Discussion on: A Failed Experiment with Python Type Annotations

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Thanks for the detailed article and the nice examples - it allows for an interesting discussion.

Here's my take on the subject (disclaimer: I'm using mypy in all my "serious" projects for a little bit than a year now)

First, maybe you missed the fact that mypy has a reveal_type function precisely so that you don't have to remember and or guess all the types.


sequence = [1, 2, 3]

def get_iter():
    res = iter(sequence)
    reveal_type(res)
    return res

$ mypy --strict foo.py
foo.py:26: error: Function is missing a type annotation
foo.py:28: error: Revealed type is 'typing.Iterator[builtins.int*]'

I know, this is a bit awkward to use, but it's there if you need it.

Second, I'm not quite sure why mypy does not infer return types - maybe it's a bug, maybe it's a design decision. The question came up for Rust by the way and I think the rationale may apply to Python too.

Finally, if you're still wondering whether to give mypy a go, I have compiled a real-world list of changes caused by the transition to static typing in this rather long blog post. You may find it interesting :)

Cheers!

edA‑qa mort‑ora‑y • Jun 12 '19

Thank you.

I'm going to give it another go soon. The issue about self-referential types is essentially fixed in an upcoming Python release, and I can import from future now.

The return types are still annoying, but the reveal types will help. I got another comment that mypy may add an option to allow inferred return types.

At the moment, doing a refactoring, I'm leaning towards typing all the return types is the lesser of evils -- when compared to hunting down type mismatches.