typing.Set (built-in set type) is aliased to collections.abc.Set (abstract set)

[layday]

• cattrs version: 22.1.0
• Python version: 3.8
• Operating System: macOS

Description

GenConverter's unstruct_collection_overrides param expects a collections.abc.Set in Python 3.9+ and a typing.Set in Python 3.8 and below. typing.Set is a generic version of the set built-in; the typing module's equivalent of collections.abc.Set is AbstractSet. In 3.8 and below, collections.abc.Set is silently ignored and typing.Set incorrectly applies to all set types. I would expect collections.abc.Set to work on all (supported) Python versions, especially considering that paremetrised overrides are not supported (Set[...] has no effect).

What I Did

from collections.abc import Set

converter = GenConverter(unstruct_collection_overrides={Set: sorted})
converter.unstructure(frozenset(()))  # Not converted to a list on Python 3.8

[Tinche]

Ooph, let's see, it's a little confusing.

On 3.8 and below, collections.abc.Set doesn't support subscripting (collections.abc.Set[int] is an error) so I don't think it's worth supporting. I believe on those versions the recommendation is to use the typing alternatives.

I think supporting typing.AbstractSet does make sense on those versions. So a collection override for typing.AbstractSet would apply to MutableSet, builtin sets, and frozen sets.

Since typing.AbstractSet was deprecated in 3.9, I don't think it's important to support it on those versions - on 3.9+ we would expect the users to switch to the collections.abc variants.

When I say I don't consider something worth supporting, I'm doing some cost/benefit analysis of the maintenance burden of all these combinations, and as you see there are a lot of them.

So in your case, your snippet (on 3.8) would need to become:

from typing import AbstractSet

converter = GenConverter(unstruct_collection_overrides={AbstractSet: sorted})
converter.unstructure(frozenset(()))

That sound good?

[layday]

On 3.8 and below, collections.abc.Set doesn't support subscripting (collections.abc.Set[int] is an error)

But collection overrides themselves don't support subscripting, right? GenConverter(unstruct_collection_overrides={collections.abc.Set[int]: sorted}) won't do anything. Arguably, the converter should raise for unsupported overrides. Is there a reason not to support collections.abc.Set across all Python versions? Anyway, either typing.AbstractSet or collections.abc.Set would be fine :)

[Tinche]

Started working on this here: #266

The docs say this feature isn't supported on versions prior to 3.9 btw, and I remember why not - it's very gnarly to get right across all the versions :/

[Tinche]

I've merged this to main, can you verify the behavior is what you'd expect?

[layday]

That's done the trick, thanks :) And I'm sorry, I missed the notice about 3.9 somehow.

[Tinche]

Cool, ok if I close this?

[layday]

Yep, thanks.