Python

Variables & Types

x = 10—Assign variable (dynamic typing)

type(x)—Get type of variable

int / float / str / bool—Primitive types

int("42")—Type casting to integer

isinstance(x, int)—Type check

None—Null value equivalent

x, y = 1, 2—Multiple assignment

f"Hello {name}"—F-string interpolation

String Methods

.strip()—Remove leading/trailing whitespace

.split(sep)—Split into list

.join(list)—Join list into string

.replace(old, new)—Replace substring

.find(sub)—Index of substring (-1 if not found)

.startswith() / .endswith()—Check prefix/suffix

.upper() / .lower()—Case conversion

.format()—String formatting

.isdigit()—Check if all digits

len(s)—String length

Lists

lst = [1, 2, 3]—Create a list

lst[0] / lst[-1]—Index from start/end

lst[1:3]—Slice (index 1 to 2)

.append(x)—Add to end

.extend(lst2)—Add all items from another list

.insert(i, x)—Insert at index

.pop(i)—Remove & return at index

.remove(x)—Remove first occurrence

.sort() / sorted()—Sort in-place / new list

.reverse()—Reverse in-place

[x for x in lst if cond]—List comprehension

len(lst)—Number of elements

Dictionaries

d = {"k": "v"}—Create dictionary

d["key"]—Access value (KeyError if missing)

d.get("key", default)—Safe access with default

d.keys() / d.values()—View keys / values

d.items()—Key-value pairs

d.update(d2)—Merge another dict

d.pop("key")—Remove and return value

"key" in d—Check if key exists

{k: v for k, v in ...}—Dict comprehension

d | d2—Merge dicts (3.9+)

Control Flow

if / elif / else—Conditional branching

for x in iterable:—For loop

while condition:—While loop

for i in range(n):—Loop n times

for i, v in enumerate(lst):—Loop with index

for k, v in d.items():—Loop over dict

break / continue—Exit or skip iteration

x if cond else y—Ternary expression

match val: case ...—Pattern matching (3.10+)

Functions

def fn(a, b):—Define function

def fn(a, b=5):—Default argument

def fn(*args):—Variable positional args

def fn(**kwargs):—Variable keyword args

return value—Return from function

lambda x: x * 2—Anonymous function

def fn() -> int:—Return type hint

@decorator—Function decorator

yield value—Generator function

Classes & OOP

class MyClass:—Define a class

def __init__(self):—Constructor method

self.attr = val—Instance attribute

class B(A):—Inheritance

super().__init__()—Call parent constructor

@staticmethod—No self/cls reference

@classmethod—Takes cls as first arg

@property—Getter decorator

__str__ / __repr__—String representation

__len__ / __getitem__—Dunder methods for len/indexing

Error Handling

try: / except:—Catch exceptions

except ValueError as e:—Catch specific error

finally:—Always executes

raise Exception(msg)—Raise an error

assert cond, msg—Assert condition

with open() as f:—Context manager

Common Built-ins

print()—Output to console

input()—Read user input

range(n)—Sequence 0 to n-1

len()—Length of collection

max() / min()—Largest / smallest

sum()—Sum of iterable

abs()—Absolute value

round()—Round number

zip(a, b)—Pair up iterables

map(fn, lst)—Apply fn to each

filter(fn, lst)—Keep matching items

any() / all()—Test iterable booleans

File & Modules

import module—Import entire module

from mod import func—Import specific name

open("f.txt", "r")—Open file for reading

f.read() / f.readlines()—Read file content

f.write(data)—Write to file

import json—JSON parsing module

json.loads(s) / json.dumps(o)—Parse / serialize JSON

os.path.join()—Join file paths safely

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