[I believe this is fundamentally a dataclass version of https://bugs.python.org/issue41249]

When using from __future__ import annotations, calling get_type_hints on the constructor of a dataclass B which inherits from a dataclass A defined in another module will error if dataclass A has type hints which are not imported in the module where dataclass B is defined.

This is best shown by example, if you have foo.py:

from __future__ import annotations

import collections
import dataclasses

@dataclasses.dataclass
class A:
  x: collections.OrderedDict

and then in bar.py:

from __future__ import annotations

import foo
import dataclasses
import typing

@dataclasses.dataclass
class B(foo.A):
  pass

typing.get_type_hints(B)

the final line will raise "NameError: name 'collections' is not defined".

This code will not error if you do either of the following:

• add import collections to bar.py.
• remove the future annotations import from both files.

I am not confident enough on the internals of dataclass to suggest a fix, but potentially a similar approach to that which solved the TypedDict equivalent https://bugs.python.org/issue41249 would work?

I can't reproduce this on Python 3.8.3, 3.9.6, 3.10.0 or 3.11.0a1+. Which versions of Python have you tried this on? (I'm able to reproduce the typing.TypedDict bug on Python 3.8, since the fix was only backported to 3.9, but not this.)

I think the example has a minor typo.

The crash is reproducible on 3.9 if the last line in bar.py is

    typing.get_type_hints(B.__init__)

instead of

    typing.get_type_hints(B)

Thanks @sergei. With that alteration, I have reproduced this on Python 3.9, 3.10 and 3.11.

Ah yes, I completely apologize about that typo, the __init__ is definitely needed (serves me right for trying to clean up a repo before posting it).

One other comment to make, perhaps obvious; manually passing a namespace to get_type_hints' globalns which contains collections does indeed solve the problem (in case anyone else sees this and is blocked). Of course determining the right values to pass is in general difficult for a user (and shouldn't be necessary).

I had some time to debug this. It happens because we treat classes and functions differently.

The difference between get_type_hints(B) and get_type_hints(B.__init__) is that we use different global contexts there:

• For B we use __mro__ entries and extract globals and locals from there:

  cpython/Lib/typing.py

  Lines 1792 to 1796 in 86dfb55

  	for base in reversed(obj.__mro__):
  	if globalns is None:
  	base_globals = getattr(sys.modules.get(base.__module__, None), '__dict__', {})
  	else:
  	base_globals = globalns

• For B.__init__ we use simplier logic

  cpython/Lib/typing.py

  Lines 1822 to 1828 in 86dfb55

  	nsobj = obj
  	# Find globalns for the unwrapped object.
  	while hasattr(nsobj, '__wrapped__'):
  	nsobj = nsobj.__wrapped__
  	globalns = getattr(nsobj, '__globals__', {})
  	if localns is None:
  	localns = globalns

  It does not know anything about __mro__ and super contexts

Funny thing, this problem goes away if we remove @dataclass decorator and convert our examples into regular classes:

# a.py
from __future__ import annotations

import collections


class A:
  x: collections.OrderedDict
  def __init__(self, x: collections.OrderedDict) -> None:
      ...

and

# b.py
from __future__ import annotations

import a
import typing

class B(a.A):
  pass

print(typing.get_type_hints(B))  
# {'x': <class 'collections.OrderedDict'>}
print(typing.get_type_hints(B.__init__))
# {'x': <class 'collections.OrderedDict'>, 'return': <class 'NoneType'>}

I am going to try to solve this with something really simple (if no one else is working on it).

@nikita, I had a go at writing some more rigorous tests regarding this issue. I found the same thing you did -- the issue seems:

• Isolated to dataclasses specifically (doesn't occur with TypedDicts, standard classes or NamedTuples)
• Isolated to the __init__ method of dataclasses
• Only occurs when you *subclass* a dataclass defined in another module.

My tests are in these two files on my cpython fork:

• https://github.com/AlexWaygood/cpython/blob/forward-annotations-bpo-45524/Lib/test/test_future_annotations.py
• https://github.com/AlexWaygood/cpython/blob/forward-annotations-bpo-45524/Lib/test/_typing_imports_helper.py

(I'm not proposing adding two new files to the cpython test suite -- just put the tests in new files so that I could isolate the new tests from the rest of the test suite and understand the problem better.)

If you try running my test script with the from __future__ import annotations line at the top commented out, however, the error is the same. (from __future__ import annotations is obviously still there in the module that's being imported by the test script.)

Is it worth also addressing the case where a @dataclass/typing.TypeDict class is defined within a function?

from __future__ import annotations

import typing
from dataclasses import dataclass


def make_A():
  import collections

  @dataclass
  class A:
    x: collections.defaultdict

  return A


A = make_A()

@dataclass
class B(A):
  y: int

# NameError: name 'collections' is not defined
print(typing.get_type_hints(B.__init__))

@sergei, I believe that's a much larger issue, and one that's quite difficult to solve. It's one of the principal reasons why from __future__ import annotations behaviour wasn't made the default in Python 3.10, as was originally the plan. (This behaviour doesn't play nicely with libraries such as pydantic that analyse annotations at runtime.) I think it's probably beyond the scope of this BPO issue :)

https://mail.python.org/archives/list/python-dev@python.org/thread/CLVXXPQ2T2LQ5MP2Y53VVQFCXYWQJHKZ/

I completelly agree that the exable above is out of scope. Because it requires locals() to be modified. While this issue is focused on globals().

Is it worth filing a separate issue for locals()?

In my experience local classes are less common than cross-module inheritance, so I suspect that the chances of someone accidentally hitting lack of locals() forwarding are quite low. However, given how confusing the error message is, it might be worth having an issue for that. Wdyt?

In my opinion, it is never a bad thing to create a new issue :)

сб, 23 окт. 2021 г. в 22:46, Sergei Lebedev <report@bugs.python.org>:

Sergei Lebedev <slebedev@google.com> added the comment:

Is it worth filing a separate issue for locals()?

In my experience local classes are less common than cross-module
inheritance, so I suspect that the chances of someone accidentally hitting
lack of locals() forwarding are quite low. However, given how confusing the
error message is, it might be worth having an issue for that. Wdyt?

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue45524\>

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