Python Cheat Sheet 2026 — Syntax, Data Structures, and Common Patterns
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Last updated: March 2026 Variables and Types name = "Alex" # str age = 25 # int height = 1.75 # float is_active = True # bool nothing = None # NoneType Type Example Notes int 42 No size limit float 3.14 64-bit decimal str "hello" Immutable bool True, False Capitalized None None Null equivalent list [1, 2, 3] Mutable, ordered tuple (1, 2, 3) Immutable, ordered dict {"a": 1} Key-value pairs set {1, 2, 3} Unique values, unordered Type Conversions int("42") # 42 float("3.14") # 3.14 str(42) # "42" list((1, 2, 3)) # [1, 2, 3] tuple([1, 2, 3]) # (1, 2, 3) set([1, 1, 2]) # {1, 2} bool(0) # False bool("") # False bool([]) # False bool("hello") # True Strings name = "Alex" f"Hello {name}" # f-string: Hello Alex f"Age: {age + 1}" # f-string with expression f"{price:.2f}" # format: 9.99 f"{name!r}" # repr: 'Alex' f"{num:,}" # thousands separator: 1,000,000 f"{value:>10}" # right-align, width 10 f"{value:<10}" # left-align, width 10 # Common methods "hello".upper() # "HELLO" "HELLO".lower() # "hello" "hello world".title() # "Hello World" " hello ".strip() # "hello" "hello world".split() # ["hello", "world"] "hello world".split("o") # ["hell", " w", "rld"] ", ".join(["a", "b", "c"]) # "a, b, c" "hello".replace("l", "r") # "herro" "hello".startswith("he") # True "hello".endswith("lo") # True "hello world".find("world") # 6 (-1 if not found) "hello world".count("l") # 3 "42".isdigit() # True # Multi-line strings text = """ First line Second line """ # Raw strings (no escape processing) path = r"C:\Users\name\folder" Lists nums = [1, 2, 3, 4, 5] # Access nums[0] # 1 (first) nums[-1] # 5 (last) nums[1:3] # [2, 3] (slice) nums[:3] # [1, 2, 3] (first 3) nums[2:] # [3, 4, 5] (from index 2) nums[::2] # [1, 3, 5] (every 2nd) nums[::-1] # [5, 4, 3, 2, 1] (reversed) # Modify nums.append(6) # add to end nums.insert(0, 0) # insert at index nums.extend([7, 8]) # add multiple nums.remove(3) # remove first occurrence nums.pop() # remove and return last nums.pop(0) # remove and return at index nums.sort() # sort in place nums.sort(reverse=True) # sort descending nums.reverse() # reverse in place nums.clear() # remove all # Functions len(nums) # length min(nums) # smallest max(nums) # largest sum(nums) # total sorted(nums) # new sorted list reversed(nums) # reversed iterator Dictionaries user = {"name": "Alex", "age": 25, "city": "Berlin"} # Access user["name"] # "Alex" (KeyError if missing) user.get("name") # "Alex" user.get("phone", "N/A") # "N/A" (default if missing) # Modify user["age"] = 26 # update user["email"] = "alex@mail.com" # add new key del user["city"] # delete key user.pop("age") # remove and return value # Iterate user.keys() # dict_keys(["name", "age", ...]) user.values() # dict_values(["Alex", 25, ...]) user.items() # dict_items([("name", "Alex"), ...]) for key, value in user.items(): print(f"{key}: {value}") # Merge (Python 3.9+) merged = dict1 | dict2 # dict2 values win on conflict Sets and Tuples # Sets — unique values, unordered colors = {"red", "green", "blue"} colors.add("yellow") colors.remove("red") # KeyError if missing colors.discard("red") # no error if missing a | b # union a & b # intersection a - b # difference a ^ b # symmetric difference # Tuples — immutable point = (10, 20) x, y = point # unpacking name, *rest = ("Alex", 25, "Berlin") # name="Alex", rest=[25, "Berlin"] Control Flow # if / elif / else if age < 18: print("minor") elif age < 65: print("adult") else: print("senior") # Ternary status = "adult" if age >= 18 else "minor" # for loop for item in items: print(item) for i in range(5): # 0, 1, 2, 3, 4 for i in range(2, 10): # 2, 3, ..., 9 for i in range(0, 10, 2): # 0, 2, 4, 6, 8 for i, item in enumerate(items): # index + value print(f"{i}: {item}") for a, b in zip(list1, list2): # parallel iteration print(a, b) # while loop while count > 0: count -= 1 # break / continue for item in items: if item == "skip": continue # skip this iteration if item == "stop": break # exit loop Pattern Matching (Python 3.10+) match status_code: case 200: print("OK") case 404: print("Not Found") case 500: print("Server Error") case _: print(f"Unknown: {status_code}") # Match with destructuring match point: case (0, 0): print("Origin") case (x, 