New submission from Joseph Perez <[email protected]>:
(This issue is already broached in https://bugs.python.org/issue38605, and a in
some way in https://bugs.python.org/issue35834, but only as a secondary
subject, that's why I've opened a ticket on this particular issue)
ForwardRef of ForwardRef are not currently evaluated by get_type_hints, only
the first level is, as illustrated in these examples:
```python
from typing import ForwardRef, Optional, get_type_hints
def func(a: "Optional[\"int\"]"):
pass
assert get_type_hints(func)["a"] == Optional[ForwardRef("int")]
# one would expect get_type_hints(func)["a"] == Optional[int]
```
```python
from __future__ import annotations
from typing import ForwardRef, Optional, get_type_hints
def func(a: Optional["int"]):
pass
assert get_type_hints(func)["a"] == Optional[ForwardRef("int")]
# one would expect get_type_hints(func)["a"] == Optional[int] (which is the
case without the import of __future__.annotations!)
```
On the one hand I find this behavior quite counter-intuitive; I rather think
ForwardRef as kind of internal (and wonder why there is no leading underscore,
like _GenericAlias where it's used) and I don't understand the purpose of
exposing it as the result of the public API get_type_hints. By the way, if
ForwardRef can be obtained by retrieving annotations without get_type_hints,
stringified annotations (especially since PEP 563) make get_type_hints kind of
mandatory, and thus make ForwardRef disappeared (only at the first level so …)
On the other hand, the second example show that adoptions of postponed
annotations can change the result of get_type_hints; several libraries relying
of get_type_hints could be broken.
An other issue raised here is that if these ForwardRef are not evaluated by
get_type_hints, how will be done their evaluatation by the user? It would
require to retrieve some globalns/localns — too bad, it's exactly what is doing
get_type_hints. And if the ForwardRef is in a class field, the class
globalns/localns will have to be kept somewhere while waiting to encounter
these random ForwardRef; that's feasible, but really tedious.
Agreeing with Guido Von Rossum (https://bugs.python.org/msg370232), this
behavior could be easily "fixed" in get_type_hints.
Actually, there would be only one line to change in ForwardRef._evaluate:
```python
# from
self.__forward_value__ = _type_check(
eval(self.__forward_code__, globalns, localns),
"Forward references must evaluate to types.",
is_argument=self.__forward_is_argument__)
# to
self.__forward_value__ = _eval_type(
_type_check(
eval(
self.__forward_code__, globalns, localns),
"Forward references must evaluate to types.",
is_argument=self.__forward_is_argument__,
),
globalns,
localns,
)
And if this fix could solve the "double ForwardRef" issue mentionned in
https://bugs.python.org/issue38605, it would also resolve
https://bugs.python.org/issue35834 in the same time, raising NameError in case
of unknown ForwardRef with postponed annotation.
----------
messages: 373960
nosy: BTaskaya, eric.smith, gvanrossum, joperez, levkivskyi, lukasz.langa,
vstinner
priority: normal
severity: normal
status: open
title: Recursive evaluation of ForwardRef (and PEP 563)
type: behavior
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue41341>
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