Python Course

Cheatsheet

A quick reference — the essential syntax of modern Python on a single page. Press Ctrl/Cmd + P to print it.

Variables and types

Assignment

x = 10           # int
pi = 3.14        # float
name = "Anna"    # str
active = True    # bool
empty = None

No let/const/var: Python has no variable declaration, only assignment.

Types and conversions

type(42)         # <class 'int'>
int("42")        # 42
str(3.14)        # '3.14'
float("1.5")     # 1.5
bool(0)          # False

Comparisons

1 == 1.0         # True
1 is 1.0         # False  (identity vs equality)
None is None     # True   (always use "is" with None)
"a" in "house"   # True

Use == for equality, is only for None/True/False.

Strings

f-string

name = "World"
msg = f"Hello, {name}!"
# Expressions and formatting:
f"pi = {3.14159:.2f}"   # 'pi = 3.14'
f"{42:>5}"              # '   42'

Common methods

len("hello")             # 5
"hello".upper()          # 'HELLO'
"  hello  ".strip()      # 'hello'
"a,b,c".split(",")       # ['a','b','c']
"-".join(["a","b","c"])  # 'a-b-c'
"hello".startswith("he") # True
"hello".replace("l", "L")

Slicing

s = "Python"
s[0]      # 'P'
s[-1]     # 'n'
s[1:4]    # 'yth'
s[:3]     # 'Pyt'
s[::-1]   # 'nohtyP'

Numbers and math

Operators

7 // 2     # 3   (integer division)
7 % 2      # 1   (remainder)
2 ** 10    # 1024 (exponentiation)
abs(-5)    # 5
round(1.6) # 2

Math module

import math
math.pi             # 3.141592653589793
math.sqrt(16)       # 4.0
math.floor(1.9)     # 1
math.ceil(1.1)      # 2
math.log(math.e)    # 1.0

Lists

Create and access

a = [1, 2, 3]
a[0]            # 1
len(a)          # 3
a.append(4)     # [1,2,3,4]
a.pop()         # removes last item
a[1:3] = [9]    # slice assignment

Comprehensions

[x * 2 for x in range(5)]              # [0,2,4,6,8]
[x for x in nums if x > 0]             # filter
[(x, y) for x in xs for y in ys]       # cartesian product

Sorting

sorted([3, 1, 2])                # [1,2,3]  (copy)
[3,1,2].sort()                   # in-place
sorted(users, key=lambda u: u["age"])

Dictionaries

Create and access

u = {"name": "Anna", "age": 30}
u["name"]            # 'Anna'
u.get("email", "?")  # default
u["email"] = "x@y"
del u["age"]

Iteration

for k in u: ...
for k, v in u.items(): ...
list(u.keys())
list(u.values())

Comprehensions

{x: x*x for x in range(4)}              # {0:0,1:1,2:4,3:9}
{k: v for k, v in u.items() if v}        # filter

Tuples and sets

Tuples (immutable)

t = (1, 2, 3)
x, y, z = t          # unpacking
t + (4, 5)           # (1,2,3,4,5)
len(t)

Sets (unique value collections)

s = {1, 2, 3}
s.add(4)
s & {2, 3, 5}   # intersection: {2,3}
s | {5}         # union
set("abracadabra")  # {'a','b','r','c','d'}

Control Flow

if / elif / else

if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")
# Ternary
label = "ok" if x > 0 else "ko"

for / while

for i in range(5):
    print(i)
for item in my_list:
    ...
for i, item in enumerate(my_list):
    ...
while cond:
    ...

break / continue

for n in nums:
    if n < 0:
        continue   # skip
    if n > 100:
        break      # exit
    ...

try / except

try:
    value = int(s)
except ValueError as err:
    print("not a number:", err)
else:
    print("ok:", value)
finally:
    print("always")

Functions

Definition

def add(a, b):
    return a + b

# Optional types (type hints)
def greet(name: str = "World") -> str:
    return f"Hello, {name}"

Variable args

def add_all(*numbers):
    return sum(numbers)
add_all(1, 2, 3)        # 6
add_all(*[1, 2, 3])     # 6

def configure(**opts):
    print(opts)
configure(host="localhost", port=8000)

lambda

square = lambda x: x * x
square(5)              # 25
sorted(items, key=lambda i: i.price)

Lambda = anonymous function with a single expression. Use def for longer blocks.

Classes

Base class

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

    def distance_origin(self):
        return (self.x ** 2 + self.y ** 2) ** 0.5

p = Point(3, 4)
p.distance_origin()  # 5.0

Inheritance

class Animal:
    def __init__(self, name):
        self.name = name
    def sound(self):
        return "..."

class Dog(Animal):
    def sound(self):
        return "woof"

Dog("Fido").sound()  # 'woof'

Dunder methods

class Money:
    def __init__(self, n):
        self.n = n
    def __repr__(self):
        return f"Money({self.n})"
    def __add__(self, other):
        return Money(self.n + other.n)

Modules and import

Import

import math
from math import sqrt, pi
from math import sqrt as sqroot
import os.path as p

Useful standard library

import json, datetime, pathlib, re, collections, itertools, csv

json.dumps({"a": 1})
datetime.date.today()
pathlib.Path("file.txt").read_text()
re.findall(r"\d+", "I have 3 apples and 12 pears")
collections.Counter("abracadabra")

Modern Python

Type hints

def total(nums: list[int]) -> int:
    return sum(nums)

def find(id: int) -> str | None:
    ...

from typing import Callable
f: Callable[[int], int] = lambda x: x + 1

Annotations only: Python does NOT enforce them at runtime. Use mypy for static checking.

dataclass

from dataclasses import dataclass, field

@dataclass
class Point:
    x: float
    y: float

@dataclass(frozen=True)
class Coord:
    lat: float
    lon: float

@dataclass
class Basket:
    items: list = field(default_factory=list)

Never default = []: use field(default_factory=list).

Context manager

with open("f.txt") as f:
    data = f.read()

from contextlib import contextmanager, suppress

@contextmanager
def timer():
    import time; t0 = time.perf_counter()
    try:
        yield
    finally:
        print(time.perf_counter() - t0)

with suppress(FileNotFoundError):
    open("maybe.txt").read()

Decorators

from functools import wraps, lru_cache

def log(func):
    @wraps(func)
    def wrapper(*a, **kw):
        print(func.__name__)
        return func(*a, **kw)
    return wrapper

@lru_cache(maxsize=None)
def fib(n):
    return n if n < 2 else fib(n-1) + fib(n-2)

Python · Cheatsheet — eLearner.app