My beloved Python cheat sheet

#python #django #flask #beginners

Here is my cheat sheet I created along my learning journey. If you have any recommendations (addition/subtraction) let me know.

Naming conventions

# Variable lower_snake
first_name = 'Mike'

# Class and module CamelCase
class InvoiceDetail:

# Constant
MAX_USER = 100 # All uppercase

# Indentation : 4 spaces
if num > 9:
    print('Small number')

Data type

name = 'Mike' # string
age = 42 # int
price = 199.99 # float
is_active = True # boolean
colors = ['red', 'green', 'blue'] # list
products = { 'name': 'iPad Pro', 'price': 199.99 } # dict
MAX_USER = 100 # Constant

Type conversion

# Python is a strong type language
number = 50 + "50" # TypeError

# Convert to string
number = 50 + int("50") # 100

# Convert to string
my_text = str(199.99)   # "199.99"

# Convert to number
my_number = int('21.99') # 21
my_number = float('21.99') # 21.99

# Get type
type(my_text) # <class 'str'>
type(my_number) # <class 'float'>

# Check if number 0 to 9
isdigit('8') # True

# Check type
isinstance(my_number, int) # True

Strings

name = 'Mike'
# or
name = "Mike"
# or
message = """This is multiline
string that is easier to
read and assign"""

# escape characters \n will do a line break
message = "Hello \nWorld"

# raw string (ignore escape characters)
message = r"https:\\example.com\index.html"

# Convert to lower case
name.lower() # mike

# Convert to upper case
name.upper() # MIKE

# Convert first char to Capital letter
name.capitalize() # Mike

# Convert first char of all words to Capital letter
name = 'mike taylor'
name.title() # Mike Taylor

# Chain methods
name = 'MIKE'
name.lower().capitalize() # Mike

name = 'Mike'

# Start with ?
name.startswith('M') # true

# End with ?
name.endswith('ke') # true

# String length
len(name) # 4

# String concatenation
full_name = first_name + ' ' + last_name

# String format
full_name = f"{first_name} {last_name}"

# Remove leading and trailing characters (like space or \n)
text = ' this is a text with white space '
text.strip() # 'this is a test with white space'

name = 'Mike'
# Get string first character
name[0] # M

# Get string last character
name[-1] # e

# Get partial string
name[1:3] # ik

# Replace
name.replace('M', 'P') # Pike

# Find (return pos or -1 if not found)
name.find('k') # 2

# List to string
colors = ['red', 'green', 'blue']
', '.join(colors) # 'red, green, blue'

Commons fonctions

# Print to console
print('Hello World')

# Print multiple string
print('Hello', 'World') # Hello World

# Multiple print
print(10 * '-') # ----------

# Variable pretty printer (for debug)
from pprint import pprint
pprint(products) # will output var with formatting

# Get keyboard input
name = input('What is your name? ')

# Random (between 0 and 1)
from random import random 
print(random()) # 0.26230234411558273

# Random beween x and y
from random import randint
print(randint(3, 9)) # 5

# Rounding
number = 4.5
print(round(number)) # 5

number = 4.5163
print(round(number, 2)) # 4.52

# Path
import os
current_file_path = __file__
folder_name = os.path.dirname(current_file_path)
new_path = os.path.join(folder_name, 'new folder')

# Round number
solution = round(12.9582, 2) # 12.96

Conditionals

if x == 4:
    print('x is 4')
elif x != 5 and x < 11:
   print('x is between 6 and 10')
else:
   print('x is 5 or greater than 10')

#In or not in
colors = ['red', 'green', 'blue', 'yellow']
if 'blue' in colors:
if 'white' not in colors:

# Ternary
print('y = 10') if y == 10 else print('y != 10') 

# ShortHand Ternary
is_valid = 'Valid'
msg = is_valid or "Not valid" # 'Valid'

# Falsy
False, None, 0, empty string "", empty list [], (), {}

# Truthy
True, not zero and not empty value

Interations

# iterating over a sequence (list, string, etc.)
for item in items:
    print(item)

# With index
for index, item in enumerate(items):
    print(index, item)

# Range
for i in range(10):  #0..9
    print(i)

for i in range(5, 10): #5..9
    print(i)

