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 <da...@drhagen.com
<mailto:da...@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.
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org <mailto:Python-Dev@python.org>
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
https://mail.python.org/mailman/options/python-dev/guido%40python.org
--
--Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
https://mail.python.org/mailman/options/python-dev/eric%2Ba-python-dev%40trueblade.com
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
