True
False or False	False

The not Operator’s Truth Table:

Expression	Evaluates to
not True	False
not False	True

Mixing Boolean and Comparison Operators

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

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

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

You can also 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

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

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 it's 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.

>>> for i in range(start=0, stop=10, step=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':
...         sys.exit()
...     print(f'You typed {feedback}.')
...
# Type exit to exit: open
# Type exit to exit: close
# Type exit to exit: exit
# You typed exit