# While loop
while x > 10:
    print(x)
    # exit loop
    if x == 5:
        break
    # Jump to next while
    if x == 3:
        continue

    x += 1

# For loop dic
for key, value in my_dict.items():
    print(key, value)

# List comprehension: 
# values = [(expression) for (value) in (collection)]
items = [value*2 for value in items]

# List comprehension filtering
# values = [expression for value in collection if condition]
even_squares = [x * x for x in range(10) if x % 2 == 0]

List and Tuple

# Create a list
fruits = ['orange', 'apple', 'melon']

# Append to List
fruits.append('banana')

# List length
nb_items = len(fruits)

# Remove from list
del fruits[1]   #remove apple

# List access
fruits[0]  # first item
fruits[-1] # last item

# Slice my_list[start:finish:step] ([::-1] reverse list) 
fruits = fruits[1:3]
fruits[:3]  # first 3
fruits[2:]  # last 2
copy_fruits = fruits[:] # copy

# List length
nb_entry = len(fruits) 

#Create list from string
colors = 'red, green, blue'.split(', ')

# Array concact
color1 = ['red', 'blue']
color2 = ['green', 'yellow']
color3 = color1 + color2

# Concat by unpacking
color3 = [*color1, *color2]

# Multiple assignment
name, price = ['iPhone', 599]

#Create a Tuple (kind of read only list)
colors = ('red', 'green', 'blue')

# Sort
colors.sort() # ['blue', 'green', 'red']
colors.sort(reverse=True) # ['red', 'green', 'blue']
colors.sort(key=lambda color: len(color)) # ['red', 'blue', 'green']

Dictionaries

# Create a empty dict
product = {}

#Create a dict with key/value
product = {'id': 100, 'name': 'iPadPro'}

# Access dict value by key
print(product['name']) # iPadPro

# Access dict
product.get('name') # if key not exist return None
product.get('name', 'default value') # if key not exist return default value

# Adding a new key/value
product['description'] = "Modern mobile device"

# Get dict keys
product.keys() # ['id', 'name', 'description']

# Get dic values
product.values() # ['100', 'iPadPro', 'Modern mobile device']

# Create a list of dict
products = [
    {'id': 100, 'name': 'iPadPro'},
    {'id': 200, 'name': 'iPhone 12'},
    {'id': 300, 'name': 'Charger'},
]

# Access list of dict
print(products[2]['name']) # Charger

# Search list dict
items_match = [item for product in products if product['id'] == 300]
# [{'id': 300, 'name': 'Charger'}]

# Sum list dict
total = sum([product['price'] for product in products])

Functions

# Create a function
def say_hello():
    print('Hello World')

# Function with argument (with default value)
def say_hello(name = 'no name'):
    print(f"Hello {name}") 

# Function with argument (with optional value)
def say_hello(name = None):
    if name:
        print(f"Hello {name}") 
    else:
        print('Hello World')

# Call a function
say_hello('Mike') # Hello Mike

# Call using keyword argument
say_hello(name = 'Mike') 

# Function returning a value
def add(num1, num2):
   return num1 + num2

num = add(10, 20) # 30

# Arbitrary numbers of arguments *args
def say_hello(*names):
    for name in names:
        print(f"Hello {name}")

# Arbitrary numbers of keywords arguments **kwargs
def say_hello(**kwargs):
    print(kwargs['name'])
    print(kwargs['age'])

say_hello(name = 'Mike', age = 45)

# Lambda function
x = lambda num : num + 10
print(x(20)) # 30

Date and time

from datetime import datetime, timedelta

# Return the current date and time.
datetime.now()

# Create a date time object
date = datetime(2020,12,31) # Dec 31 2020

# Add to date/time (weeks, days, hours, minutes, seconds) 
new_year = date + timedelta(days=1) # Jan 1 2021

# Format a date to string
new_year.strftime('%Y/%m/%d %H %M %S') # 2021/01/01 00 00 00 
new_year.strftime('%A, %b %d') # Friday, Jan 01

