Skip to main content

Python Cheatsheet

Harish Balaji
EditReport

This page is a quick reference for Python concepts and methods that show up constantly in DSA and competitive programming. If you're just starting out, don't worry, every snippet here is explained line by line ๐Ÿ˜Š

Core Syntaxโ€‹

Swap, Unpacking, Multiple Assignmentโ€‹

Swap and unpack values in Python
a, b = b, a                           # Swaps a and b without a temp variable
x, y, z = 1, 2, 3                     # Assigns 1->x, 2->y, 3->z
first, *mid, last = [10, 20, 30, 40]  # first=10, mid=[20,30], last=40

Ternary Operatorโ€‹

Ternary expression in Python
ans = x if x > y else y  # Returns x if x > y, otherwise returns y

Built-ins Functionsโ€‹

zip, enumerateโ€‹

zip and enumerate in Python
for i, v in enumerate(arr):  # i=index (0,1,2...), v=value (arr[0],arr[1],...)
    pass

for a, b in zip(A, B):       # Pairs elements from A and B, stops at shortest
    pass

map, filterโ€‹

map and filter in Python
nums = list(map(int, input().split()))          # Reads space-separated integers from input, e.g "1 2 3" -> [1, 2, 3]
evens = list(filter(lambda n: n % 2 == 0, nums)) # Keeps only even numbers, e.g [1,2,3,4] -> [2,4]

sorted, reversedโ€‹

sorted and reversed in Python
sorted_nums = sorted(nums)                      # Returns a new sorted list, original unchanged
sorted_by_key = sorted(pairs, key=lambda p: p[1]) # Sorts pairs by second element

for x in reversed(nums):                        # Iterates in reverse order without creating a new list
    pass

any, all, sum, roundโ€‹

any, all, sum and round in Python
ok = any(x < 0 for x in nums)   # Returns True if at least one element is negative
ok2 = all(x >= 0 for x in nums) # Returns True if all elements are non-negative
s = sum(nums)                    # Returns sum of all elements
r = round(3.14159, 2)            # Rounds to 2 decimal places, i.e r = 3.14

Comprehensionsโ€‹

Creating/Manipulating List / Set / Dictionary with Conditionsโ€‹

Comprehensions in Python
squares = [x * x for x in range(10)]              # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
evens = [x for x in range(20) if x % 2 == 0]      # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
unique = {c.lower() for c in "AaBbCC"}             # {'a', 'b', 'c'} โ€” duplicates removed
pos = {i: v for i, v in enumerate(nums) if v > 0} # Dict of index->value for positive elements only

collections (Main concepts/functions for DSA)โ€‹

collections imports in Python
from collections import defaultdict, Counter, deque, namedtuple

defaultdictโ€‹

defaultdict in Python
g = defaultdict(list) # Returns [] for missing keys instead of raising KeyError
g[u].append(v)        # Safe to append without checking if key exists first

Counterโ€‹

Counter in Python
freq = Counter("banana")   # Counts occurrences: {'a':3, 'n':2, 'b':1}
top = freq.most_common(2)  # Returns top 2 most frequent: [('a', 3), ('n', 2)]

deque (O(1) pops from both ends)โ€‹

deque in Python
q = deque([1, 2, 3]) # deque = [1, 2, 3]
q.append(4)          # Add to right,  deque = [1, 2, 3, 4]
q.appendleft(0)      # Add to left,   deque = [0, 1, 2, 3, 4]
q.pop()              # Remove right,  deque = [0, 1, 2, 3]
q.popleft()          # Remove left,   deque = [1, 2, 3]

namedtupleโ€‹

namedtuple in Python
Point = namedtuple("Point", ["x", "y"]) # Creates a lightweight class with fields x and y
p = Point(2, 5)                         # p.x = 2, p.y = 5

Stringsโ€‹

Formatting (f-strings)โ€‹

f-strings in Python
name = "Ada"
score = 42
msg = f"{name} scored {score}" # msg = "Ada scored 42"

