How Can I Efficiently Transpose a List of Lists in Python?

Transposing a List of Lists: A Pythonic Approach

Given a list of lists, such as:

l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

we aim to transpose it, resulting in:

r = [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

Zipping and Unpacking to the Rescue

Python provides a powerful tool called zip that allows us to combine multiple iterables, creating a zip object that iterates over the elements in parallel. The key insight is to unzip the list of lists using zip and then convert the resulting tuples into lists.

Python 3 and Jagged Tables

For Python 3, we have two options:

list(map(list, zip(*l)))

This approach may short-circuit for jagged tables (where lists have different lengths).

list(map(list, itertools.zip_longest(*l, fillvalue=None)))

This handles jagged tables by filling shorter lists with None.

Python 2 and Homogeneous Tables

In Python 2, the simpler approach works well for homogeneous tables:

map(list, zip(*l))

Unveiling the Magic

To understand the process, consider the first approach in Python 3:

• We unpack the list of lists using *l, passing each inner list as a separate argument to zip.
• zip creates a zip object that combines the elements of the inner lists, yielding tuples: (1, 4, 7), (2, 5, 8), (3, 6, 9).
• The map function applies the list function to each tuple, converting it back to a list.
• The result is a list of lists, as desired.

Handling Jagged Tables

itertools.zip_longest handles jagged tables by filling shorter lists with the specified fill value (default: None). It ensures that all lists in the transposed result have the same length.

Conclusion

Transposing a list of lists is a common task in data manipulation. By leveraging the power of zip and understanding its behavior, we can implement efficient solutions in Python.

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