Quick reference

Ordering elements: sort() vs. sorted()

Both functions accept key and reverse parameters. reverse=True flips the order; key lets you pick a criterion (e.g., a field in nested data).

In-place ordering with sort()

nums = [0, 3, 2, 1, 4, 9, 6, 8, 7, 5]
nums.sort()
print(nums)           # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

nums.sort(reverse=True)
print(nums)           # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Non-destructive ordering with sorted()

src = [0, 3, 2, 1, 4, 9, 6, 8, 7, 5]
asc = sorted(src)
desc = sorted(src, reverse=True)

print(src)  # original untouched
print(asc)  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(desc) # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Custom key example

matrix = [[1, 2, 3], [9, 8, 7], [2, 4, 6], [3, 1, 9]]

by_first = sorted(matrix)                     # default: first element
by_second = sorted(matrix, key=lambda r: r[1])  # sort by second column

print(by_first)   # [[1, 2, 3], [2, 4, 6], [3, 1, 9], [9, 8, 7]]
print(by_second)  # [[3, 1, 9], [1, 2, 3], [2, 4, 6], [9, 8, 7]]

Reversing a list

Slice trick (creates a new list)

seq = [0, 1, 2, 3, 4, 5]
rev = seq[::-1]

print(seq)  # [0, 1, 2, 3, 4, 5]
print(rev)  # [5, 4, 3, 2, 1, 0]

reverse() method (in-place)

seq = [0, 1, 2, 3, 4, 5]
seq.reverse()
print(seq)  # [5, 4, 3, 2, 1, 0]

Copying lists safely

Shallow copies

src = [0, 1, 2, 3, 4, 5]

c1 = src[:]          # slice
c2 = src.copy()      # built-in method
c3 = list(src)       # constructor

print(c1, c2, c3)    # all equal to src

Deep copy for nested structures

import copy

nested = [0, 1, 2, 3, 4, [100, 200]]

shallow = nested[:]          # shares inner list
deep = copy.deepcopy(nested) # fully independent

nested[-1][0] = 999

print(nested)  # [0, 1, 2, 3, 4, [999, 200]]
print(shallow) # [0, 1, 2, 3, 4, [999, 200]]
print(deep)    # [0, 1, 2, 3, 4, [100, 200]]

Loctaing and counting elements

words = ['apple', 'banana', 'banana', 'orange']

print(words.index('banana'))  # 1  (first match)
print(words.count('banana'))  # 2
print(len(words))             # 4

String concatenation with join()

lines = ['hello world', 'I am oxxo', 'how are you?']
result = ', '.join(lines)
print(result)  # hello world, I am oxxo, how are you?

Membership tests

words = ['apple', 'banana', 'orange']
print('orange' in words)   # True
print('melon' in words)    # False
print('grape' not in words) # True

Comparing lists

a = [1, 2, 3, 4]
b = [1, 2, 3, 4, 5]

print(a == b)  # False
print(a != b)  # True
print(a < b)   # True  (shorter list is smaller)
print(a >= b)  # False

Repeating elements with *

base = ['apple', 'banana']
triple = base * 3
print(triple)  # ['apple', 'banana', 'apple', 'banana', 'apple', 'banana']