Chris Castiglione Teacher at One Month. Faculty at Columbia University where I teach Digital Literacy. I write about coding, the internet, and social impact.

The Ultimate Beginner’s Python Cheat Sheet

4 min read

Dog Coding Python

Python is one of the most popular programming languages in the world. Some of the world’s most famous companies use Python like Netflix, Google, and Uber. But if you’ve seen our article on developer’s confessions then you already know developers use cheat sheets all the time!

To help you learn Python, we here at OneMonth wanted to offer this free Python cheat sheet that you can use anytime to look up python variables, functions, tuples, and more. Enjoy!

Python Primitives

Variables

Variables are used for storing values. A string is a series of characters, surrounded by quotes.

x = 1 #integer 
y = 1.1 #float 
z = 1 + 2j #complex 
a = "this is a string" #string
b = True #boolean 
v1 = 1
v2 = 2
v1, v2 = v2, v1 #variable swapping

String Manipulation

String manipulation is for accessing specific characters within a string.

this_is_a_string = “Python is awesome”
len(this_is_a_string) #returns the length of a string
this_is_a_string[0] #returns the first char of a string
this_is_a_string[1] #returns the second char of a string
this_is_a_string[0:4] #returns the first 5 chars of a string

Escape Sequences

Escape sequences are used for indicating special characters that are used in the languages, such as quotes.

\"
\’
\\
\n

Type Conversions

Type conversions are used for converting between different types of values.

int(value) #converts value to an integer
float(value) #converts value to a float
bool(value) #converts value to a boolean
string(value) #converts value to a string

Useful Number Functions

A couple of useful number related functions.

abs(value) #returns an absolute value
round(value) #returns a rounded value

Useful String Methods

Useful and common string functions.

str.upper() #returns the string as uppercase
str.lower() #returns the string as lowercase
str.strip() #returns the string with both leading and trailing characters removed
str.find("x") #finds “x” in str
str.replace("x", "y") #replaces “x” with “y”
"a" in str #checks if “a” exists within str

Formatting Strings

Formatting strings is the most common way to format a string.

print("Hello, I am {} years old !".format(18))
#this prints the following string: Hello, I am  18 years old!

Falsy Values

Falsy values are values that evaluate to false.

“”
0
None

Logic Flow

Regular Conditions

Conditions control the logic flow within a program.

if person == "me":
print("it’s me")
elif person == "you":
print("it’s you")
else:
print("no, it’s someone else")

Ternary Condition

Ternary condition is the short version of an if-else condition statement.

person = "it’s me" if who == "me" else "it’s you or someone else"

 

Chaining Comparison

Chaining Comparison is a way to chain two conditions into one.

if 18 <= tariff < 28:

For Loops

Loops repeat a block of code for a specific number of iterations.

for number in range(0, 99):

Python While Loops

Repeats a block of code until a specific condition is true.

while number > 0:

Boolean Logic

Determines what is true or false.

red and blue #both should be true
red or blue #at least one true
not red #inverses a boolean

Equality

Checks if two items are equal or not.

== #equal
!= #not equal

Functions

How to Define a Function

A function is a named block of code designed to do one specific job.

def add(n1, n2):
return n1 + n2

Variable Number of Arguments

Variable number of parameters that are passed to a function.

def print_items(*items):
for item in items:
print item

print_items(1, 2, 3, 4, 5, 6)

Variable Number of Keyword Arguments

def say_something(**person):
...
say_something(firstname="John", lastname="Doe")

Keyword Arguments

say_something("John", lastname="Doe")

Python Lists

Creation

Stores a series of items in a particular order.

items = ["red", "green", "blue"]
num_range = list(range(10))
matrix_list = [["red", "green"], ["green”, "blue"]]
ones = [1] * 5
combined = ones + items

Access

items = ["red", "green", "blue"]
items[0] # returns “x”
items[-1] # returns “z”

Unpacking

item1, *others = items

Looping

Items within a list are accessed using an index, or within a loop.

for item in items:

for idx, item in enumerate(items):
...

