Python 3.15 Cheat Sheet

$ uv run --python 3.15 python

$ python -m profiling.sampling run app.py
$ python -m profiling.sampling attach 5171

$ python

$ python my_script.py

# This is a comment
# print("This code will not run.")
print("This will run.")  # Comments are ignored by Python

type(42)                # <class 'int'>
type(3.14)              # <class 'float'>
type("Hello")           # <class 'str'>
type(None)              # <class 'NoneType'>

isinstance(3.14, float)  # True
issubclass(int, object)  # True - all inherit

int("42")                # 42
float("3.14")            # 3.14
str(42)                  # "42"
list("abc")              # ["a", "b", "c"]

name = "Leo"           # String
age = 7                # Integer
height = 5.6           # Float
is_cat = True          # Boolean
flaws = None           # None type

x, y = 10, 20          # Assign multiple values
a = b = c = 0          # Same value, 3 names

counter += 1
numbers += [4, 5]
permissions |= write

single = 'Hello'
double = "World"
multi = """Multiple
line string"""

greeting = "me" + "ow!"  # "meow!"
repeat = "Meow!" * 3     # "Meow!Meow!Meow!"
length = len("Python")   # 6

"a".upper()                     # "A"
"A".lower()                     # "a"
" a ".strip()                   # "a"
"abc".replace("bc", "ha")       # "aha"
"a b".split()                   # ["a", "b"]
"-".join(["a", "b"])            # "a-b"

text = "Python"
text[0]      # "P" (first)
text[-1]     # "n" (last)
text[1:4]    # "yth" (slice)
text[:3]     # "Pyt" (from start)
text[3:]     # "hon" (to end)
text[::2]    # "Pto" (every 2nd)
text[::-1]   # "nohtyP" (reverse)

# f-strings
name = "Aubrey"
age = 2
f"Hello, {name}!"               # "Hello, Aubrey!"
f"{name} is {age} years old"    # "Aubrey is 2 years old"
f"Debug: {age=}"                # "Debug: age=2"
f"{3.14159:.2f}"                # "3.14"

# Raw strings keep escape sequences
r"C:\new"                       # "C:\\new"

10 + 3    # 13
10 - 3    # 7
10 * 3    # 30
10 / 3    # 3.3333333333333335
10 // 3   # 3
10 % 3    # 1
2 ** 3    # 8

abs(-5)              # 5
round(3.7)           # 4
round(3.14159, 2)    # 3.14
min(3, 1, 2)         # 1
max(3, 1, 2)         # 3
sum([1, 2, 3])       # 6

if age < 13:
    category = "child"
elif age < 20:
    category = "teenager"
else:
    category = "adult"

x == y    # Equal to
x != y    # Not equal to
x < y     # Less than (x <= y for or equal)
x > y     # Greater than (x >= y for or equal)

if age >= 18 and has_car:
    print("Roadtrip!")

if is_weekend or is_holiday:
    print("No work today.")

# Loop through range
for i in range(5):      # 0, 1, 2, 3, 4
    print(i)

# Loop through collection
fruits = ["apple", "banana"]
for fruit in fruits:
    print(fruit)

# With enumerate for index
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

while True:
    answer = input("Enter 'quit' to exit: ")
    if answer == "quit":
        break
    print(f"You entered: {answer}")

def greet():
    return "Hello!"

def greet_person(name):
    return f"Hello, {name}!"

def add(x, y=10):    # Default parameter
    return x + y

greet()                   # "Hello!"
add(5, 3)                 # 8
add(7)                    # 17

def get_min_max(numbers):
    return min(numbers), max(numbers)

minimum, maximum = get_min_max([1, 5, 3])

MISSING = sentinel("MISSING")

def get(config, key, default=MISSING):
    value = config.get(key, default)
    if value is MISSING:
        raise KeyError(key)
    return value

square = lambda x: x**2
result = square(5)  # 25

numbers = [1, 2, 3, 4]
squared = list(map(lambda x: x**2, numbers))
evens = list(filter(lambda x: x % 2 == 0, numbers))

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says Woof!"

# Create instance
my_dog = Dog("Frieda", 3)
print(my_dog.bark())  # Frieda says Woof!

class Cat:
    species = "Felis catus"   # Class attribute

    def __init__(self, name):
        self.name = name      # Instance attribute

    def meow(self):
        return f"{self.name} says Meow!"

