Python

The Python cheat sheet is a one-page reference sheet for the Python 3 programming language.

#Getting Started

#Introduction

#Hello World

>>> print("Hello, World!")
Hello, World!

The famous "Hello World" program in Python

#Variables

x = 4        # x is of type int
x = "Sally"  # x is now of type str
print(x)

Python has no command for declaring a variable.

#Data Types

str Text
int, float, complex Numeric
list, tuple, range Sequence
dict Mapping
set, frozenset Set
bool Boolean
bytes, bytearray, memoryview Binary

See: Data Types

#Slicing String

>>> b = "Hello, World!"
>>> print(b[2:5])
llo

See: Strings

#Lists

mylist = []
mylist.append(1)
mylist.append(2)
for x in mylist:
    print(x) # prints out 1,2

See: Lists

#If Else

a = 200
if a > 0:
    print("a is greater than 0")
else:
    print("a is not greater than 0")

See: Flow control

#Loops

for x in range(6):
    if x == 3: break
    print(x)
else:
    print("Finally finished!")

See: Loops

#Functions

>>> def my_function():
...     print("Hello from a function")
...
>>> my_function()
Hello from a function

See: Functions

#File Handling

with open("myfile.txt", "r", encoding='utf8') as file:
    for x in file:
        print(x)

See: File Handling

#Arithmetic

result = 10 + 30  # =>  40
result = 40 - 10  # =>  30
result = 50 * 5   # =>  250
result = 16 / 4   # =>  4
result = 25 % 2   # =>  1
result = 5 ** 3   # =>  125

#Plus-Equals

counter = 0
counter += 10           # => 10
counter = 0
counter = counter + 10  # => 10

message = "Part 1."
# => Part 1.Part 2.
message += "Part 2."   

#Data Types

#Strings

s = "Hello World"
s = 'Hello World'

a = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """

See: Strings

#Numbers

x = 1    # int
y = 2.8  # float
z = 1j   # complex

>>> print(type(x))
<class 'int'>

#Booleans

a = True 
b = False

bool(0)     # => False
bool(1)     # => True

#Lists

list1 = ["apple", "banana", "cherry"]
list2 = [True, False, False]
list3 = [1, 5, 7, 9, 3]
list4 = list((1, 5, 7, 9, 3))

See: Lists

#Tuple

a = (1, 2, 3)
a = tuple((1, 2, 3))

Similar to List but immutable

#Set

set1 = {"a", "b", "c"}   
set2 = set(("a", "b", "c"))

Set of unique items/objects

#Dictionary

>>> empty_dict = {}
>>> a = {"one": 1, "two": 2, "three": 3}
>>> a["one"]
1
>>> a.keys()
dict_keys(['one', 'two', 'three'])
>>> a.values()
dict_values([1, 2, 3])
>>> a.update({"four": 4})
>>> a.keys()
dict_keys(['one', 'two', 'three', 'four'])
>>> a['four']
4

Key: Value pair, JSON like object

#Casting

#Integers

x = int(1)   # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3

#Floats

x = float(1)     # x will be 1.0
y = float(2.8)   # y will be 2.8
z = float("3")   # z will be 3.0
w = float("4.2") # w will be 4.2

#Strings

x = str("s1") # x will be 's1'
y = str(2)    # y will be '2'
z = str(3.0)  # z will be '3.0'

#Strings

#Array-like

>>> a = "Hello, World"
>>> print(a[1])
e
>>> print(a[len(a)-1])
d

Get the character at position 1

#Looping

>>> for x in "abc":
...     print(x)
a
b
c

Loop through the letters in the word "banana"

#Slicing string

 ┌───┬───┬───┬───┬───┬───┬───┐
 | m | y | b | a | c | o | n |
 └───┴───┴───┴───┴───┴───┴───┘
 0   1   2   3   4   5   6   7
-7  -6  -5  -4  -3  -2  -1

>>> s = 'mybacon'
>>> s[2:5]
'bac'
>>> s[0:2]
'my'
>>> s = 'mybacon'
>>> s[:2]
'my'
>>> s[2:]
'bacon'
>>> s[:2] + s[2:]
'mybacon'
>>> s[:]
'mybacon'
>>> s = 'mybacon'
>>> s[-5:-1]
'baco'
>>> s[2:6]
'baco'

#With a stride

>>> s = '12345' * 5
>>> s
'1234512345123451234512345'
>>> s[::5]
'11111'
>>> s[4::5]
'55555'
>>> s[::-5]
'55555'
>>> s[::-1]
'5432154321543215432154321'

#String Length

>>> a = "Hello, World!"
>>> print(len(a))
13

The len() function returns the length of a string

#Multiple copies

>>> s = '===+'
>>> n = 8
>>> s * n
'===+===+===+===+===+===+===+===+'

#Check String

>>> s = 'spam'
>>> s in 'I saw spamalot!'
True
>>> s not in 'I saw The Holy Grail!'
True

#Concatenates

>>> s = 'spam'
>>> t = 'egg'
>>> s + t
'spamegg'
>>> 'spam' 'egg'
'spamegg'

