Lists — Reference | Guide to Python

Lists are mutable, ordered sequences. They can hold items of any type and are one of Python's most versatile data structures.

Creating Lists

python

empty = []
numbers = [1, 2, 3, 4, 5]
mixed = [1, "two", 3.0, True]
nested = [[1, 2], [3, 4]]
from_range = list(range(5))  # [0, 1, 2, 3, 4]

Accessing Elements

python

nums = [10, 20, 30, 40, 50]

nums[0]      # 10 — first element
nums[-1]     # 50 — last element
nums[1:3]    # [20, 30] — slice
nums[:3]     # [10, 20, 30] — from start
nums[2:]     # [30, 40, 50] — to end
nums[::2]    # [10, 30, 50] — every 2nd
nums[::-1]   # [50, 40, 30, 20, 10] — reversed

Python Playground

nums = [10, 20, 30, 40, 50]

print(f"First: {nums[0]}")
print(f"Last: {nums[-1]}")
print(f"Slice [1:3]: {nums[1:3]}")
print(f"Every 2nd: {nums[::2]}")
print(f"Reversed: {nums[::-1]}")

Output

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Adding Elements

Method	Description	Example
`append(x)`	Add to end	`[1,2].append(3)` → `[1,2,3]`
`insert(i, x)`	Insert at index	`[1,3].insert(1, 2)` → `[1,2,3]`
`extend(iter)`	Add all from iterable	`[1,2].extend([3,4])` → `[1,2,3,4]`
`+`	Concatenate (new list)	`[1,2] + [3,4]` → `[1,2,3,4]`

Removing Elements

Method	Description	Example
`remove(x)`	Remove first occurrence	`[1,2,3,2].remove(2)` → `[1,3,2]`
`pop()`	Remove & return last	`[1,2,3].pop()` → `3`
`pop(i)`	Remove & return at index	`[1,2,3].pop(0)` → `1`
`clear()`	Remove all elements	`[1,2,3].clear()` → `[]`
`del lst[i]`	Delete at index	`del lst[0]`

Python Playground

fruits = ["apple", "banana"]

# Adding
fruits.append("cherry")
print(f"After append: {fruits}")

fruits.insert(1, "blueberry")
print(f"After insert: {fruits}")

# Removing
removed = fruits.pop()
print(f"Popped: {removed}")
print(f"After pop: {fruits}")

fruits.remove("banana")
print(f"After remove: {fruits}")

Output

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Searching and Counting

Method	Description
`index(x)`	Index of first `x` (raises `ValueError` if not found)
`count(x)`	Number of occurrences of `x`
`x in lst`	`True` if `x` is in list

Sorting and Reversing

python

nums = [3, 1, 4, 1, 5, 9]

# In-place sort
nums.sort()                  # [1, 1, 3, 4, 5, 9]
nums.sort(reverse=True)      # [9, 5, 4, 3, 1, 1]

# New sorted list
sorted(nums)                 # returns new list
sorted(nums, key=abs)        # sort by absolute value

# Reverse in place
nums.reverse()

# New reversed
list(reversed(nums))

List Comprehensions

A concise way to create lists:

python

# Basic
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

# With transformation
words = ["hello", "world"]
upper = [w.upper() for w in words]

# Nested
matrix = [[1, 2], [3, 4], [5, 6]]
flat = [x for row in matrix for x in row]

Python Playground

# List comprehensions
squares = [x**2 for x in range(1, 11)]
print(f"Squares: {squares}")

evens = [x for x in range(20) if x % 2 == 0]
print(f"Evens: {evens}")

words = ["hello", "world", "python"]
upper = [w.upper() for w in words]
print(f"Upper: {upper}")

Output

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Useful Built-in Functions

Function	Description	Example
`len(lst)`	Number of elements	`len([1,2,3])` → `3`
`sum(lst)`	Sum of elements	`sum([1,2,3])` → `6`
`min(lst)`	Smallest element	`min([3,1,2])` → `1`
`max(lst)`	Largest element	`max([3,1,2])` → `3`
`any(lst)`	`True` if any truthy	`any([0, False, 1])` → `True`
`all(lst)`	`True` if all truthy	`all([1, True, "a"])` → `True`
`enumerate(lst)`	Index-value pairs	`list(enumerate(["a","b"]))` → `[(0,"a"),(1,"b")]`
`zip(a, b)`	Pair elements	`list(zip([1,2], ["a","b"]))` → `[(1,"a"),(2,"b")]`

Copying Lists

python

original = [1, 2, 3]

# Shallow copies
copy1 = original.copy()
copy2 = original[:]
copy3 = list(original)

# Deep copy (for nested lists)
import copy
deep = copy.deepcopy(original)