Python cheatsheet

To help C++ background reader, corresponding C++ syntax is also provided whenever necessary

Variables

a = 1       # integer
b = 1.1     # float
c = 1 + 2j  # complex number (a + bi)
d = 'a'     # string
e = True    # boolean (True / False)

Strings

x = "Python"
len(x)
x[0]
x[-1]
x[0:3]

# Formatted strings
name = f"{first} {last}"

# Escape sequences
\” \’ \\ \n

# String methods
x.upper()
x.lower()
x.title()
x.strip()
x.find(“p”)
x.replace(“a”, “b”)
“a” in x

Type Conversion

int(x)  
float(x) 
bool(x) 
string(x)

False Values

0
""
[]

False

Conditional Statements

if x == 1:  
    print(“a”)
elif x == 2:  
    print(“b”)
else:   
    print(“c”)

# Ternary operator 
x = "a" if n > 1 else "b"
# x = n > 1 ? "a" : "b" # C++

# Chaining comparison operators
if 18 <= age < 65:

Loops

for n in range(1, 10): 
    print(n)

while n < 10: 
    print(n)
    n += 1

Functions

def increment(number, by=1):   
    return number + by

# Keyword arguments 
increment(2, by=1) # ok

increment(2, 1)  # ok
increment(2, b=1) # ERR TypeError: increment() got an unexpected keyword argument 'b'
'

# Variable number of arguments 
def multiply(*numbers): 
    for number in numbers: 
        print number 


multiply(1, 2, 3, 4)

# Variable number of keyword arguments 
def save_user(**user):  
    ...


save_user(id=1, name="Mosh")

Lists

# Creating lists
letters = ["a", "b", "c"]     
matrix = [[0, 1], [1, 2]]
zeros = [0] * 5
combined = zeros + letters
numbers = list(range(20))

# Accessing items
letters = ["a", "b", "c", "d"]
letters[0]  # "a"
letters[-1] # "d"

# Slicing lists 
letters[0:3]   # "a", "b", "c"
letters[:3]    # "a", "b", "c"
letters[0:]    # "a", "b", "c", "d"
letters[:]     # "a", "b", "c", "d"
letters[::2]   # "a", "c"
letters[::-1]  # "d", "c", "b", "a" 

# Unpacking 
first, second, *other = letters 

# Looping over lists 
for letter in letters: 
    ... 

for index, letter in enumerate(letters): 
    ... 

# Adding items 
letters.append("e")
letters.insert(0, "-")

# Removing items 
letters.pop()
letters.pop(0)
letters.remove("b")
del letters[0:3]

# Finding items 
if "f" in letters: 
    letters.index("f")

# Sorting lists 
letters.sort()
letters.sort(reverse=True) 
x=[3,1,2]; sorted(x) # output:[1,2,3]; but x is unchanged
# Custom sorting 
items = [
    ("Product1", 10),
    ("Product2", 9),
    ("Product3", 11)
]

items.sort(key=lambda item: item[1])
#sort(items.begin(), items.end(), [](auto& a, auto& b){return a[1] < b[1];}); #C++
# Map and filter 
prices = list(map(lambda item: item[1], items))
expensive_items = list(filter(lambda item: item[1] >= 10, items))

# List comprehensions 
prices = [item[1] for item in items]
expensive_items = [item for item in items if item[1] >= 10]

# Zip function 
list1 = [1, 2, 3]
list2 = [10, 20, 30]
combined = list(zip(list1, list2))    # [(1, 10), (2, 20)]

Tuples

point = (1, 2, 3)
point(0:2)     # (1, 2)
x, y, z = point 
if 10 in point: 
    ... 

# Swapping variables 
x = 10
y = 11
x, y = y, x 

Arrays

from array import array 

numbers = array("i", [1, 2, 3])

Sets

first = {1, 2, 3, 4}
second = {1, 5}

first | second  # {1, 2, 3, 4, 5}
first & second  # {1}
first - second  # {2, 3, 4}
first ^ second  # {2, 3, 4, 5}

if 1 in first: 
    ... 

Dictionaries

point = {"x": 1, "y": 2}
point = dict(x=1, y=2)
point["z"] = 3
if "a" in point: 
    ... 
point.get("a", 0)   # 0
del point["x"]
for key, value in point.items(): 
   ... 

# Dictionary comprehensions 
values = {x: x * 2 for x in range(5)}

Generator Expressions

values = (x * 2 for x in range(10000))
len(values)  # Error
for x in values: 

Unpacking Operator

first = [1, 2, 3]
second = [4, 5, 6]
combined = [*first, "a", *second]

first = {"x": 1}
second = {"y": 2}
combined = {**first, **second}

Exceptions

# Handling Exceptions 
try: 
  …

except (ValueError, ZeroDivisionError):
  …
else: 
  # no exceptions raised
finally:
  # cleanup code 

# Raising exceptions 
if x < 1: 
    
    raise ValueError(“…”)

# The with statement 
with open(“file.txt”) as file: 
   
   … 

Classes

# Creating classes
class Point: 
    def __init__(self, x, y):        
        self.x = x
        self.y = y 

    def draw(self): 
        
        …

# Instance vs class attributes
class Point: 
    default_color = “red”

    def __init__(self, x, y): 
        
        self.x = x

# Instance vs class methods
class Point: 
    def draw(self): 
       
        …
    
    @classmethod 
    def zero(cls): 
        
        return cls(0, 0)


# Magic methods
__str__()

__eq__()
__cmp__()
... 

# Private members 
class Point: 
    def __init__(self, x): 
        
        self.__x = x


# Properties 
class Point: 
    def __init__(self, x): 
        
        self.__x = x

    @property
    def x(self):    
        return self.__x     

    @property.setter:
    def x.setter(self, value): 
        self.__x = value 


# Inheritance
class FileStream(Stream): 
    def open(self): 
       
         super().open()
         … 

# Multiple inheritance 
class FlyingFish(Flyer, Swimmer): 
    … 

# Abstract base classes
from abc import ABC, abstractmethod

class Stream(ABC): 
    @abstractmethod
    def read(self): 
        pass  

# Named tuples 
from collections import namedtuple

Point = namedtuple(“Point”, [“x”, “y”])
point = Point(x=1, y=2)

Reference:

https://programmingwithmosh.com/python/python-3-cheat-sheet/