None is accepted as start and stop index in slices. a[None:i] is equivalent to a[:i] and a[i:None] is equivalent to a[i:], independently from the type of the sequence. It is also accepted as the start or end index in the str, bytes and bytesrray methods index(), count(), startswith() and endswith(). This is not documented and officially supported feature. The documentation contains something like index(value[, start[, end]]), pydoc shows this as well, and inspect.signature() fails for these methods

The start and end indices in the tuple, list, memoryview, array and deque method index() only accept integers (including negative values). It was previously discussed in

• list.index does not accept None as start or stop #57549
• list and tuple index methods should accept None parameters #74121

and support of None was even merged, but then reverted.

Sequence.index() supports None since #67275 (although it seems there is a bug for start=None).

This inconsistency is just an annoying nuisance, except that it is difficult to express the default value for the stop index. For in-memory collections we can use sys.maxsize, but currently inspect and pydoc show it as a huge integer value:

>>> inspect.signature(tuple.index)
<Signature (self, value, start=0, stop=9223372036854775807, /)>

I think this is an argument to support None as the stop index (and as the start index for consistency). The possible drawback is that when None was the result of error, this error will no longer be detected.

For now, there were two open ways:

1. Declare official support of None in methods which support it de-facto. inspect.signature() will support this. Also, add support of None in the index() methods of other classes (this is very easy now).
2. Deprecate support of None in methods which support it. This was perhaps an unintentional effect of using a particular private C API. We need support of multisignatures in the inspect and pydoc modules to get and show adequate signatures, but we need this for tuple.index() etc:

   (value, start=0, /)
   (value, start, stop, /)
   

cc @rhettinger