python / python/typing

Allow Ellipsis as annotation for inferred type

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topic: feature
Dominant language
Python
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Description

PEP 484 says that if an annotation is missing then the type assumed to be Any

def f(x) -> None:
    reveal_type(x) # Revealed type is 'Any'
def g(x: int):
    pass
reveal_type(g(1)) # Revealed type is 'Any'

However, it is not clear how to say to type checker that it should infer a missing type, rather than assume that it is Any. It was proposed by @ncoghlan to use Ellipsis for this purpose:

def f(x: ...) -> None:
    ...
def g(x: int) -> ...:
    ...

I am opening this issue, so that this idea will not be forgotten.

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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 by reading PEP 484 and the examples in this issue; no implementation file or test is identified. Clarify the intended Ellipsis semantics and affected type-checker behavior before defining acceptance tests, since the issue records a proposal rather than a scoped change.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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