Adding

items.append("dark red") # items is now ["red", "green", "blue", "dark red"]
items.insert(0, "white") # items is now ["white", "red", "green", "blue", "dark red"]

Removing

items.pop(0) # items is now ["red", "green", "blue", "dark red"]
items.remove("green") # items is now ["red", "blue", "dark red"]
del items[1:2] # items is now ["red"]
items = ["red", "green", "blue"] # re-assigning the original items

Finding

if "red" in items:
items.index("red")

Sorting

items.sort() # orders items as red, green, blue
items.sort(reverse=True) # orders items as blue, green, red

List Zipping

a = [1, 2]
b = [10, 20]
c = list(zip(a, b)) # returns [(1, 10), (2, 20)]

Python Dictionaries, Sets & Tuples

Dictionaries

Dictionaries store connections between pieces of information. Each item is a key-value pair.

dic = {"a": 1, "b": 2} # returns {"a": 1, "b": 2}
dic = dict(a=1, b=2) # returns {"a": 1, "b": 2}
dic["c"] = 3 # {"a": 1, "b": 2, "c": 3}
if "a" in dic:
...
del dic["c"] # {"a": 1, "b": 2}
for key, value in dic.items():
...

Sets

a = {1, 2, 3, 4}
b = {1, 5}
a | b # {1, 2, 3, 4, 5}
a & b # {1}
a - b # {2, 3, 4}

Python Tuples

Similar to lists, but the items can’t be modified.

t = 1, 2, 3
t = (1, 2, 3)
t = (1,)
t = ()
t(0:3)
a, b, c = t
if 1 in t:

Generators

List

items = [a * 2 for a in range(15)]
items = [a * 2 for a in range(15) if a % 2 == 0]

Set

items = {a * 2 for a in range(15)}

Dictionary

items = {a: a * 2 for a in range(15)}

Exceptions

Handling

Exceptions help respond appropriately to errors that are likely to occur.

try:

except (ValueError, ZeroDivisionError):

else:
# no exceptions raised
finally:
# code cleanup here

Raising

if x < 1:
raise ValueError("…")
With
with open("file.txt") as file: # any exception produced, it is handled inside this statement

Python Classes

Creating

A class defines the behavior of an object and the kind of information an object can store.

class Test:
def __init__(self, a, b):
self.a = a
self.b = b
def something(self):

Attributes

The information in a class is stored in attributes.

class Test:
name = "Cristiano Ronaldo" # attribute
def __init__(self, a, b):
self.a = a

 

Instance / Class / Statics Methods

Functions that belong to a class are called methods.

class Test:
def something(self): # instance method

@classmethod          # class method
def anotherthing(cls):

@staticmethod         # static method
def onemorething():

Private Members

class Test:
def __init__(self, a):
self.__a = a # __a is a private member

Properties

class Point:
def __init__(self, a):
self.__a = a # __a is a private member
@property
def a(self): # defines the ‘a’ property for the __a is a private member
return self.__a
@property.setter:
def a.setter(self, value): # defines the setter for property ‘a’
self.__a = value

Inheritance

A child class inherits the attributes and methods from its parent class.

class CsvFileReader(FileReader): # CsvFileReader inherits from FileReader
def open(self):
super().open() # Invokes the open() method from FileReader

 

Multiple Inheritance

class CsvFileReader(FileReader, GenericReader): # CsvFileReader inherits from two classes

Named Tuples Example

from collections import namedtuple
Item = namedtuple("Item", ["a", "a"])
item= Item(a=1, b=2)

Importing Python Libraries

Importing

import sys # Import all the sys library
# Imports namedtuple from the collections library
from collections import namedtuple 

Learn to Code Comment Avatar
Chris Castiglione Teacher at One Month. Faculty at Columbia University where I teach Digital Literacy. I write about coding, the internet, and social impact.

Leave a Reply

Your email address will not be published. Required fields are marked *