How does repetition operator work on list in Python?

In Python, the * symbol serves as the repetition operator when used with lists. Instead of multiplication, it creates multiple copies of a list and concatenates them together into a single new list.

Basic Syntax

The repetition operator follows this pattern ?

# Syntax: list * number
new_list = original_list * repetition_count

Single Element Repetition

Create a list with repeated single elements ?

numbers = [0] * 5
print(numbers)

[0, 0, 0, 0, 0]

The list [0] contains one element. The repetition operator makes 5 copies and joins them into a single list.

Multiple Element Repetition

Repeat lists containing multiple elements ?

numbers = [0, 1, 2] * 3
print(numbers)

[0, 1, 2, 0, 1, 2, 0, 1, 2]

The entire sequence [0, 1, 2] is repeated 3 times in order.

Practical Examples

Creating Default Values

# Initialize list with default values
scores = [0] * 10
print("Scores:", scores)

# Create a list of empty strings
names = [""] * 5
print("Names:", names)

Scores: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Names: ['', '', '', '', '']

Pattern Creation

# Create repeating patterns
pattern = ["A", "B"] * 4
print("Pattern:", pattern)

# Mixed data types
mixed = [1, "hello", True] * 2
print("Mixed:", mixed)

Pattern: ['A', 'B', 'A', 'B', 'A', 'B', 'A', 'B']
Mixed: [1, 'hello', True, 1, 'hello', True]

Important: Shallow Copy Behavior

The repetition operator creates shallow copies. For mutable objects like lists, this means all copies reference the same object ?

# Be careful with mutable objects
matrix = [[0] * 3] * 2
print("Original:", matrix)

# Modifying one affects all copies
matrix[0][0] = 5
print("After change:", matrix)

Original: [[0, 0, 0], [0, 0, 0]]
After change: [[5, 0, 0], [5, 0, 0]]

To avoid this behavior with nested lists, use list comprehension instead ?

# Correct way for nested lists
matrix = [[0] * 3 for _ in range(2)]
print("Original:", matrix)

matrix[0][0] = 5
print("After change:", matrix)

Original: [[0, 0, 0], [0, 0, 0]]
After change: [[5, 0, 0], [0, 0, 0]]

Conclusion

The repetition operator * is useful for creating lists with repeated elements or patterns. Remember that it creates shallow copies, so use list comprehension when working with mutable objects to avoid unintended side effects.