0): print(f"On x-axis at {x}") case (0, y): print(f"On y-axis at {y}") case (x, y): print(f"Point at ({x}, {y})") # Match with guards match user: case {"role": "admin", "name": name}: print(f"Admin: {name}") case {"role": "user", "name": name} if name != "blocked": print(f"User: {name}") Comprehensions # List comprehension squares = [x**2 for x in range(10)] evens = [x for x in nums if x % 2 == 0] pairs = [(x, y) for x in range(3) for y in range(3)] # Dict comprehension word_lengths = {word: len(word) for word in words} filtered = {k: v for k, v in data.items() if v > 0} # Set comprehension unique_lengths = {len(word) for word in words} # Generator expression (lazy, memory-efficient) total = sum(x**2 for x in range(1000000)) Functions # Basic function def greet(name): return f"Hello {name}" # Default parameters def greet(name="World"): return f"Hello {name}" # *args and **kwargs def func(*args, **kwargs): print(args) # tuple of positional args print(kwargs) # dict of keyword args # Lambda double = lambda x: x * 2 sorted(users, key=lambda u: u["age"]) # Type hints (Python 3.10+) def greet(name: str) -> str: return f"Hello {name}" def process(items: list[int]) -> dict[str, int]: return {"sum": sum(items), "count": len(items)} Classes class User: def __init__(self, name: str, age: int): self.name = name self.age = age def greet(self) -> str: return f"Hi, I'm {self.name}" def __repr__(self) -> str: return f"User({self.name!r}, {self.age})" user = User("Alex", 25) # Dataclass (Python 3.7+) — auto-generates __init__, __repr__, __eq__ from dataclasses import dataclass @dataclass class User: name: str age: int city: str = "Unknown" # Inheritance class Admin(User): def __init__(self, name, age, level): super().__init__(name, age) self.level = level Error Handling try: result = 10 / 0 except ZeroDivisionError: print("Cannot divide by zero") except (ValueError, TypeError) as e: print(f"Error: {e}") else: print("Success") # runs if no exception finally: print("Always runs") # cleanup # Raise an exception raise ValueError("Invalid input") File I/O # Read entire file with open("file.txt", "r") as f: content = f.read() # Read lines with open("file.txt", "r") as f: lines = f.readlines() # list of lines # or for line in f: print(line.strip()) # Write with open("file.txt", "w") as f: # overwrite f.write("Hello\n") with open("file.txt", "a") as f: # append f.write("More text\n") # JSON import json data = json.loads('{"name": "Alex"}') # parse string text = json.dumps(data, indent=2) # to string with open("data.json", "r") as f: data = json.load(f) # parse file with open("data.json", "w") as f: json.dump(data, f, indent=2) # write file Async / Await import asyncio async def fetch_data(url: str) -> str: # simulate async work await asyncio.sleep(1) return f"Data from {url}" # Run async function async def main(): result = await fetch_data("https://api.example.com") print(result) asyncio.run(main()) # Run tasks in parallel async def main(): results = await asyncio.gather( fetch_data("https://api1.example.com"), fetch_data("https://api2.example.com"), ) print(results) # both complete in ~1 second, not 2 Common Built-in Functions Function Description Example len(x) Length len([1,2,3]) → 3 range(n) Sequence 0..n-1 range(5) → 0,1,2,3,4 enumerate(x) Index + value enumerate(["a","b"]) zip(a, b) Pair elements zip([1,2], ["a","b"]) map(fn, x) Apply function map(str, [1,2,3]) filter(fn, x) Filter elements filter(bool, [0,1,"",2]) any(x) True if any truthy any([False, True]) → True all(x) True if all truthy all([True, True]) → True sorted(x) New sorted list sorted([3,1,2]) → [1,2,3] reversed(x) Reversed iterator list(reversed([1,2,3])) isinstance(x, type) Type check isinstance(42, int) → True type(x) Get type type(42) → <class 'int'> dir(x) List attributes dir([]) help(x) Show docs help(str.split) Virtual Environments python -m venv .venv # create source .venv/bin/activate # activate (macOS/Linux) .venv\Scripts\activate # activate (Windows) pip install requests # install package pip freeze > requirements.txt # save dependencies pip install -r requirements.txt # install from file deactivate # deactivate Common Mistakes Mutable default arguments — def add(item, items=[]) shares the same list across all calls. Use def add(item, items=None) and items = items or [] inside the function. ...