Скачать или смотреть How to Type Annotate Variables Inside a Loop When Using itertools.zip_longest in Python

Learn how to properly type annotate loop variables when iterating with itertools.zip_longest to avoid mypy errors and workaround current mypy v1.14.0 limitations.
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Introduction

When using Python's itertools.zip_longest() to iterate over multiple sequences, you might encounter mypy errors demanding explicit type annotations for your loop variables. This is common, especially with empty sequences passed as fillvalue.

The Problem

Consider the following loop:

[[See Video to Reveal this Text or Code Snippet]]

mypy might raise errors such as:

[[See Video to Reveal this Text or Code Snippet]]

This error appears to stem from mypy v1.14.0, especially when an empty sequence (like an empty tuple ()) is used in fillvalue or when fillvalue is an empty mutable sequence.

Why This Happens

mypy struggles to infer the type of variables coming from zip_longest when fillvalue is an empty sequence.

Other type checkers like pyright do not have this issue.

Using a non-empty default fillvalue or a non-sequence as fillvalue also circumvents this.

Recommended Workaround

You can avoid this by not specifying fillvalue explicitly and handling None values inside the loop:

[[See Video to Reveal this Text or Code Snippet]]

This means:

Omit the fillvalue parameter (defaults to None).

Inside the loop or comprehension, check if args or kwargs is None.

Safely process only if they are not None.

Why This Works

It avoids using problematic empty sequence fillvalues that confuse mypy.

By handling None explicitly, type inference becomes straightforward and clean.

Additional Tips

You cannot directly add type annotations for args and kwargs inline in the for-loop syntax (e.g., for args: Type, kwargs: Type in ... is invalid).

Use type annotations on the variables before the loop or add type comments if necessary:

[[See Video to Reveal this Text or Code Snippet]]

Consider using static types from your program logic to replace Any for more precise annotations.

Summary

mypy v1.14.0 has a known limitation with zip_longest when fillvalue is an empty sequence.

The best approach is to avoid specifying fillvalue and handle possible None values inside your loop.

Annotate variables separately if needed, but you cannot annotate loop variables directly in the for-loop header.

This workaround keeps your code clean, type-safe, and compatible with the current mypy behavior.



Note: This advice is specific to mypy v1.14.0 and may change as type checkers evolve.