As of python3.6, passing None to the start/end parameters of `list.index` and `tuple.index` raises the following exception:
"slice indices must be integers or None or have an __index__ method"

This suggests that the intent is to support None as a valid input. This would be quite useful for the end parameter, where the sensible default is len(self) rather than a constant. Note also that str, bytes, and bytearray all support None.

I suggest that CPython be patched to support None for start/end. Otherwise, at the very least the exception message should be changed.

Accepting None will make the optional start/end parameters for this method more consistent across the types, which is especially helpful when using type annotations / checking.

The error message should be fixed and the API should be left alone.  A *None* value doesn't do a good jog of communicating intent.

I concur with Raymond. Proposed patch fixes error messages and doesn't change the public API.

I think it is a mistake not to support None values. Please consider:

The existing message clearly suggests that the intent is to support the same set of values as the start/stop parameters of the slice type. str, bytes, and bytearray all support None for `index`, as well as `count`, `find`, `rfind`, and `rindex`.

Not supporting None makes the type signature of list.index and tuple.index subtly different from those of str, bytes, and bytearray, when they could otherwise be interchangeable. It makes the type signature of index inconsistent across the types.

Furthermore, it makes converting slice start/stop to these arguments a pain. Compare the following:

    seq.index(start=slice.start, end=slice.stop) # accepting None

    seq.index(start=(0 if slice.start is None else slice.start), end=(len(seq) if slice.stop is None else slice.stop)) # current behavior.

I do not buy the argument that the latter communicates intent better, and it is adds bug-prone baggage to every call site.


Similarly, being able to pass None for end/stop parameters is very convenient in the case of optional passthrough arguments, saving the programmer from this:
def f(seq, end=None):
  i = seq.index(start=0, end=(len(seq) if end is None else end)) # current behavior.
  ...

Note that for the programmer writing `f`, None (or some other distinct sentinel value) is a *requirement* for correctness, because end=len(seq) is incorrect.

I would understand opposition to this on the basis of not changing existing behavior in any way, but "communicating intent" seems like a thin argument in comparison to the above.

I agree with George that supporting None here is the better option.

This problem also applies to collections.deque. tuple, list, and deque all have very similar index implementations, and it would be nice to merge their argument parsing boilerplate.

Sorry, modifying these mature APIs is a really bad idea.  They are worked well as-is of over a decade.  Seeing people use None for these arguments makes the code less readable. 

Python has a number of APIs where the number of arguments controls the behavior.  For example, type() with one argument does something very different that type() with three arguments.  Likewise, iter() behaves differently with two arguments than three.  The pattern used by index() shows-up in a number of places where we have optional start and end arguments.  These APIs are a long standing Python norm, both for the core and for third-party modules.  Type checking will have to adapt to norms rather than having all of Python change for the convenience of typing.

Serhiy, please go ahead with the doc fix.

If the OP really wants to insist on changing old and stable APIs through out the language in a way that I consider harmful to readability, we can battle it out in a PEP and on mailing lists.

Thank you Raymond for your review.

In any case we should first fix error messages in maintain releases. Changing signatures of methods of basic type should be discussed on the mailing lists. I don't think this needs a PEP, unless this change will be extended to many other methods and types.

New changeset d4edfc9abffca965e76ebc5957a92031a4d6c4d4 by Serhiy Storchaka in branch 'master':
bpo-29935: Fixed error messages in the index() method of tuple, list and deque (#887)
https://github.com/python/cpython/commit/d4edfc9abffca965e76ebc5957a92031a4d6c4d4

New changeset bf4bb2e43030661e568d5d4b046e8b9351cc164c by Serhiy Storchaka in branch '3.6':
bpo-29935: Fixed error messages in the index() method of tuple, list and deque (#887) (#907)
https://github.com/python/cpython/commit/bf4bb2e43030661e568d5d4b046e8b9351cc164c

New changeset 8b8bde44f3912d8b2df5591ffc31d2d8c6dcca6c by Serhiy Storchaka in branch '3.5':
bpo-29935: Fixed error messages in the index() method of tuple, list and deque (#887) (#907) (#909)
https://github.com/python/cpython/commit/8b8bde44f3912d8b2df5591ffc31d2d8c6dcca6c

New changeset 079f21f873b99f9564a41c5b7f3e0d7035847ae5 by Serhiy Storchaka in branch '2.7':
bpo-29935: Fixed error messages in the index() method of tuple and list (#887) (#907) (#910)
https://github.com/python/cpython/commit/079f21f873b99f9564a41c5b7f3e0d7035847ae5