dataclasses should allow frozendict default value

[gianni]

BPO	44674
Nosy	@rhettinger, @ericvsmith, @CCLDArjun
PRs	bpo-44674: Use unhashability as a proxy for mutability for default dataclass __init__ arguments. #29867

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

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2021-12-11.21:13:04.100>
created_at = <Date 2021-07-19.14:06:32.276>
labels = ['type-bug', 'library', '3.9']
title = 'dataclasses should allow frozendict default value'
updated_at = <Date 2021-12-12.09:27:52.990>
user = 'https://bugs.python.org/gianni'

bugs.python.org fields:

activity = <Date 2021-12-12.09:27:52.990>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2021-12-11.21:13:04.100>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2021-07-19.14:06:32.276>
creator = 'gianni'
dependencies = []
files = []
hgrepos = []
issue_num = 44674
keywords = ['patch']
message_count = 10.0
messages = ['397799', '397800', '398098', '398105', '398118', '403103', '403108', '408335', '408365', '408370']
nosy_count = 4.0
nosy_names = ['rhettinger', 'eric.smith', 'CCLDArjun', 'gianni']
pr_nums = ['29867']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44674'
versions = ['Python 3.9']

[gianni]

Using a frozendict as a default value should not cause an error in dataclasses. The check for mutability is:

   isinstance(f.default, (list, dict, set))

It appears frozendict has been changed to have a dict base class and it now raises an exception.

There should be a way to indicate object mutability as the purpose of the isinstance(f.default, (list, dict, set)) check is for mutable default values.

Using default_factory to work around this issue is cumbersome.

[ericvsmith]

I agree that would be an improvement.

[CCLDArjun]

Which frozendict does your message concern? I found this in some test:

cpython/Lib/test/test_builtin.py

Line 741 in bb3e0c2

class frozendict(dict):

or are you suggesting any user defined immutable object?

I'm not sure indicating that an object should be immutable to dataclasses will be less cumbersome than default_factory.

Currently, you have to do this:

x: frozendict = field(default_factory=frozendict)

But, indicating mutability would be something like this:

x: frozendict = field(mutable=false)

[ericvsmith]

When I originally read this, I read it as frozenset. I agree that there's no good way of telling if an arbitrary class is immutable, so I'm not sure we can do anything here.

So I think we should close this as a rejected idea.

[gianni]

@arjun - this is about default values (See the bug description - Using a frozendict as a default value)

Try this:

from frozendict import frozendict
from dataclasses import dataclass

@dataclass
class A:
    a: frozendict = frozendict(a=1)

This used to work until frozendict became a subclass of dict.

Perhaps another fix is to convert any dict to a frozendict? Maybe not.

How would you handle this case? The only thing I figured was:
from frozendict import frozendict, field
from dataclasses import dataclass

AD=frozendict(a=1)
@dataclass
class A:
    a: frozendict = field(default_factory=lambda:AD)

Which imho is cumbersome.

[rhettinger]

[Eric]

I agree that there's no good way of telling if an
arbitrary class is immutable, so I'm not sure we can
do anything here.

Consider using non-hashability as a proxy indicator for immutability.

-  isinstance(f.default, (list, dict, set))
+  f.default.__hash__ is None

While this is imperfect, it would be more reliable than what we have now.

[ericvsmith]

That's a good idea, Raymond.

>>> [x.__hash__ is None for x in (list, dict, set, frozenset)]
[True, True, True, False]

I don't think this change would cause any backward compatibility issues, except it would now allow a default of something bad like:

>>> class BadList(list):
...   def __hash__(self): return 0
...
>>> isinstance(BadList(), list), BadList.__hash__ is None
(True, False)

I can't say I care too much about now allowing things that didn't used to be allowed, especially if they're poorly designed like BadList.

I'll put together a PR.