Python cheat sheet

Introduction

• Python (python.org)
• Python Document (docs.python.org)
• Learn X in Y minutes (learnxinyminutes.com)
• Regex in python (cheatsheets.zip)

Hello World

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

The famous "Hello World" program in Python

Variables

age = 18 # age is of type int
name = "John" # name is now of type str
print(name)

Python can't declare a variable without assignment.

Data Types

See: Data Types

Slicing String

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

See: Strings

Lists

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

See: Lists

If Else

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

See: Flow control

Loops

for item in range(6):
if item == 3: break
print(item)
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 line in file:
print(line)

See: File Handling

Arithmetic

result = 10 + 30 # => 40
result = 40 - 10 # => 30
result = 50 * 5 # => 250
result = 16 / 4 # => 4.0 (Float Division)
result = 16 // 4 # => 4 (Integer Division)
result = 25 % 2 # => 1
result = 5 ** 3 # => 125

The / means quotient of x and y, and the // means floored quotient of x and y, also see StackOverflow

Plus-Equals

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

message = "Part 1."

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

f-Strings (Python 3.6+)

>>> website = 'Quickref.ME'
>>> f"Hello, {website}"
"Hello, Quickref.ME"

>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

See: Python F-Strings

Strings

hello = "Hello World"
hello = 'Hello World'

multi_string = """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

my_bool = True
my_bool = 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

my_tuple = (1, 2, 3)
my_tuple = 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'

Heaps

import heapq

myList = [9, 5, 4, 1, 3, 2]
heapq.heapify(myList) # turn myList into a Min Heap
print(myList) # => [1, 3, 2, 5, 9, 4]
print(myList[0]) # first value is always the smallest in the heap

heapq.heappush(myList, 10) # insert 10
x = heapq.heappop(myList) # pop and return smallest item
print(x) # => 1

Negate all values to use Min Heap as Max Heap

myList = [9, 5, 4, 1, 3, 2]
myList = [-val for val in myList] # multiply by -1 to negate
heapq.heapify(myList)

x = heapq.heappop(myList)
print(-x) # => 9 (making sure to multiply by -1 again)

Heaps are binary trees for which every parent node has a value less than or equal to any of its children. Useful for accessing min/max value quickly. Time complexity: O(n) for heapify, O(log n) push and pop. See: Heapq

Stacks and Queues {.row-span-3}

from collections import deque

q = deque() # empty
q = deque([1, 2, 3]) # with values

q.append(4) # append to right side
q.appendleft(0) # append to left side
print(q) # => deque([0, 1, 2, 3, 4])

x = q.pop() # remove & return from right
y = q.popleft() # remove & return from left
print(x) # => 4
print(y) # => 0
print(q) # => deque([1, 2, 3])

q.rotate(1) # rotate 1 step to the right
print(q) # => deque([3, 1, 2])

Deque is a double-ended queue with O(1) time for append/pop operations from both sides. Used as stacks and queues. See: Deque

Array-like

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

Get the character at position 1 or last

Looping

>>> for char in "foo":
... print(char)
f
o
o

Loop through the letters in the word "foo"

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

>>> hello = "Hello, World!"
>>> print(len(hello))
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 {.col-span-2}

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

f-Strings usage

>>> website = 'Quickref.ME'
>>> f"Hello, {website}"
"Hello, Quickref.ME"

>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

>>> f"""He said {"I'm John"}"""
"He said I'm John"

>>> f'5 {"{stars}"}'
'5 {stars}'
>>> f'{{5}} {"stars"}'
'{5} stars'

>>> name = 'Eric'
>>> age = 27
>>> f"""Hello!
... I'm {name}.
... I'm {age}."""
"Hello!\n I'm Eric.\n I'm 27."

it is available since Python 3.6, also see: Formatted string literals

f-Strings Fill Align

>>> f'{"text":10}' # [width]
'text '
>>> f'{"test":*>10}' # fill left
'******test'
>>> f'{"test":*<10}' # fill right
'test******'
>>> f'{"test":*^10}' # fill center
'***test***'
>>> f'{12345:0>10}' # fill with numbers
'0000012345'

f-Strings Type

>>> f'{10:b}' # binary type
'1010'
>>> f'{10:o}' # octal type
'12'
>>> f'{200:x}' # hexadecimal type
'c8'
>>> f'{200:X}'
'C8'
>>> f'{345600000000:e}' # scientific notation
'3.456000e+11'
>>> f'{65:c}' # character type
'A'
>>> f'{10:#b}' # [type] with notation (base)
'0b1010'
>>> f'{10:#o}'
'0o12'
>>> f'{10:#x}'
'0xa'

F-Strings Others

>>> f'{-12345:0=10}' # negative numbers
'-000012345'
>>> f'{12345:010}' # [0] shortcut (no align)
'0000012345'
>>> f'{-12345:010}'
'-000012345'
>>> import math # [.precision]
>>> math.pi
3.141592653589793
>>> f'{math.pi:.2f}'
'3.14'
>>> f'{1000000:,.2f}' # [grouping_option]
'1,000,000.00'
>>> f'{1000000:_.2f}'
'1_000_000.00'
>>> f'{0.25:0%}' # percentage
'25.000000%'
>>> f'{0.25:.0%}'
'25%'

