Dataclass initialization inconsistently raises an error if a lazily imported annotation cannot be reified

[brittanyrey]

Bug report

Bug/Inconsistency

Dataclass initialization errors out only if a lazily imported annotation that is represented as a string (such as when from __future__ import annotations is present) cannot be reified.

Examples

Repl without strs (success)

>>> from dataclasses import dataclass
>>> lazy import fake
>>> @dataclass
... class config:
...     data: fake | None = None
...
>>>

Repl using str annotations (failure)

>>> from __future__ import annotations
>>> lazy import fake
>>> @dataclass
... class config:
...     data: fake | None = None
...
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    lazy import fake
ImportError: lazy import of 'fake' raised an exception during resolution

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<python-input-8>", line 1, in <module>
    @dataclass
     ^^^^^^^^^
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1458, in dataclass
    return wrap(cls)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1448, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
                          frozen, match_args, kw_only, slots,
                          weakref_slot)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1066, in _process_class
    a_type_annotation = _get_type_from_annotation(type, cls)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 809, in _get_type_from_annotation
    root = getattr(root, path_item.strip(), None)
ModuleNotFoundError: No module named 'fake'

>>> from dataclasses import dataclass
>>> lazy import fake
>>> @dataclass
... class config:
...     data: 'fake'
...
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    lazy import fake
ImportError: lazy import of 'fake' raised an exception during resolution

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    @dataclass
     ^^^^^^^^^
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1458, in dataclass
    return wrap(cls)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1448, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
                          frozen, match_args, kw_only, slots,
                          weakref_slot)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 1066, in _process_class
    a_type_annotation = _get_type_from_annotation(type, cls)
  File "/home/breynoso/cpython/Lib/dataclasses.py", line 809, in _get_type_from_annotation
    root = getattr(root, path_item.strip(), None)
ModuleNotFoundError: No module named 'fake'

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-158936: Resolve lazy imports consistently for dataclass annotations #158939
• [3.15] gh-158936: Resolve lazy imports consistently for dataclass annotations (GH-158939) #158946