# Extract from date
year = new_year.year # 2021
month = new_year.month # 01

File

# Reading a file and storing its lines
filename = 'demo.txt'
with open(filename) as file:
    lines = file.readlines()

for line in lines:
    print(line)

# Writing to a file
filename = 'settings.txt'
with open(filename, 'w') as file:
    file.write("MAX_USER = 100")

# File exist?
from os import path
path.exists('templates/index.html') # True/False

# CSV
import csv
csv_file = 'export.csv'
csv_columns = products[0].keys() # ['id', 'name']
with open(csv_file, 'w') as csvfile:
    writer = csv.DictWriter(csvfile, fieldnames=csv_columns)
    writer.writeheader()
    for item in products:
        writer.writerow(item)

Catching an exception

age_string = input('Your age? ')

try:
    age = int(age_string)
except ValueError:
    print("Please enter a numeric value")
else:
    print("Your age is saved!")

OOP

# Create a class
class Product:
    pass

# Class attribute
class Product:
    nb_products = 0

print(Product.nb_products) # 0

# Create new object instance
product_1 = Product()

# Object instance attributes
class Product:
    def __init__(self, name, price):
        self.name = name
        self.price = price

# Create instance with attributes
product_1 = Product('iPadPro', 699.99)
product_2 = Product('iPhone12', 799.99)
print(product_1.name) # iPadPro

# instance method
class Product()
    def display_price(self):
        return f"Price : {self.price}"

print(product_1.display_price())

# class method
class Product:
    # ... 
    @classmethod
    def create_default(cls):
        # create a instance 
        return cls('Product', 0) # default name, default price

product_3 = Product.create_default() 

# static method
class Product:
    # ... 
    @staticmethod
    def trunc_text(word, nb_char):
        return word[:nb_char] + '...' 

product_3 = Product.trunc_text('This is a blog', 5) # This i... 

# Python Inheritance
class WebProduct(Product):
    def __init__(self, name, price, web_code):
        super().__init__(name, price)
        self.web_code = web_code

# Private scope (naming convention only)
def __init__(self, price):
    self.__price = price

# Getter and setter
class Product:
    def __init__(self):
        self.__price = 0

    @property
    def price(self):
        return self.__price

    @price.setter
    def price(self, value):
        self.__price = value

# Mixins
class Mixin1(object):
    def test(self):
        print "Mixin1"

class Mixin2(object):
    def test(self):
        print "Mixin2"

class MyClass(Mixin2, Mixin1, BaseClass):
    pass

obj = MyClass()
obj.test() # Mixin2

Top comments (6)

Hekitakai • Jan 25 '21 • Edited

It's very good cheat cheet, but it need some improvements:

you missed kwargs in say_hello function
in file and exception paragraph indentation is wrong
Naming scope is also a little wrong. Single underscore is more protected value than private. It can still be accessed from outside. Private is double underscore, which triggers name mangling.
Mixin part seems wrong to me - mixin should add functionality, not overwrite. I thing it would make more sense to use foo and bar instead of test. As it is now it looks more like multiple inheritance and mro at work.

To your list you can add:

slicing and range - you can use 3rd parameter to change order and how list is sliced: [1, 2, 3, 4, 5][::-2] -> [5, 3, 1]
datatypes: missing sets and tuples (I know this is later, but still)
else statement for for loops
more verbose variable unpacking (enumerate and items use it) and cool things you can do with it: a, b = b, a; a, _, _, d = (1, 2, 3, 4)
while you were on list unpacking you could add updating dictionary (Python 3 only): a = {'a': 1, 'b': 2}; b = {'b': 3, 'c': 4}; c = {**a, **b} => {'a': 1, 'b': 3, 'c': 4}

John Copella • Jan 23 '21

Good stuff! One recommendation: any cheat sheet or beginner's guide that talks about default arguments should also link to or describe the infamous "mutable default argument" gotcha, because it will get you:

florimond.dev/blog/articles/2018/0...

or:

docs.python-guide.org/writing/gotc...