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

    def speak(self):
        pass

class Dog(Animal):
    def speak(self):
        return f"{self.name} barks!"

try:
    number = int(input("Enter a number: "))
    result = 10 / number
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")
else:
    print(f"Result: {result}")
finally:
    print("Calculation attempted")

ValueError         # Invalid value
TypeError          # Wrong type
IndexError         # List index out of range
KeyError           # Dict key not found
FileNotFoundError  # File doesn't exist

>>> ratings = [4.5, 3.8]
>>> ratings.push(4.9)
AttributeError: 'list' object has no
attribute 'push'. Did you mean '.append'?

def validate_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

# Creating lists
empty = []
nums = [5]
mixed = [1, "two", 3.0, True]

# List methods
nums.append("x")         # Add to end
nums.insert(0, "y")      # Insert at index 0
nums.extend(["z", 5])    # Extend with iterable
nums.remove("x")         # Remove first "x"
last = nums.pop()        # Pop returns last element

# List indexing and checks
fruits = ["banana", "apple", "orange"]
fruits[0]                # "banana"
fruits[-1]               # "orange"
"apple" in fruits        # True
len(fruits)              # 3

# Creating tuples
point = (3, 4)
single = (1,)    # Note the comma!
empty = ()

# Basic tuple unpacking
point = (3, 4)
x, y = point
x                # 3
y                # 4

# Extended unpacking
first, *rest = (1, 2, 3, 4)
first            # 1
rest             # [2, 3, 4]

# Creating Sets
a = {1, 2, 3}
b = set([3, 4, 4, 5])

# Set Operations
a | b            # {1, 2, 3, 4, 5}
a & b            # {3}
a - b            # {1, 2}
a ^ b            # {1, 2, 4, 5}

# Creating Dictionaries
empty = {}
pet = {"name": "Leo", "age": 42}

# Dictionary Operations
pet["sound"] = "Purr!"   # Add key and value
pet["age"] = 7           # Update value
age = pet.get("age", 0)  # Get with default
del pet["sound"]         # Delete key
pet.pop("age")           # Remove and return

# Dictionary Views
pet.keys()         # dict_keys(['name'])
pet.values()       # dict_values(['Leo'])
pet.items()        # dict_items([('name', 'Leo')])

>>> config = frozendict({"debug": True})
>>> config | {"debug": False}
frozendict({'debug': False})
>>> config["debug"] = False
TypeError: 'frozendict' object does not
support item assignment

# Basic
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

# Nested
matrix = [[i*j for j in range(3)]
          for i in range(3)]

# Dictionary comprehension
words = ["hello", "world"]
lengths = {word: len(word) for word in words}

# Set comprehension
unique = {len(word) for word in words}

# Generator expression
sum_squares = sum(x**2 for x in range(1000))

>>> temps = [[18.2, 21.7], [17.9]]
>>> [*t for t in temps]
[18.2, 21.7, 17.9]
>>> parts = [{"host": "db"}, {"port": 5432}]
>>> {**p for p in parts}
{'host': 'db', 'port': 5432}

# Read an entire file
with open("file.txt", mode="r", encoding="utf-8") as file:
    content = file.read()

# Write a file
with open("out.txt", mode="w", encoding="utf-8") as file:
    file.write("Hello, World!\n")

# Append to a File
with open("log.txt", mode="a", encoding="utf-8") as file:
    file.write("New log entry\n")

# 3.15 defaults to UTF-8 for text files,
# whatever the operating system's locale says
open("notes.txt").read()

# Opt back in to the locale encoding
open("legacy.txt", encoding="locale")

# Import entire module
import math
result = math.sqrt(16)

# Import specific function
from math import sqrt
result = sqrt(16)

# Import with alias
import numpy as np
array = np.array([1, 2, 3])

import sys

lazy import json        # Bound, not loaded

"json" in sys.modules   # False
json.dumps({"a": 1})    # Imports on first use
"json" in sys.modules   # True

$ python -m venv .venv

PS> .venv\Scripts\activate

$ source .venv/bin/activate

$ python -m pip install requests

$ python -m pip freeze > requirements.txt
$ python -m pip install -r requirements.txt

# Swap variables
a, b = b, a

# Remove duplicates
unique_unordered = list(set(my_list))

# Remove duplicates, preserve order
unique = list(dict.fromkeys(my_list))