Sphinx not respecting __signature__

[asmeurer]

I am having an issue with Sphinx 3.0.3 where classes that define __signature__ are being shown with *args (the naive signature from __init__). This is the project https://github.com/Quansight/ndindex/tree/master/docs.

See for example https://quansight.github.io/ndindex/api.html#slice. The signature on the class in the docs says "Slice(*args)". But if you call inspect.signature, you get

>>> from ndindex import Slice
>>> import inspect
>>> inspect.signature(Slice)
<Signature (start, stop=<class 'ndindex.slice.default'>, step=None)>

You can see __signature__ defined on the superclass here.

This was supposedly fixed in #2828, so either there was a regression, or else somehow the code doesn't handle what I am doing properly

[tk0miya]

Note: ndindex provides Slice.__signature__ via classproperty. But autodoc refers Slice.__init__.__signature__ instead. This will not be fixed even if #7384 merged.

https://github.com/Quansight/ndindex/blob/6367b37d4d89094038568e43788208caa1f6de38/ndindex/ndindex.py#L98-L107

[asmeurer]

I use a class property so that help() shows the signature on the class, not just on instances. Looking at the __signature__ of the init method itself doesn't seem helpful. Is that even something that can be overridden?

[eric-wieser]

This ought to be fairly straightforward to add another special-case for.

The problem is that Sphinx cannot use inspect.signature directly, as doing so would skip its hooks that allow it to parse # type: comments. So Sphinx tries to implement its own version of inspect.signature that lines up with the regular implementation in the cases that were worth thinking about.

In this case, that implementation is missing the code path for a __signature__ attribute.