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No (public) way to dynamically introspect if an annotation is a TypedDict #751

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pxeger opened this issue Sep 14, 2020 · 6 comments
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No (public) way to dynamically introspect if an annotation is a TypedDict #751

pxeger opened this issue Sep 14, 2020 · 6 comments

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@pxeger
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@pxeger pxeger commented Sep 14, 2020 •
edited

I want to know the Origin of an annotation. This works fine for basically everything, but there is no way to check if an annotation is a TypedDict subclass without resorting to (semantically) private API (by checking if it is an instance of _TypedDictMeta), because issubclass(x, TypedDict) fails. I believe this is because you don't want people thinking isinstance(x, TypedDict) will work to actually validate the keys on a dict. However, I don't think there's any reason to prevent issubclass on it

https://github.com/python/cpython/blob/1b4552c/Lib/typing.py#L1901

Here is an example use-case:

from typing import Dict, get_origin

def validate(data, annotation) -> bool:
    """check if `data` conforms to the type given in `annotation`"""
    # this works
    if get_origin(annotation) is Dict:
        return isinstance(data, dict)
    # but this doesn't...
    elif is_a_typed_dict(annotation):
        return isinstance(data, dict) and validate_typeddict_keys(data, annotation)

    ...

This could be solved in one of the following ways:

  • allow issubclass on TypedDicts
    • or allow it only on TypedDict itself and not subclasses of it, e.g. using a _root classvar like NamedTuple does for its magic
      • although I see no reason not to allow it in that case either
  • make _TypedDictMeta public API (rename to TypedDictMeta)
  • make get_origin work on TypedDicts (not ideal because it isn't really a type)
@pxeger pxeger changed the title No (public) way to dynamically introspect if an object is a TypedDict No (public) way to dynamically introspect if an annotation is a TypedDict Sep 14, 2020
@gvanrossum
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@gvanrossum gvanrossum commented Sep 14, 2020

This is reasonable. Can you show us a function that when added to typing.py would solve your problem? (E.g. is_typed_dict() -- how should it be implemented?)
@pxeger
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@pxeger pxeger commented Sep 14, 2020

Simply:

def is_typed_dict(cls: type) -> bool:
    return isinstance(cls, _TypedDictMeta)

would do. Is there a reason that we can't just allow issubclass on TypedDicts though?
@gvanrossum
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@gvanrossum gvanrossum commented Sep 14, 2020

PEP 589 explicitly rejects supporting isinstance or issubclass for TypedDict, so let's not go there.

Are you willing to make a contribution? There would actually have to be several -- this should be added to typing.py in the CPython 3.10 stdlib, to typing.py and typing_extensions in the typing repo. (Hm, maybe only to typing_extensions? typing.py there is only relevant for Python versions that are no longer supported, like 2.7 and 3.4.)
@pxeger
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@pxeger pxeger commented Sep 14, 2020

typing.py there is only relevant for Python versions that are no longer supported, like 2.7 and 3.4.)

https://github.com/python/typing/#workflow-for-cpython-changes says:

At Guido's discretion, he will from time to time copy typing.py and test_typing.py from the python/typing GitHub repo to the cpython repo.

Is that note out of date then?
@gvanrossum
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@gvanrossum gvanrossum commented Sep 14, 2020

Ouch, yes. That is totally out of date. The "master" copy of those files now lives in the CPython repo and the version here is only maintained (or at least not killed) for the benefit of Python 2.7 users of mypy and pytype.
pxeger added a commit to pxeger/cpython that referenced this issue Sep 15, 2020
@pxeger
add `is_typeddict` function to typing.py
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3e1c6ff 
python/typing#751
@pxeger pxeger mentioned this issue Sep 15, 2020
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@pxeger
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@pxeger pxeger commented Sep 15, 2020 •
edited

Should I create a BPO for this? It seems like the CPython pull request workflow is heavily centered around using those issue numbers.

Done, see bpo-41792
@pxeger pxeger mentioned this issue Sep 15, 2020
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xzy3 pushed a commit to xzy3/cpython that referenced this issue Oct 18, 2020
@pxeger Seth Sims
bpo-41792: Add is_typeddict function to typing.py (pythonGH-22254)
29ef8f2 
Closes issue41792.

Also closes python/typing#751.
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