is_dataclass returns True if getattr always succeeds.

[JohanHidding]

BPO	37868
Nosy	@ericvsmith, @serhiy-storchaka, @miss-islington
PRs	bpo-37868: Improve is_dataclass for instances. #15325 [3.8] bpo-37868: Improve is_dataclass for instances. (GH-15325) #15339 [3.7] bpo-37868: Improve is_dataclass for instances. (GH-15325) #15340

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2019-08-20.06:05:07.986>
created_at = <Date 2019-08-15.12:02:15.512>
labels = ['3.7', 'type-bug', 'library']
title = '`is_dataclass` returns `True` if `getattr` always succeeds.'
updated_at = <Date 2019-08-20.06:05:07.985>
user = 'https://bugs.python.org/JohanHidding'

bugs.python.org fields:

activity = <Date 2019-08-20.06:05:07.985>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2019-08-20.06:05:07.986>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2019-08-15.12:02:15.512>
creator = 'Johan Hidding'
dependencies = []
files = []
hgrepos = []
issue_num = 37868
keywords = ['patch']
message_count = 7.0
messages = ['349802', '349821', '349822', '349832', '349989', '349990', '349991']
nosy_count = 4.0
nosy_names = ['eric.smith', 'serhiy.storchaka', 'miss-islington', 'Johan Hidding']
pr_nums = ['15325', '15339', '15340']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37868'
versions = ['Python 3.7']

[JohanHidding]

Given a class A that overloads __getattr__

class A:
  def __getattr__(self, key):
    return 0

An instance of this class is always identified as a dataclass.

from dataclasses import is_dataclass

a = A()
print(is_dataclass(a))

gives the output True.

Possible fix: check for the instance type.

is_dataclass(type(a))

does give the correct answer.

[ericvsmith]

I'm guessing I'm looking up the attribute on the instance, not the class.

[serhiy-storchaka]

I am not sure that it is good idea to accept a type and an instance, but if it is a goal, is_dataclass() should be defined as:

def is_dataclass(obj):
    cls = obj if isinstance(obj, type) else type(obj)
    return hasattr(cls, _FIELDS)

_is_dataclass_instance() should be changed too:

def _is_dataclass_instance(obj):
    return hasattr(type(obj), _FIELDS)

[ericvsmith]

Yeah, I agree it's not an awesome design to work with classes or instances, but it's documented that way. As soon as I write some tests I'll check this in.

[ericvsmith]

New changeset b0f4dab by Eric V. Smith in branch 'master':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
b0f4dab

[miss-islington]

New changeset 1271ee8 by Miss Islington (bot) in branch '3.8':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
1271ee8

[miss-islington]

New changeset 02c1457 by Miss Islington (bot) in branch '3.7':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
02c1457