The Python cheat sheet is a one-page reference sheet for the Python 3 programming language.
>>> print("Hello, World!")
Hello, World!
The famous "Hello World" program in Python
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.
str |
Text |
int , float , complex |
Numeric |
list , tuple , range |
Sequence |
dict |
Mapping |
set , frozenset |
Set |
bool |
Boolean |
bytes , bytearray , memoryview |
Binary |
See: Data Types
mylist = []
mylist.append(1)
mylist.append(2)
for item in mylist:
print(item) # prints out 1,2
See: Lists
num = 200
if num > 0:
print("num is greater than 0")
else:
print("num is not greater than 0")
See: Flow control
for item in range(6):
if item == 3: break
print(item)
else:
print("Finally finished!")
See: Loops
>>> def my_function():
... print("Hello from a function")
...
>>> my_function()
Hello from a function
See: Functions
with open("myfile.txt", "r", encoding='utf8') as file:
for line in file:
print(line)
See: File Handling
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
counter = 0
counter += 10 # => 10
counter = 0
counter = counter + 10 # => 10
message = "Part 1."
# => Part 1.Part 2.
message += "Part 2."
>>> website = 'Quickref.ME'
>>> f"Hello, {website}"
"Hello, Quickref.ME"
>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'
See: Python F-Strings
hello = "Hello World"
hello = 'Hello World'
multi_string = """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'>
my_bool = True
my_bool = 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
my_tuple = (1, 2, 3)
my_tuple = 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(['one', 'two', 'three', 'four'])
>>> a['four']
4
Key: Value pair, JSON like object
x = int(1) # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3
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
x = str("s1") # x will be 's1'
y = str(2) # y will be '2'
z = str(3.0) # z will be '3.0'
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
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
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
>>> hello = "Hello, World"
>>> print(hello[1])
e
>>> print(hello[-1])
d
Get the character at position 1 or last
>>> for char in "foo":
... print(char)
f
o
o
Loop through the letters in the word "foo"
┌───┬───┬───┬───┬───┬───┬───┐
| 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'
>>> s = '12345' * 5
>>> s
'1234512345123451234512345'
>>> s[::5]
'11111'
>>> s[4::5]
'55555'
>>> s[::-5]
'55555'
>>> s[::-1]
'5432154321543215432154321'
>>> hello = "Hello, World!"
>>> print(len(hello))
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 = "John"
print("Hello, %s!" % name)
name = "John"
age = 23
print("%s is %d years old." % (name, age))
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)
>>> 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
>>> 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'{"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'{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'{-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'{12345:+}' # [sign] (+/-)
'+12345'
>>> f'{-12345:+}'
'-12345'
>>> f'{-12345:+10}'
' -12345'
>>> f'{-12345:+010}'
'-000012345'
>>> 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]
>>> 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]
>>> 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]
Syntax of list slicing:
a_list[start:end]
a_list[start:end:step]
>>> 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']
>>> 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']
['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']
>>> 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
>>> 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]
>>> li = [3, 1, 3, 2, 5]
>>> li.sort()
>>> li
[1, 2, 3, 3, 5]
>>> li.reverse()
>>> li
[5, 3, 3, 2, 1]
>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2
>>> li = ["re"] * 3
>>> li
['re', 're', 're']
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.")
>>> 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")
x = 1
match x:
case 0:
print("zero")
case 1:
print("one")
case _:
print("multiple")
primes = [2, 3, 5, 7]
for prime in primes:
print(prime)
Prints: 2 3 5 7
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
x = 0
while x < 4:
print(x)
x += 1 # Shorthand for x = x + 1
Prints: 0 1 2 3
x = 0
for index in range(10):
x = index * 10
if index == 5:
break
print(x)
Prints: 0 10 20 30 40
for index in range(3, 8):
x = index * 10
if index == 5:
continue
print(x)
Prints: 30 40 60 70
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
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,
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
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)
Type of "args" is tuple.
def keyword_args(**kwargs):
return kwargs
# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")
Type of "kwargs" is dict.
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)
class MyNewClass:
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 MyClass:
class_variable = "A class variable!"
# => A class variable!
print(MyClass.class_variable)
x = MyClass()
# => A class variable!
print(x.class_variable)
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
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!
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
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)
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)
def calc_summary(*args: int):
return sum(args)
print(calc_summary(3, 1, 4)) # => 8
Indicate all arguments' type is int.
def say_hello(name) -> str:
return "Hello, " + name
var = "Python"
print(say_hello(var)) # => Hello, Python
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.
def calc_summary(**kwargs: int):
return sum(kwargs.values())
print(calc_summary(a=1, b=2)) # => 3
Indicate all parameters' value type is int.
def resp200() -> (int, str):
return 200, "OK"
returns = resp200()
print(returns) # => (200, 'OK')
print(type(returns)) # tuple
def resp200(meaningful) -> int | str:
return "OK" if meaningful else 200
Since Python 3.10
class Employee:
name: str
age: int
def __init__(self, name, age):
self.name = name
self.age = age
self.graduated: bool = False
class Employee:
name: str
def set_name(self, name) -> "Employee":
self.name = name
return self
def copy(self) -> 'Employee':
return type(self)(self.name)
from typing import Self
class Employee:
name: str
age: int
def set_name(self: Self, name) -> Self:
self.name = name
return self
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)
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)
# 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)
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")