On 1/5/2018 12:58 PM, Guido van Rossum wrote:
Hm. I don't know that people will conclude that checking for a dataclass
is an anti-pattern. They'll probably just invent a myriad of different
hacks like the one you showed. I recommend making it public.
I'm trying to track down the original discussion. We got bogged down on
whether it worked for classes or instances or both, then we got tied up
in naming it (surprise!), then it looks like we decided to just not
include it since you could make those decisions for yourself.
I think the discussion is buried in this thread:
https://mail.python.org/pipermail/python-dev/2017-November/150966.html
Which references:
https://github.com/ericvsmith/dataclasses/issues/99
So, ignoring the naming issue, I think if we want to revive it, the
question is: should isdataclass() return True on just instances, just
classes, or both? And should it ever raise an exception, or just return
False?
I still worry a bit about ClassVar and InitVar being potentially useful
but I concede I have no use case so I'll drop it.
IIRC, we decided that we could add a parameter to dataclasses.fields()
if we ever wanted to return pseudo-fields. But no one came up with a use
case.
Eric.
On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith <e...@trueblade.com
<mailto:e...@trueblade.com>> wrote:
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>
<mailto: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>
<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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