Cheatsheets / Learn Python 3
Loops
break Keyword
In a loop, the break keyword escapes the loop,
regardless of the iteration number. Once break
numbers = [0, 254, 2, -1, 3]
executes, the program will continue to execute after the
loop. for num in numbers:
In this example, the output would be: if (num < 0):
print("Negative number detected!")
●
0 break
print(num)
●
254
●
2 # 0
●
Negative number detected! # 254
# 2
# Negative number detected!
Python List Comprehension
Python list comprehensions provide a concise way for
creating lists. It consists of brackets containing an # List comprehension for the squares of
expression followed by a for clause, then zero or more all even numbers between 0 and 9
for or if clauses: [EXPRESSION for ITEM in result = [x**2 for x in range(10) if x % 2
LIST <if CONDITIONAL>] . == 0]
The expressions can be anything - any kind of object can
go into a list. print(result)
A list comprehension always returns a list. # [0, 4, 16, 36, 64]
Python For Loop
A Python for loop can be used to iterate over a list of
items and perform a set of actions on each item. The for <temporary variable> in <list
syntax of a for loop consists of assigning a temporary variable>:
value to a variable on each successive iteration. <action statement>
When writing a for loop, remember to properly <action statement>
indent each action, otherwise an
IndentationError will result. #each num in nums will be printed below
nums = [1,2,3,4,5]
for num in nums:
print(num)
/
The Python continue Keyword
In Python, the continue keyword is used inside a
loop to skip the remaining code inside the loop code big_number_list = [1, 2, -1, 4, -5, 5, 2,
block and begin the next loop iteration. -9]
# Print only positive numbers:
for i in big_number_list:
if i < 0:
continue
print(i)
Python for Loops
Python for loops can be used to iterate over and
perform an action one time for each element in a list. dog_breeds = ["boxer", "bulldog", "shiba
Proper for loop syntax assigns a temporary value, the inu"]
current item of the list, to a variable on each successive
iteration: for <temporary value> in <a # Print each breed:
list>: for breed in dog_breeds:
print(breed)
for loop bodies must be indented to avoid an
IndentationError .
Python Loops with range().
In Python, a for loop can be used to perform an
action a specific number of times in a row. # Print the numbers 0, 1, 2:
The range() function can be used to create a list for i in range(3):
that can be used to specify the number of iterations in a print(i)
for loop.
# Print "WARNING" 3 times:
for i in range(3):
print("WARNING")
Infinite Loop
An infinite loop is a loop that never terminates. Infinite
loops result when the conditions of the loop prevent it
from terminating. This could be due to a typo in the
conditional statement within the loop or incorrect logic.
To interrupt a Python program that is running forever,
press the Ctrl and C keys together on your
keyboard.
/
Python while Loops
In Python, a while loop will repeatedly execute a
code block as long as a condition evaluates to True .
# This loop will only run 1 time
hungry = True
The condition of a while loop is always checked first
while hungry:
before the block of code runs. If the condition is not met
initially, then the code block will never run. print("Time to eat!")
hungry = False
# This loop will run 5 times
i = 1
while i < 6:
print(i)
i = i + 1
Python Nested Loops
In Python, loops can be nested inside other loops.
Nested loops can be used to access items of lists which groups = [["Jobs", "Gates"], ["Newton",
are inside other lists. The item selected from the outer "Euclid"], ["Einstein", "Feynman"]]
loop can be used as the list for the inner loop to iterate
over. # This outer loop will iterate over each
list in the groups list
for group in groups:
# This inner loop will go through each
name in each list
for name in group:
print(name)
