Python One-Liners

Flatten a nested list

flat = [item for sublist in nested for item in sublist]

Example: [[1,2], [3,4], [5]] → [1,2,3,4,5]

Count word frequency in a string

from collections import Counter; Counter(text.lower().split())

Example: "the cat sat on the mat" → Counter({'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1})

Reverse a string

reversed_str = s[::-1]

Example: "hello" → "olleh"

Check if string is palindrome

is_palindrome = s == s[::-1]

Example: "racecar" → True

Get unique elements while preserving order

list(dict.fromkeys(items))

Example: [1,2,2,3,1,4] → [1,2,3,4]

Transpose a matrix (list of lists)

transposed = list(map(list, zip(*matrix)))

Example: [[1,2], [3,4]] → [[1,3], [2,4]]

Find most common element

max(set(items), key=items.count)

Example: [1,2,2,3,2,4] → 2

Merge two dicts (Python 3.9+)

merged = dict1 | dict2

Example: {'a': 1} | {'b': 2} → {'a': 1, 'b': 2}

Filter a dictionary by keys

{k: v for k, v in d.items() if k in keys}

Example: {'a': 1, 'b': 2, 'c': 3} with keys ['a', 'c'] → {'a': 1, 'c': 3}

Group by key in list of dicts

from itertools import groupby; {k: list(g) for k, g in groupby(sorted(data, key=lambda x: x['id']), key=lambda x: x['id'])}

Example: [{'id': 1}, {'id': 2}, {'id': 1}] → {1: [{'id': 1}, {'id': 1}], 2: [{'id': 2}]}

Find all prime numbers up to N (Sieve)

[x for x in range(2, n) if all(x % y != 0 for y in range(2, int(x**0.5) + 1))]

Example: n=10 → [2, 3, 5, 7]

Chunk a list into smaller lists of size N

[items[i:i + n] for i in range(0, len(items), n)]

Example: items=[1,2,3,4,5], n=2 → [[1,2], [3,4], [5]]

Intersection of two lists (preserving duplicates/count)

from collections import Counter; list((Counter(a) & Counter(b)).elements())

Example: a=[1,2,2,3], b=[2,2,3,4] → [2,2,3]

Sort list of strings by length

sorted(strings, key=len)

Example: ['apple', 'pie', 'banana'] → ['pie', 'apple', 'banana']

Convert a list of strings to a single comma-separated string

csv_string = ",".join(items)

Example: ['a', 'b', 'c'] → "a,b,c"

Get all indices of an element in a list

indices = [i for i, x in enumerate(items) if x == target]

Example: items=[1,2,3,2,1], target=2 → [1, 3]

Check if all elements in a list are unique

is_unique = len(items) == len(set(items))

Example: [1,2,3] → True, [1,2,2] → False

Remove all falsy values from a list

filtered = list(filter(None, items))

Example: [0, 1, False, 2, '', 3] → [1, 2, 3]

Find the first non-repeated element in a list

next((x for x in items if items.count(x) == 1), None)

Example: [1, 2, 1, 3, 2] → 3

Deep flatten a nested list (recursive approach in one line)

def deep_flat(l): return [item for sub in l for item in (deep_flat(sub) if isinstance(sub, list) else [sub])]

Example: [1, [2, [3, 4], 5], 6] → [1, 2, 3, 4, 5, 6]

Get the Cartesian product of multiple lists

from itertools import product; list(product(*lists))

Example: [[1,2], ['a','b']] → [(1,'a'), (1,'b'), (2,'a'), (2,'b')]

Calculate Fibonacci series up to N elements

from functools import reduce; fib = lambda n: reduce(lambda x, _: x + [x[-1] + x[-2]], range(n - 2), [0, 1])

Example: n=5 → [0, 1, 1, 2, 3]

Convert decimal to binary

bin(n).replace("0b", "")

Example: n=10 → "1010"

Calculate age from birthdate

from datetime import date; (date.today() - birthdate).days // 365

Example: date(1990, 1, 1) → 36 (approx)

