python / python/mypy

Getting 'Untyped decorator makes function "*" untyped' when using decorator inside a function

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bug
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Python
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Description

Sorry for the long title! I found a small issue when using disallow-untyped-decorators and while having a decorated function inside another function, here's the reproduction:

https://mypy-play.net/?mypy=latest&python=3.9&flags=disallow-untyped-decorators&gist=315f6ee827fbb1373b854cd317ee5882

from typing import Callable

def mydeco(f: Callable) -> Callable:
    return f

# Failing case

def something():
    class Query:
        @mydeco
        def search(self, input: str) -> str:
            return input

    reveal_type(Query.search)

# Failing case

def something():
    @mydeco
    def example() -> str:
        return "str"
        
    reveal_type(example)

# Ok case

class Query:
    @mydeco
    def search(self, input: str) -> str:
        return input
        
reveal_type(Query.search)

# Ok case

@mydeco
def example() -> str:
    return "str"
    
reveal_type(example)

I guess this is not a big deal since we don't normall define functions inside functions, but I got this issue while writing some tests and it'd be nice to get it fixed 😊

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked mypy-play reproduction and compare decorator handling for functions defined at module scope versus inside another function or class. Trace the type-checking path for nested decorated definitions; done means the reproduction no longer reports an untyped decorator and the reveal_type results remain appropriately typed.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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