`typing.get_type_hints()` raises on invalid types only if they're wrapped in a ForwardRef #133959

JelleZijlstra · 2025-05-13T02:01:38Z

>>> import typing
>>> def f(x: typing.ClassVar[int]): pass
... 
>>> typing.get_type_hints(f)
{'x': typing.ClassVar[int]}
>>> def f(x: "typing.ClassVar[int]"): pass
... 
>>> typing.get_type_hints(f)
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    typing.get_type_hints(f)
    ~~~~~~~~~~~~~~~~~~~~~^^^
  File "/Users/jelle/py/cpython/Lib/typing.py", line 2390, in get_type_hints
    hints[name] = _eval_type(value, globalns, localns, type_params, format=format, owner=obj)
                  ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jelle/py/cpython/Lib/typing.py", line 448, in _eval_type
    return evaluate_forward_ref(t, globals=globalns, locals=localns,
                                type_params=type_params, owner=owner,
                                _recursive_guard=recursive_guard, format=format)
  File "/Users/jelle/py/cpython/Lib/typing.py", line 993, in evaluate_forward_ref
    type_ = _type_check(
        value,
    ...<2 lines>...
        allow_special_forms=forward_ref.__forward_is_class__,
    )
  File "/Users/jelle/py/cpython/Lib/typing.py", line 204, in _type_check
    raise TypeError(f"{arg} is not valid as type argument")
TypeError: typing.ClassVar[int] is not valid as type argument

(On main but also reproduced on 3.13.)

This seems inconsistent and I don't think it's helpful for users. We should either raise in both cases or neither. I prefer to not raise in either case, because it's less disruptive for callers (they already have to deal with ClassVar etc. potentially appearing in annotations), and provides more information.

The text was updated successfully, but these errors were encountered:

As explained in python#133960, this removes most of the behavior differences with ForwardRef.evaluate. The remaining difference is about recursive evaluation of forwardrefs; this is practically useful in cases where an annotation refers to a type alias that itself is string-valued. This also improves several edge cases that were previously not handled optimally. For example, the function now takes advantage of the partial evaluation behavior of ForwardRef.evaluate() to evaluate more ForwardRefs in the FORWARDREF format. This also fixes python#133959 as a side effect, because the buggy behavior in python#133959 derives from evaluate_forward_ref().

As explained in #133960, this removes most of the behavior differences with ForwardRef.evaluate. The remaining difference is about recursive evaluation of forwardrefs; this is practically useful in cases where an annotation refers to a type alias that itself is string-valued. This also improves several edge cases that were previously not handled optimally. For example, the function now takes advantage of the partial evaluation behavior of ForwardRef.evaluate() to evaluate more ForwardRefs in the FORWARDREF format. This also fixes #133959 as a side effect, because the buggy behavior in #133959 derives from evaluate_forward_ref().

As explained in pythonGH-133960, this removes most of the behavior differences with ForwardRef.evaluate. The remaining difference is about recursive evaluation of forwardrefs; this is practically useful in cases where an annotation refers to a type alias that itself is string-valued. This also improves several edge cases that were previously not handled optimally. For example, the function now takes advantage of the partial evaluation behavior of ForwardRef.evaluate() to evaluate more ForwardRefs in the FORWARDREF format. This also fixes pythonGH-133959 as a side effect, because the buggy behavior in pythonGH-133959 derives from evaluate_forward_ref(). (cherry picked from commit 57fef27) Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

…34663) gh-133960: Improve typing.evaluate_forward_ref (GH-133961) As explained in GH-133960, this removes most of the behavior differences with ForwardRef.evaluate. The remaining difference is about recursive evaluation of forwardrefs; this is practically useful in cases where an annotation refers to a type alias that itself is string-valued. This also improves several edge cases that were previously not handled optimally. For example, the function now takes advantage of the partial evaluation behavior of ForwardRef.evaluate() to evaluate more ForwardRefs in the FORWARDREF format. This also fixes GH-133959 as a side effect, because the buggy behavior in GH-133959 derives from evaluate_forward_ref(). (cherry picked from commit 57fef27) Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

JelleZijlstra mentioned this issue May 13, 2025

Document difference between typing.get_type_hints and inspect.get_annotations(eval_str=True) #102405

Open

JelleZijlstra added topic-typing type-bug An unexpected behavior, bug, or error 3.13 bugs and security fixes 3.14 bugs and security fixes 3.15 new features, bugs and security fixes labels May 13, 2025

JelleZijlstra mentioned this issue May 13, 2025

Simplify typing.evaluate_forward_ref #133960

Closed

JelleZijlstra mentioned this issue May 13, 2025

gh-133960: Improve typing.evaluate_forward_ref #133961

Merged

picnixz added the stdlib Python modules in the Lib dir label May 15, 2025

JelleZijlstra closed this as completed in #133961 May 25, 2025

miss-islington mentioned this issue May 25, 2025

[3.14] gh-133960: Improve typing.evaluate_forward_ref (GH-133961) #134663

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`typing.get_type_hints()` raises on invalid types only if they're wrapped in a ForwardRef #133959

`typing.get_type_hints()` raises on invalid types only if they're wrapped in a ForwardRef #133959

JelleZijlstra commented May 13, 2025 •

edited by AlexWaygood

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typing.get_type_hints() raises on invalid types only if they're wrapped in a ForwardRef #133959

typing.get_type_hints() raises on invalid types only if they're wrapped in a ForwardRef #133959

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JelleZijlstra commented May 13, 2025 • edited by AlexWaygood Loading Uh oh! There was an error while loading. Please reload this page.

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`typing.get_type_hints()` raises on invalid types only if they're wrapped in a ForwardRef #133959

`typing.get_type_hints()` raises on invalid types only if they're wrapped in a ForwardRef #133959

JelleZijlstra commented May 13, 2025 •

edited by AlexWaygood

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