A list of lists in Python

Python Tip: Creating a List of Lists

When creating a list of lists in Python, it's important to understand how list multiplication works.

Using:

m = [[]] * 7

creates seven references to the same list. Modifying one list will affect all others because they reference the same object.

Instead, use list comprehension to ensure each list is independent:

m = [[] for _ in range(7)]

This way, each empty list in 'm' is a separate object, avoiding unwanted side effects.