[issue28197] range.index mismatch with documentation

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee: rhettinger -> 

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[issue28197] range.index mismatch with documentation

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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stage:  -> needs patch

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[issue28197] range.index mismatch with documentation

[Raymond Hettinger]

Raymond Hettinger added the comment:

> Why can't this go into 3.6?

Because it is a new feature and we're already past beta 1; because nothing is 
actually broken; and because it is a very minor issue (x isn't 100% consistent 
with y); because this would depend on new code and tests that we haven't 
written yet; and because the use case for the optional arguments is to do 
repeated searches and that doesn't make much sense in the context of a range 
object.  That said, I think it is harmless to add this to 3.7 even though it 
isn't very useful.

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assignee:  -> rhettinger

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[issue28197] range.index mismatch with documentation

[Vedran Čačić]

Vedran Čačić added the comment:

Why can't this go into 3.6? To me this situation seems like a bug.

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[issue28197] range.index mismatch with documentation

[Raymond Hettinger]

Raymond Hettinger added the comment:

>  2) Add the support of stop and start arguments to range() in 3.7.

Yes, the whole point of adding these bogus methods in the first place was to 
harmonize the range object with other sequence types.

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[issue28197] range.index mismatch with documentation

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Sorry for poor words. I was not going to blame anybody. But changing interface 
always is dangerous. It makes third-party classes that implemented the 
interface no longer valid substitutions.

I think two things are worth to be done:

1) Explicitly document (including the docstring) that the support of stop and 
start arguments is optional. Not all sequences implements it. 3.5+.

2) Add the support of stop and start arguments to range() in 3.7.

And would be nice to provide a way for testing if the sequence supports 
extended index(). If this is possible.

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[issue28197] range.index mismatch with documentation

[Raymond Hettinger]

Raymond Hettinger added the comment:

> I'd argue that range can implement the 3-arg .index much better 
> than generic Sequence.

Yes, the range index() method should implement all three arguments.  That 
shouldn't be difficult.

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assignee: docs@python -> 
type: behavior -> enhancement
versions:  -Python 3.5, Python 3.6

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[issue28197] range.index mismatch with documentation

[Vedran Čačić]

Vedran Čačić added the comment:

Yes, I agree these are useless _if you know you're dealing with range_. 
However, if you have a Sequence, it would be very useful not to have range be a 
special case.

Of course, one solution is to have a default .index implementation in the 
Sequence ABC itself, but still I'd argue that range can implement the 3-arg 
.index much better than generic Sequence.

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[issue28197] range.index mismatch with documentation

[Raymond Hettinger]

Raymond Hettinger added the comment:

> These broke some concrete Sequence implementations.

Poor choice of words.  The concrete implementations didn't change at all. 

Perhaps the concrete implementation need to be brought more in-sync with the 
ABC.  That would be reasonable; afteralll the goal is substitutability  -- that 
is the only reason that the old xrange morphed into something with nearly 
useless count() and index() methods in the first place.

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[issue28197] range.index mismatch with documentation

[Vedran Čačić]

Vedran Čačić added the comment:

Yes, that's the precise reason I caught this. I was implementing some tools to 
do with Sequences (seqtools, like itertools but having non-ephemeral output 
given non-ephemeral input), and the Sequence that was easiest to test quickly 
was range. In `positions` (like .index but gives all the places a value 
appears), it raised TypeError.

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[issue28197] range.index mismatch with documentation

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Parameters start and stop were added to the Sequence.index() mixin method in 
issue23086. These broke some concrete Sequence implementations.

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nosy: +Devin Jeanpierre, rhettinger, serhiy.storchaka

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[issue28197] range.index mismatch with documentation

[SilentGhost]

Changes by SilentGhost :


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versions: +Python 3.7

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[issue28197] range.index mismatch with documentation

[Vedran Čačić]

New submission from Vedran Čačić:

Look at this:

>>> from collections.abc import Sequence

>>> help(Sequence.index)
index(self, value, start=0, stop=None)
S.index(value, [start, [stop]]) -> integer -- return first index of 
value.
Raises ValueError if the value is not present.

>>> issubclass(range, Sequence)
True

>>> help(range.index)
index(...)
rangeobject.index(value, [start, [stop]]) -> integer -- return index of 
value.
Raise ValueError if the value is not present.

So far, so good. But:

>>> range(9).index(2, 1, 5)
TypeError: index() takes exactly one argument (3 given)

Of course it's not essential, but the docs shouldn't lie. And if range _is_ a 
Sequence, then it should have the complete interface of a Sequence. Including 
start and end arguments for .index: they are optional from the point of call, 
not from the point of implementation. :-)

--
assignee: docs@python
components: Documentation, Library (Lib)
messages: 276908
nosy: docs@python, veky
priority: normal
severity: normal
status: open
title: range.index mismatch with documentation
type: behavior
versions: Python 3.5, Python 3.6

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