Lists and Sequences¶

A list in Python is an ordered, mutable sequence of items. Lists are written as a list of comma-separated values between square brackets [...]. Items within a list can be of different data types.

Creating Lists¶

# A list of integers
numbers = [1, 2, 3, 4, 5]

# A list of strings
fruits = ["apple", "banana", "cherry"]

# A mixed data type list
mixed = ["Python", 3.14, 42, True]

# An empty list
empty_list = []

Modifying Lists (Mutability)¶

Unlike strings, lists are mutable. You can change items directly:

Example: Modifying List Elements

letters = ['a', 'b', 'c', 'd']

# Change a single item
letters[0] = 'A'
print(letters)  # ['A', 'b', 'c', 'd']

# Change a slice of items
letters[1:3] = ['B', 'C']
print(letters)  # ['A', 'B', 'C', 'd']

# Clear items by slice assignment
letters[2:] = []
print(letters)  # ['A', 'B']

Common List Methods¶

Method	Description	Example
`list.append(x)`	Adds an item to the end of the list	`nums.append(10)`
`list.extend(iterable)`	Appends all elements from another iterable	`nums.extend([20, 30])`
`list.insert(i, x)`	Inserts item `x` at given index `i`	`nums.insert(0, 5)`
`list.remove(x)`	Removes the first occurrence of item `x`	`nums.remove(10)`
`list.pop([i])`	Removes and returns item at index `i` (default last item)	`last = nums.pop()`
`list.clear()`	Removes all items from the list	`nums.clear()`
`list.index(x)`	Returns zero-based index of first occurrence of `x`	`idx = nums.index(20)`
`list.count(x)`	Returns number of times `x` appears in list	`cnt = nums.count(5)`
`list.sort()`	Sorts the items of the list in-place	`nums.sort()`
`list.reverse()`	Reverses the elements of the list in-place	`nums.reverse()`
`list.copy()`	Returns a shallow copy of the list	`copy_list = nums.copy()`

Example: List Methods in Practice

colors = ["red", "blue"]

colors.append("green")
print("After append:", colors)

colors.insert(1, "yellow")
print("After insert:", colors)

popped_color = colors.pop()
print("Popped:", popped_color)
print("After pop:", colors)

Output

After append: ['red', 'blue', 'green'] After insert: ['red', 'yellow', 'blue', 'green'] Popped: green After pop: ['red', 'yellow', 'blue']

Assignment vs Copying (Reference Trap)¶

In Python, assigning a list variable to another variable creates a reference to the same underlying object in memory, not a new copy:

Example: List References vs Shallow Copies

# 1. Assignment creates a reference to the same list:
a = [1, 2, 3]
b = a
b.append(4)
print("a:", a)  # a is modified too: [1, 2, 3, 4]

# 2. Creating an independent copy:
c = a.copy()    # or c = a[:]
c.append(99)
print("a:", a)  # [1, 2, 3, 4] (unchanged)
print("c:", c)  # [1, 2, 3, 4, 99]

Built-in Functions with Lists¶

values = [15, 3, 89, 42]

print("Length: ", len(values))   # 4
print("Minimum:", min(values))   # 3
print("Maximum:", max(values))   # 89
print("Sum:    ", sum(values))   # 149