On 3/26/18 11:08 AM, Nick Coghlan wrote:
On 27 March 2018 at 00:40, Eric V. Smith <e...@trueblade.com> wrote:
https://bugs.python.org/issue33141 points out an interesting issue with
dataclasses and descriptors.
Given this code:
from dataclasses import *
class D:
"""A descriptor class that knows its name."""
def __set_name__(self, owner, name):
self.name = name
def __get__(self, instance, owner):
if instance is not None:
return 1
return self
@dataclass
class C:
d: int = field(default=D(), init=False)
C.d.name is not set, because d.__set_name__ is never called. However, in
this case:
class X:
d: int = D()
X.d.name is set to 'd' when d.__set_name__ is called during type.__new__.
The problem of course, is that in the dataclass case, when class C is
initialized, and before the decorator is called, C.d is set to a Field()
object, not to D(). It's only when the dataclass decorator is run that I
change C.d from a Field to the value of D(). That means that the call to
d.__set_name__(C, 'd') is skipped. See
https://www.python.org/dev/peps/pep-0487/#implementation-details for details
on how type.__new__ works.
The only workaround I can think of is to emulate the part of PEP 487 where
__set_name__ is called. I can do this from within the @dataclass decorator
when I'm initializing C.d. I'm not sure how great this solution is, since
it's moving the call from class creation time to class decorator time. I
think in 99+% of cases this would be fine, but you could likely write code
that depends on side effects of being called during type.__new__.
Unless anyone has strong objections, I'm going to make the call to
__set_name__ in the @datacalss decorator. Since this is such a niche use
case, I don't feel strongly that it needs to be in today's beta release, but
if possible I'll get it in. I already have the patch written. And if it does
get in but the consensus is that it's a bad idea, we can back it out.
Would it be feasible to define `Field.__set_name__`, and have that
call `default.__set_name__` when the latter exists, and be a no-op
otherwise?
A clever idea! I'll look in to it.
Eric.
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