F-Strings Sign

>>> f'{12345:+}' # [sign] (+/-)
'+12345'
>>> f'{-12345:+}'
'-12345'
>>> f'{-12345:+10}'
' -12345'
>>> f'{-12345:+010}'
'-000012345'

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 {.col-span-2}

>>> 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']

Basic

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

match case

x = 1
match x:
case 0:
print("zero")
case 1:
print("one")
case _:
print("multiple")

Basic

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

Prints: 2 3 5 7

With index

animals = ["dog", "cat", "mouse"]
# enumerate() adds counter to an iterable
for i, value in enumerate(animals):
print(i, value)

Prints: 0 dog 1 cat 2 mouse

While

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

Prints: 0 1 2 3

Break

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

Prints: 0 10 20 30 40

Continue

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

Prints: 30 40 60 70

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()

words = ['Mon', 'Tue', 'Wed']
nums = [1, 2, 3]
# Use zip to pack into a tuple list
for w, n in zip(words, nums):
print('%d:%s, ' %(n, w))

Prints: 1:Mon, 2:Tue, 3:Wed,

for/else

nums = [60, 70, 30, 110, 90]
for n in nums:
if n > 100:
print("%d is bigger than 100" %n)
break
else:
print("Not found!")

Also see: Python Tips

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)

Type of "args" is tuple.

Keyword arguments

def keyword_args(**kwargs):
return kwargs

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

Type of "kwargs" is dict.

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)

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)

Read file

Line by line

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

With line number

file = open('myfile.txt', 'r')
for i, line in enumerate(file, 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")

Defining

class MyNewClass:
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 MyClass:
class_variable = "A class variable!"

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

x = MyClass()

# => A class variable!
print(x.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!

Variable & Parameter

string: str = "ha"
times: int = 3

# wrong hit, but run correctly
result: str = 1 + 2
print(result) # => 3

def say(name: str, start: str = "Hi"):
return start + ", " + name

print(say("Python")) # => Hi, Python

Built-in date type

from typing import Dict, Tuple, List

bill: Dict[str, float] = {
"apple": 3.14,
"watermelon": 15.92,
"pineapple": 6.53,
}
completed: Tuple[str] = ("DONE",)
succeeded: Tuple[int, str] = (1, "SUCCESS")
statuses: Tuple[str, ...] = (
"DONE", "SUCCESS", "FAILED", "ERROR",
)
codes: List[int] = (0, 1, -1, -2)

Built-in date type (3.10+)

bill: dict[str, float] = {
"apple": 3.14,
"watermelon": 15.92,
"pineapple": 6.53,
}
completed: tuple[str] = ("DONE",)
succeeded: tuple[int, str] = (1, "SUCCESS")
statuses: tuple[str, ...] = (
"DONE", "SUCCESS", "FAILED", "ERROR",
)
codes: list[int] = (0, 1, -1, -2)

Positional argument

def calc_summary(*args: int):
return sum(args)

print(calc_summary(3, 1, 4)) # => 8

Indicate all arguments' type is int.

Returned

def say_hello(name) -> str:
return "Hello, " + name

var = "Python"
print(say_hello(var)) # => Hello, Python

Union returned

from typing import Union

def resp200(meaningful) -> Union[int, str]:
return "OK" if meaningful else 200

Means returned value type may be int or str.

Keyword argument

def calc_summary(**kwargs: int):
return sum(kwargs.values())

print(calc_summary(a=1, b=2)) # => 3

Indicate all parameters' value type is int.

Multiple returns

def resp200() -> (int, str):
return 200, "OK"

returns = resp200()
print(returns) # => (200, 'OK')
print(type(returns)) # tuple

Union returned (3.10+)

def resp200(meaningful) -> int | str:
return "OK" if meaningful else 200

Since Python 3.10

Property

class Employee:
name: str
age: int

def __init__(self, name, age):
self.name = name
self.age = age
self.graduated: bool = False

Self instance

class Employee:
name: str

def set_name(self, name) -> "Employee":
self.name = name
return self

def copy(self) -> 'Employee':
return type(self)(self.name)

Self instance (3.11+)

from typing import Self

class Employee:
name: str
age: int

def set_name(self: Self, name) -> Self:
self.name = name
return self

Type & Generic

from typing import TypeVar, Type

T = TypeVar("T")

# "mapper" is a type, like int, str, MyClass and so on.
# "default" is an instance of type T, such as 314, "string", MyClass() and so on.
# returned is an instance of type T too.
def converter(raw, mapper: Type[T], default: T) -> T:
try:
return mapper(raw)
except:
return default

raw: str = input("Enter an integer: ")
result: int = converter(raw, mapper=int, default=0)

Function

from typing import TypeVar, Callable, Any

T = TypeVar("T")

def converter(raw, mapper: Callable[[Any], T], default: T) -> T:
try:
return mapper(raw)
except:
return default

# Callable[[Any], ReturnType] means a function declare like:
# def func(arg: Any) -> ReturnType:
# pass

# Callable[[str, int], ReturnType] means a function declare like:
# def func(string: str, times: int) -> ReturnType:
# pass

# Callable[..., ReturnType] means a function declare like:
# def func(*args, **kwargs) -> ReturnType:
# pass

def is_success(value) -> bool:
return value in (0, "OK", True, "success")

resp = dict(code=0, message="OK", data=[])
successed: bool = converter(resp.message, mapper=is_success, default=False)

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")