Common Methods: join, split, replace, stripโ€‹

String methods in Python
parts = "a,b,c".split(",")              # Splits by comma, parts = ['a', 'b', 'c']
s = "-".join(parts)                     # Joins with dash, s = "a-b-c"
t = "hello world".replace("world", "python") # t = "hello python"
u = "  padded \n".strip()              # Removes leading/trailing whitespace, u = "padded"

Functional Toolsโ€‹

lambdaโ€‹

lambda in Python
key_fn = lambda p: (p[0], -p[1]) # Anonymous function: sorts by first element asc, second element desc

functools.partialโ€‹

functools partial in Python
from functools import partial

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

add10 = partial(add, 10) # Fixes first argument as 10
add10(5)                 # i.e 10 + 5 = 15

lru_cache (Memoization)โ€‹

lru_cache memoization in Python
from functools import lru_cache

@lru_cache(maxsize=None) # Caches results of previous calls, avoids recomputation
def fib(n):
    if n < 2:
        return n
    return fib(n - 1) + fib(n - 2) # fib(5) -> 5, computed once and cached

Iteration Patterns (itertools)โ€‹

itertools import in Python
import itertools as it

combinations, permutations, productโ€‹

Combinations permutations and product in Python
list(it.combinations([1, 2, 3, 4], 2)) # All 2-element combos (order doesn't matter): [(1,2),(1,3),...]
list(it.permutations([1, 2, 3], 2))    # All 2-element arrangements (order matters): [(1,2),(1,3),...]
list(it.product([0, 1], repeat=3))     # Cartesian product: [(0,0,0),(0,0,1),(0,1,0),...]

Error Handling and Context Managersโ€‹

try/except/finallyโ€‹

try except finally in Python
try:
    x = int("42")  # Attempts to parse string to int
except ValueError:
    x = 0          # Runs if parsing fails
finally:
    pass           # Always runs regardless of exception

with (Context Managers)โ€‹

Context manager with statement in Python
with open("input.txt", "r", encoding="utf-8") as f: # Automatically closes file after block
    data = f.read()                                  # Reads entire file content into data

OOP Basicsโ€‹

Classes, Inheritanceโ€‹

Class inheritance in Python
class Base:
    def ping(self):
        return "base"   # Default implementation

class Child(Base):
    def ping(self):
        return "child"  # Overrides Base's ping()

c = Child()
print(c.ping()) # "child"

Abstract Base Classes (ABC)โ€‹

Abstract base class in Python
from abc import ABC, abstractmethod

class Solver(ABC):
    @abstractmethod
    def solve(self) -> int: # Subclasses must implement solve(), cannot instantiate Solver directly
        raise NotImplementedError

__slots__ (Memory Optimization)โ€‹

slots in Python
class Node:
    __slots__ = ("val", "next") # Restricts attributes to only val and next, reduces memory usage
    def __init__(self, val, nxt=None):
        self.val = val
        self.next = nxt

n = Node(1)  # n.val = 1, n.next = None

Advanced Topicsโ€‹

Generatorsโ€‹

Generator function in Python
def gen(n):
    for i in range(n):
        yield i * i  # Yields one value at a time instead of building a full list

list(gen(4))  # [0, 1, 4, 9] โ€” generated lazily

Coroutines (Async Basics)โ€‹

Async coroutine in Python
import asyncio

async def task():
    await asyncio.sleep(0.1) # Non-blocking wait, allows other tasks to run
    return 1

dataclassesโ€‹

dataclass in Python
from dataclasses import dataclass

@dataclass
class Edge:
    u: int
    v: int
    w: int = 1  # Default weight is 1

e = Edge(0, 1)    # e.u=0, e.v=1, e.w=1
e2 = Edge(0, 1, 5) # e2.u=0, e2.v=1, e2.w=5

Walrus Operator (:=)โ€‹

Walrus operator in Python
if (n := len(nums)) > 0:  # Assigns len(nums) to n and checks if > 0 in one step
    avg = sum(nums) / n   # Reuses n without calling len() again

Referencesโ€‹

Telemetry Integration

Completed working through this block? Sync progress to workspace.