Generate a random hex color

import random; "#{:06x}".format(random.randint(0, 0xFFFFFF))

Example: "#e3a1b2"

Check if a list is sorted

all(items[i] <= items[i+1] for i in range(len(items)-1))

Example: [1, 2, 3] → True, [3, 2, 1] → False

Flatten a nested list (one level)

flattened = [item for sublist in nested_list for item in sublist]

Example: [[1, 2], [3, 4]] → [1, 2, 3, 4]

Get most frequent element in a list

most_frequent = max(set(items), key=items.count)

Example: [1, 2, 3, 1, 2, 1] → 1

Transpose a 2D matrix

transposed = [list(row) for row in zip(*matrix)]

Example: [[1, 2], [3, 4]] → [[1, 3], [2, 4]]

Merge two lists into a dictionary

merged_dict = dict(zip(keys, values))

Example: keys=['a', 'b'], values=[1, 2] → {'a': 1, 'b': 2}

Capitalize the first letter of each word in a string

' '.join(word.capitalize() for word in s.split())

Example: "hello world" → "Hello World"

Swap two variables without a temporary one

a, b = b, a

Example: a=5, b=10 → a=10, b=5

Remove duplicates from a list while preserving order

seen = set(); unique = [x for x in items if not (x in seen or seen.add(x))]

Example: [1, 2, 2, 3, 1] → [1, 2, 3]

Convert a list of dictionaries to a single dictionary (by key)

merged = {item['id']: item for item in list_of_dicts}

Example: [{'id': 1, 'name': 'A'}, {'id': 2, 'name': 'B'}] → {1: {'id': 1, 'name': 'A'}, 2: {'id': 2, 'name': 'B'}}

Calculate factorial of a number

import math; fact = math.factorial(n)

Example: n=5 → 120

Find all substrings of a string

[s[i:j] for i in range(len(s)) for j in range(i + 1, len(s) + 1)]

Example: "abc" → ["a", "ab", "abc", "b", "bc", "c"]

Find all permutations of a string

from itertools import permutations; ["".join(p) for p in permutations(s)]

Example: "abc" → ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']

Flatten a list of dicts values into a single list

[val for d in list_of_dicts for val in d.values()]

Example: [{'a': 1}, {'b': 2}, {'c': 3}] → [1, 2, 3]

Sort a list of dictionaries by multiple keys

sorted(data, key=lambda x: (x['priority'], x['name']))

Example: [{'priority': 2, 'name': 'B'}, {'priority': 1, 'name': 'A'}] → [{'priority': 1, 'name': 'A'}, {'priority': 2, 'name': 'B'}]

Zip two lists into a list of tuples

zipped = list(zip(list1, list2))

Example: ['a', 'b'], [1, 2] → [('a', 1), ('b', 2)]

Convert a string of hex values to bytes

bytes.fromhex(hex_string)

Example: "48656c6c6f" → b'Hello'

Convert a list of integers into a single integer

res = int("".join(map(str, items)))

Example: [1, 2, 3] → 123

Rotate a list by N positions

rotated = items[n:] + items[:n]

Example: items=[1,2,3,4,5], n=2 → [3,4,5,1,2]

Download a file using requests

import requests; open('file.ext', 'wb').write(requests.get(url).content)

Example: requests.get('https://example.com/logo.png') → saves as file.ext

Convert a list of strings to integers

ints = [int(x) for x in strings if x.isdigit()]

Example: ["1", "2", "a", "3"] → [1, 2, 3]

Unzip a list of tuples

list1, list2 = zip(*list_of_tuples)

Example: [('a', 1), ('b', 2)] → ('a', 'b'), (1, 2)

Remove specific character from string

s = s.replace(char, "")

Example: "apple", "p" → "ale"

Remove duplicates from list while preserving order

seen = set(); result = [x for x in my_list if not (x in seen or seen.add(x))]

Example: [1, 2, 2, 3, 1] → [1, 2, 3]