python cheatsheet

• Python(python.org)
• Learn X in Y minutes(learnxinyminutes.com)
• Regex in python(quickref.me)

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

The famous "Hello World" program in Python

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.

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

See: Strings

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

See: Lists

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

See: Flow control

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

See: Loops

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

See: Functions

result = 10 + 30  # =>  40
result = 40 - 10  # =>  30
result = 50 * 5   # =>  150
result = 16 / 4   # =>  4
result = 25 % 2   # =>  1
result = 5 ** 3   # =>  125

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

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

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

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

See: Strings

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

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

a = True 
b = False

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

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

See: Lists

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

Similar to List but immutable

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

Set of unique items/objects

>>> 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(['brand', 'colors', 'four'])
>>> a['four']
4

Key: Value pair, JSON like object

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'

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

Get the character at position 1

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

Loop through the letters in the word "banana"

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

The len() function returns the length of a string

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

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

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

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

Get input data from console

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

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

>>> 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]

>>> 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]

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

>>> 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

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

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

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.")

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

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

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

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

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

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

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

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

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

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

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

add(5, 6)    # => 11

def varargs(*args):
    return args

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

def keyword_args(**kwargs):
    return kwargs

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

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

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

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

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

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

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

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

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

from math import *

import math as m

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

import math
dir(math)

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))

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)

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)

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

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

import os
os.rmdir("myfolder")

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

# Class Instantiation
my = MyNewClass()

class Animal:
  def __init__(self, voice):
    self.voice = voice

cat = Animal('Meow')
print(cat.voice)    # => Meow

dog = Animal('Woof') 
print(dog.voice)    # => Woof

class Dog:

  # Method of the class
  def bark(self):
    print("Ham-Ham")

charlie = Dog()
charlie.bark()   # => "Ham-Ham"

class Employee:
  def __init__(self, name):
    self.name = name

  def __repr__(self):
    return self.name

john = Employee('John')
print(john)  # => John

class CustomError(Exception):
  pass

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

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

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!

# 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.
'''

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

Generators help you make lazy code.

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

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