Python Cheatsheet

Variables

Variable names use underscores between words for complex names

number = 5
my_number = 7

Spacing

Python enforces consistent tabbing. So, when you’re working with loops inside loops or functions inside classes, etc., you must preserve the hierarchy using meaningful whitespace.

if this_thing == True: print(“this!”)

for thing in many_things: if this_is == True: print(‘This!’) else: print(“This instead”)

Conditional Tests (if)

Place if, elif, and else on separate lines. Each logic line must end in a colon, and the subsequent lines must be indented. You do not need to put an “end” keyword at the end of the sequence. In Python 3 you must indent with five spaces (though most text editors will do this for you).

if number == 5:
  print("Success")
elif number == 7:
  print("Praxis FTW")
else:
  print("FAIL")

Operators

Coming soon!

Method Calls

Parentheses are required (though will change in Python 2 vs 3). Additionally, if you you do not give parentheses at the end of a function it will not run.

print("Word")

run_a_function <-- not run
run_a_function() <-- run!

Method Definitions

Methods can return values which can be assigned to a variable. The function must end with a return statement, something that signifies what the function is giving back when called.

def make_positive(number):
  if number < 0:
    return -number

print(make_positive(-5))

Dictionaries

Dictionaries hold objects and are referenced by their key assignment.

dictionary = {}
other_dictionary = {"VA": "Virginia", "NC": "North Carolina"}

len(other_dictionary)

other_dictionary.keys()
other_dictionary['VA']
other_dictionary.values()

Lists

Lists hold objects and are referenced by their position, starting with position 0. You can use a ‘:’ to grab everything before or after particular index positions.

empty_list = []
another_list = [1, 2, "three"]

empty_list[0]   # <= error
another_list[2] # <= "three"

another_list[-1]
another_list[0] = "nonsense" # <= sets value at position 0 to
"nonsense"
another_list[1:] # <= everything from the first element on.
another_list[:2] # <= everything before the 2nd element

For Loops

Loops allow you to apply the same thing to each element in a list. You assign a variable name to refer to the item in each index position. The following both do the same thing.

a_list = [1, 2, "three"]

for item in a_list:
     print(item)

for foo in a_list:
     print(foo)

List Comprehension

List comprehensions are a way of neatly organizing a particular loop type that happens often. These do the same thing.

numbers = [1,2,3,4,5]
results = []

for item in numbers:     	
     results.append(item + 5)

results_as_a_list_comprehension = [item + 5 for item in numbers]