[Python-Dev] Postponed annotations break inspection of dataclasses
Eric V. Smith
eric at trueblade.com
Sat Sep 22 14:29:21 EDT 2018
On 9/22/2018 12:41 PM, Guido van Rossum wrote:
> This is a good catch -- thanks for bringing it up. I'm adding Eric Smith
> (author of dataclasses) and Ivan Levkivskyi (co-author of typing) as
> well as Łukasz Langa (author of PEP 563) to the thread to see if they
> have further insights.
I don't see Ivan and Łukasz cc'd, so I'm adding them here.
> Personally I don't think it's feasible to change PEP 563 to use lambdas
> (if it were even advisable, which would be a long discussion), but I do
> think we might be able to make small improvements to the dataclasses
> and/or typing modules to make sure your use case works.
>
> Probably a bugs.python.org <http://bugs.python.org> issue is a better
> place to dive into the details than python-dev.
Agreed that opening a bug would be good.
And then I'll ruin that suggestion by answering here, too:
I think this problem is endemic to get_type_hints(). I've never
understood how you're supposed to use the globals and locals arguments
to it, but this works:
print(get_type_hints(Bar.__init__, globals()))
as does:
print(get_type_hints(Bar.__init__, Bar.__module__))
But that seems like you'd have to know a lot about how a class were
declared in order to call get_type_hints on it. I'm not sure __module__
is always correct (but again, I haven't really thought about it).
The docs for get_type_hints() says: "In addition, forward references
encoded as string literals are handled by evaluating them in globals and
locals namespaces."
Every once in a while someone will bring up the idea of delayed
evaluation, and the answer is always "use a lambda". If we ever wanted
to do something more with delayed evaluation, this is a good use case
for it.
Eric
>
> Thanks again,
>
> --Guido (top-poster in chief)
>
> On Sat, Sep 22, 2018 at 8:32 AM David Hagen <david at drhagen.com
> <mailto:david at drhagen.com>> wrote:
>
> The new postponed annotations have an unexpected interaction with
> dataclasses. Namely, you cannot get the type hints of any of the
> data classes methods.
>
> For example, I have some code that inspects the type parameters of a
> class's `__init__` method. (The real use case is to provide a
> default serializer for the class, but that is not important here.)
>
> ```
> from dataclasses import dataclass
> from typing import get_type_hints
>
> class Foo:
> pass
>
> @dataclass
> class Bar:
> foo: Foo
>
> print(get_type_hints(Bar.__init__))
> ```
>
> In Python 3.6 and 3.7, this does what is expected; it prints
> `{'foo': <class '__main__.Foo'>, 'return': <class 'NoneType'>}`.
>
> However, if in Python 3.7, I add `from __future__ import
> annotations`, then this fails with an error:
>
> ```
> NameError: name 'Foo' is not defined
> ```
>
> I know why this is happening. The `__init__` method is defined in
> the `dataclasses` module which does not have the `Foo` object in its
> environment, and the `Foo` annotation is being passed to `dataclass`
> and attached to `__init__` as the string `"Foo"` rather than as the
> original object `Foo`, but `get_type_hints` for the new annotations
> only does a name lookup in the module where `__init__` is defined
> not where the annotation is defined.
>
> I know that the use of lambdas to implement PEP 563 was rejected for
> performance reasons. I could be wrong, but I think this was
> motivated by variable annotations because the lambda would have to
> be constructed each time the function body ran. I was wondering if I
> could motivate storing the annotations as lambdas in class bodies
> and function signatures, in which the environment is already being
> captured and is code that usually only runs once.
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> --
> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
>
>
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