#Formatting

name = "John"
print("Hello, %s!" % name)
name = "John"
age = 23
print("%s is %d years old." % (name, age))

#format() Method

txt1 = "My name is {fname}, I'm {age}".format(fname = "John", age = 36)
txt2 = "My name is {0}, I'm {1}".format("John",36)
txt3 = "My name is {}, I'm {}".format("John",36)

#Input

>>> name = input("Enter your name: ")
Enter your name: Tom
>>> name
'Tom'

Get input data from console

#Join

>>> "#".join(["John", "Peter", "Vicky"])
'John#Peter#Vicky'

#Endswith

>>> "Hello, world!".endswith("!")
True

#Lists

#Defining

>>> li1 = []
>>> li1
[]
>>> li2 = [4, 5, 6]
>>> li2
[4, 5, 6]
>>> li3 = list((1, 2, 3))
>>> li3
[1, 2, 3]
>>> li4 = list(range(1, 11))
>>> li4
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

#Generate

>>> list(filter(lambda x : x % 2 == 1, range(1, 20)))
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

>>> [x ** 2 for x in range (1, 11) if  x % 2 == 1]
[1, 9, 25, 49, 81]

>>> [x for x in [3, 4, 5, 6, 7] if x > 5]
[6, 7]

>>> list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))
[6, 7]

#Append

>>> li = []
>>> li.append(1)
>>> li
[1]
>>> li.append(2)
>>> li
[1, 2]
>>> li.append(4)
>>> li
[1, 2, 4]
>>> li.append(3)
>>> li
[1, 2, 4, 3]

#List Slicing

Syntax of list slicing:

a_list[start:end]
a_list[start:end:step]

#Slicing

>>> a = ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[2:5]
['bacon', 'tomato', 'ham']
>>> a[-5:-2]
['egg', 'bacon', 'tomato']
>>> a[1:4]
['egg', 'bacon', 'tomato']

#Omitting index

>>> a[:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[0:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[2:]
['bacon', 'tomato', 'ham', 'lobster']
>>> a[2:len(a)]
['bacon', 'tomato', 'ham', 'lobster']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[:]
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

#With a stride

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[0:6:2]
['spam', 'bacon', 'ham']
>>> a[1:6:2]
['egg', 'tomato', 'lobster']
>>> a[6:0:-2]
['lobster', 'tomato', 'egg']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[::-1]
['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']

#Remove

>>> li = ['bread', 'butter', 'milk']
>>> li.pop()
'milk'
>>> li
['bread', 'butter']
>>> del li[0]
>>> li
['butter']

#Access

>>> li = ['a', 'b', 'c', 'd']
>>> li[0]
'a'
>>> li[-1]
'd'
>>> li[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

#Concatenating

>>> odd = [1, 3, 5]
>>> odd.extend([9, 11, 13])
>>> odd
[1, 3, 5, 9, 11, 13]
>>> odd = [1, 3, 5]
>>> odd + [9, 11, 13]
[1, 3, 5, 9, 11, 13]

#Sort & Reverse

>>> li = [3, 1, 3, 2, 5]
>>> li.sort()
>>> li
[1, 2, 3, 3, 5]
>>> li.reverse()
>>> li
[5, 3, 3, 2, 1]

#Count

>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2

#Repeating

>>> li = ["re"] * 3
>>> li
['re', 're', 're']

#Flow control

#Basic

a = 5
if a > 10:
    print("a is totally bigger than 10.")
elif a < 10:
    print("a is smaller than 10.")
else:
    print("a is indeed 10.")

#One line

>>> a = 330
>>> b = 200
>>> r = "a" if a > b else "b"
>>> print(r)
a

#else if

value = True
if not value:
    print("Value is False")
elif value is None:
    print("Value is None")
else:
    print("Value is True")

#Loops

#Basic

primes = [2, 3, 5, 7]
for prime in primes:
    print(prime)

#With index

animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
    print(i, value)

#While

x = 0
while x < 4:
    print(x)
    x += 1  # Shorthand for x = x + 1

#Break

x = 0
for index in range(10):
    x = index * 10
    if index == 5:
    	break
    print(x)

#Continue

for index in range(3, 8): 
    x = index * 10
    if index == 5:
    	continue
    print(x)

#Range

for i in range(4):
    print(i) # Prints: 0 1 2 3

for i in range(4, 8):
    print(i) # Prints: 4 5 6 7

for i in range(4, 10, 2):
    print(i) # Prints: 4 6 8

#With zip()

name = ['Pete', 'John', 'Elizabeth']
age = [6, 23, 44]
for n, a in zip(name, age):
    print('%s is %d years old' %(n, a))

#List Comprehension

result = [x**2 for x in range(10) if x % 2 == 0]
 
print(result)
# [0, 4, 16, 36, 64]

#Functions

#Basic

def hello_world():  
    print('Hello, World!')

