PEP 563, PEP 649 and the future of pydantic and FastAPI

Re:PEP 563, PEP 649 and the future of pydantic and FastAPI by travisjungroth on Thursday April 15, 2021 @11:48PM (#26829218)

You and I seem to have different definitions of "experimental feature" then. By your definition, I don't think they did anything wrong. dataclasses in the standard library also rely on runtime type annotations. (how they don't have the same issue with 3.10 I don't know). I would hope people would start relying on new language features years after their release.

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Re:PEP 563, PEP 649 and the future of pydantic and FastAPI by Mehdi2277 on Friday April 16, 2021 @12:27AM (#26829451)

Data classes do not do runtime type checking. They are a way for static type checkers to do validation. So there’s no issue for data classes in 3.10.

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Re:PEP 563, PEP 649 and the future of pydantic and FastAPI by nemetroid on Friday April 16, 2021 @06:55AM (#26832280)

Data classes don't do runtime type checking, but they do provide dynamic type info through dataclasses.fields(). This function returns a tuple of Field objects, and according to the documentation[1], Field includes the attribute "type":

  type: The type of the field.

In Python 3.10, this now returns a string. In my opinion that's a bug (perhaps in the docs).

I verified this using a short test program:

  import dataclasses
  
  @dataclasses.dataclass
  class Apa:
      bepa: int
  
  first_field = dataclasses.fields(Apa)[0]
  print(type(first_field.type))

Running with 3.9 and 3.10:

  $ python test.py  # Python 3.9
  <class 'type'>
  $ python3.10 test.py
  <class 'str'>

1: https://docs.python.org/3.10/library/dataclasses.html#datacl...

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Re:PEP 563, PEP 649 and the future of pydantic and FastAPI by nerdponx on Friday April 16, 2021 @12:43PM (#26836148)

That is indeed annoying. Thanks for the example.

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Re:PEP 563, PEP 649 and the future of pydantic and FastAPI by travisjungroth on Friday April 16, 2021 @12:44PM (#26836156)

The variable annotations are used at runtime to define the fields. They aren't type checked.

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