Python3 cheat sheet

Style

# Variables

counter = 0
my_counter = 0
my_other_counter = 0

# Functions

def work(): ...

def do_something(): ...

def do_something_else(): ...

# Classes

class FooClass: ...

Variables

Declaration

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

# With type hints (3.6+)

a: int = 1
b: float = 1.1
c: complex = 1 + 2j
d: str = "python"
e: bool = True

Falsy and empty values

a = None
b = 0
c = ""
d = []

Type convertion

int(x)
float(x) 
bool(x) 
str(x)

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

Control flow

Conditional statements

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

# Ternary operator 
x = "a" if n > 1 else "b"

# 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)

# 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="username")


# With type hints (3.6+)

def increment(number: float, by: float = 1.0) -> float:
    return number + by


def do_something() -> None:  # When the function returns nothing
    ...

Exceptions

# Handling Exceptions 
try:
    do_something()
except (ValueError, ZeroDivisionError):
    do_something_else()
else: 
    ...  # no exceptions raised
finally:
    do_cleanup()  # cleanup code

# Raising exceptions
if x < 1: raise ValueError("...")

# The with statement 
with open("file.txt") as file:
    ...

Classes

class Point:

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

    def draw(self):
        ...


# Instance and class attributes
class Point:

    default_color = "red"

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


# Instance and class methods
class Point:

    def draw(self):
        ...

    @classmethod
    def zero(cls):
        return cls(0, 0)


# 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

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


# Inheritance

class FileStream(Stream):

    def open(self):
        super().open()


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