#Return

def add(x, y):
    print("x is %s, y is %s" %(x, y))
    return x + y

add(5, 6)    # => 11

#Positional arguments

def varargs(*args):
    return args

varargs(1, 2, 3)  # => (1, 2, 3)

#Keyword arguments

def keyword_args(**kwargs):
    return kwargs

# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")

#Returning multiple

def swap(x, y):
    return y, x

x = 1
y = 2
x, y = swap(x, y)  # => x = 2, y = 1

#Default Value

def add(x, y=10):
    return x + y

add(5)      # => 15
add(5, 20)  # => 25

#Anonymous functions

# => True
(lambda x: x > 2)(3)

# => 5
(lambda x, y: x ** 2 + y ** 2)(2, 1)

#Modules

#Import modules

import math
print(math.sqrt(16))  # => 4.0

#From a module

from math import ceil, floor
print(ceil(3.7))   # => 4.0
print(floor(3.7))  # => 3.0

#Import all

from math import *

#Shorten module

import math as m

# => True
math.sqrt(16) == m.sqrt(16)

#Functions and attributes

import math
dir(math)

#File Handling

#Read file

#Line by line

with open("myfile.txt") as file:
    for line in file:
        print(line)

#With line number

input = open('myfile.txt', 'r')
for i,line in enumerate(input, start=1):
    print("Number %s: %s" % (i, line))

#String

#Write a string

contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
    file.write(str(contents))

#Read a string

with open('myfile1.txt', "r+") as file:
    contents = file.read()
print(contents)

#Object

#Write an object

contents = {"aa": 12, "bb": 21}
with open("myfile2.txt", "w+") as file:
    file.write(json.dumps(contents))

#Read an object

with open('myfile2.txt', "r+") as file:
    contents = json.load(file)
print(contents)

#Delete a File

import os
os.remove("myfile.txt")

#Check and Delete

import os
if os.path.exists("myfile.txt"):
    os.remove("myfile.txt")
else:
    print("The file does not exist")

#Delete Folder

import os
os.rmdir("myfolder")

#Classes & Inheritance

#Defining

class MyNewClass:
    '''This is a docstring.'''
    pass

# Class Instantiation
my = MyNewClass()

#Constructors

class Animal:
    def __init__(self, voice):
    self.voice = voice
 
cat = Animal('Meow')
print(cat.voice)    # => Meow
 
dog = Animal('Woof') 
print(dog.voice)    # => Woof

#Method

class Dog:

    # Method of the class
    def bark(self):
    print("Ham-Ham")
 
charlie = Dog()
charlie.bark()   # => "Ham-Ham"

#Class Variables

class my_class:
    class_variable = "A class variable!"
  
x = my_class()
y = my_class()
 
# => A class variable!
print(x.class_variable)

# => A class variable!
print(y.class_variable)

#Super() Function

class ParentClass:
    def print_test(self):
        print("Parent Method")
 
class ChildClass(ParentClass):
    def print_test(self):
        print("Child Method")
        # Calls the parent's print_test()
        super().print_test() 

>>> child_instance = ChildClass()
>>> child_instance.print_test()
Child Method
Parent Method

#repr() method

class Employee:
    def __init__(self, name):
        self.name = name
 
    def __repr__(self):
        return self.name
 
john = Employee('John')
print(john)  # => John

#User-defined exceptions

class CustomError(Exception):
    pass

#Polymorphism

class ParentClass:
    def print_self(self):
        print('A')
 
class ChildClass(ParentClass):
    def print_self(self):
        print('B')
 
obj_A = ParentClass()
obj_B = ChildClass()
 
obj_A.print_self() # => A
obj_B.print_self() # => B

#Overriding

class ParentClass:
    def print_self(self):
        print("Parent")
 
class ChildClass(ParentClass):
    def print_self(self):
        print("Child")
 
child_instance = ChildClass()
child_instance.print_self() # => Child

#Inheritance

class Animal: 
    def __init__(self, name, legs):
        self.name = name
        self.legs = legs
        
class Dog(Animal):
    def sound(self):
        print("Woof!")
 
Yoki = Dog("Yoki", 4)
print(Yoki.name) # => YOKI
print(Yoki.legs) # => 4
Yoki.sound()     # => Woof!

#Miscellaneous

#Comments

# This is a single line comments.
""" Multiline strings can be written
    using three "s, and are often used
    as documentation.
"""
''' Multiline strings can be written
    using three 's, and are often used
    as documentation.
'''

#Generators

def double_numbers(iterable):
    for i in iterable:
        yield i + i

Generators help you make lazy code.

#Generator to list

values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)

# => [-1, -2, -3, -4, -5]
print(gen_to_list)

#Handle exceptions

try:
    # Use "raise" to raise an error
    raise IndexError("This is an index error")
except IndexError as e:
    pass                 # Pass is just a no-op. Usually you would do recovery here.
except (TypeError, NameError):
    pass                 # Multiple exceptions can be handled together, if required.
else:                    # Optional clause to the try/except block. Must follow all except blocks
    print("All good!")   # Runs only if the code in try raises no exceptions
finally:                 # Execute under all circumstances
    print("We can clean up resources here")