On 1/5/2018 11:24 AM, Guido van Rossum wrote:
On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith <e...@trueblade.com
<mailto:e...@trueblade.com>> wrote:
On 1/2/2018 12:01 AM, Guido van Rossum wrote:
Yes, there's a class variable (__dataclass_fields__) that
identifies the parent fields. The PEP doesn't mention this or
the fact that special methods (like __repr__ and __init__) can
tell whether a base class is a dataclass. It probably should
though. (@Eric)
I think that's covered in this section:
https://www.python.org/dev/peps/pep-0557/#inheritance
<https://www.python.org/dev/peps/pep-0557/#inheritance>
I was specifically talking about the name and contents of
__dataclass_fields__, which are not documented by the PEP. I expect it's
inevitable that people will be looking at this (since they can see it in
the source code). Or do you recommend that people use
dataclasses.fields() and catch ValueError?
The expectation is to use dataclasses.fields(). Both it and
__dataclass_fields__ contain the fields for this class and the parents.
The only difference is the pseudo-fields.
I can add some words describing .fields() returning which fields are
present.
I notice that _isdataclass()
exists but is private and I don't recall why.
I think the argument was that it's an anti-pattern, and if you really
want to know, just call dataclasses.fields() and catch the TypeError. I
have this in a helper file:
def isdataclass(obj):
"""Returns True for dataclass classes and instances."""
try:
dataclasses.fields(obj)
return True
except TypeError:
return False
(Also now I'm curious what
the "pseudo-fields" are that fields() ignores, but that's OT.)
ClassVar and InitVar "fields". dataclasses.fields() doesn't return them.
Eric.
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