What does this 9223372036854775807 mean? I have idea, but this looks totally confusing.

And there is an integer overflow on 32-bit platform.

I found it somewhere (in cpython), though I can't remember where. I'll try to readjust with clinic, taking list_index() as an example, and commit the updates asap.

EDIT: Updated.

An arbitrary Python code can be executed in PyObject_RichCompareBool(). It can change the size of the array. As result, the index can become out of the array limits.

Use the same loop condition as in list.index to prevent this.

Yes, I was able to reproduce this, using an object that contained a reference to the array, which in its __eq__ mutates the array, causing an assertion failure. At this point in time, my code has boiled down to an almost identical copy of list_index, except I do not see the reason for truncating the stop index, as is done there.

Remove redundant empty lines.

"else if" should be on a separate line (PEP 7). But actually this branch is not needed due to the comment below.

I think the words "but the returned index is still relative to the start of the array, not the subsection" are redundant since this directly follows from the first sentence.

*start* and *stop*

Perhaps this is redundant too. This is common behavior of index() methods.

You can document the default start value:

array.index(x, start=0[, stop])

I would prefer to specify start=0, stop=sys.maxsize, but there were objections against this, because this looks like start and stop are keyword parameters. There were issues about changing the documentation back to [, start[, stop]].

Is it possible to not document the default value at Python level? Try to remove "= sys.maxsize".

Maybe you should put "/" (currently the following line) between start and stop?

It would be better to not make stop the keyword argument until other index() methods accept keyword arguments.

The signature of array.index() matches the signature of list.index() and this is the best that we can to do now. Due to limitations of pydoc, inspect and Argument Clinic the default value for stop is displayed as a concrete number, but this is a separate issue.

Oh, right, I didn't notice that other methods don't accept keywords.

>>> "abc".index("c", stop=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: index() takes no keyword arguments

I like the idea of accepting keyword arguments in other index() methods :-)

I think there is an open issue for this. But first I want unify the code for all index() methods. Currently index() methods of str/bytes/bytearray use slightly different code.

Usually we don't document the exact exception raised by a method. Please remove this addition.

Maybe we should start to document them, but I would prefer to have a discussion about that, before starting to document them.

Docstrings of other index() methods document this.

What is stop is still negative here?

Maybe you can use PySlice_AdjustIndices() here with step=1? What do you think @serhiy-storchaka?

All correct if stop is still negative. The following loop just will not be executed.

I guess that you kept the "< Py_SIZE()" check if the array is mutated during the PyObject_RichCompareBool() call? If yes, please add a short comment to explain that, since it can be surprising.

I suggested to do this. This is an exact copy from list.index(). Do you want to add a comment in list.index()?