Lists

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

Creating Lists

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

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

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]}")

Click "Run" to execute your code

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]

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}")

Click "Run" to execute your code

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

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:

# 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]

# 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}")

Click "Run" to execute your code

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

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)