Improve itertools.starmap

[collinwinter]

BPO	1498370
Nosy	@rhettinger, @jackdied
Files	better_starmap.patch: Improve itertools.startmap(), against r47103

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2009-01-27.05:14:58.369>
created_at = <Date 2006-05-31.17:41:09.000>
labels = ['extension-modules']
title = 'Improve itertools.starmap'
updated_at = <Date 2009-01-27.05:14:58.368>
user = 'https://bugs.python.org/collinwinter'

bugs.python.org fields:

activity = <Date 2009-01-27.05:14:58.368>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2009-01-27.05:14:58.369>
closer = 'rhettinger'
components = ['Extension Modules']
creation = <Date 2006-05-31.17:41:09.000>
creator = 'collinwinter'
dependencies = []
files = ['7299']
hgrepos = []
issue_num = 1498370
keywords = ['patch']
message_count = 8.0
messages = ['50390', '50391', '50392', '50393', '50394', '50395', '50396', '80621']
nosy_count = 5.0
nosy_names = ['anthonybaxter', 'collinwinter', 'rhettinger', 'quiver', 'jackdied']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1498370'
versions = ['Python 2.5']

[collinwinter]

As it currently stands, the iterator argument to
itertools.starmap() must yield tuples, even those any
iterable can be *-expanded in function calls. The
attached patch changes starmap()'s behaviour (as well
as docs and tests) to allow the provided iterator to
return any iterable object.

The patch is against r46582.

[quiver]

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[jackdied]

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The patch can be simplified by always calling
PySequence_Tuple() and doing the decref instead of special
casing tuples. If you pass PySequence_Tuple() a tuple it
will incref it and hand it back to you.

[collinwinter]

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I've updated the patch per jackdied's suggestion, plus
improved the test suite a bit.

The patch is now against r47103.

[rhettinger]

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Since starmap() is documented to behave like "yield
function(*iterable.next())", I think this fixup can be
considered a bugfix. If Anthony is okay with this, it
should into Py2.5.

While Jack's suggested modification leads to a
conceptually simpler patch, I prefer an alternate version
that only calls PySequence_Tuple() if PyTuple_CheckExact()
fails. That will make sure that we don't slow down the
most common case.

Also, I prefer slightly different updates to the docs and
test suite. If Anthony approves for inclusion in Py2.5, I
will upload my own version of a patch to starmap().

[jackdied]

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Raymond's intuition for bare metal is correct. My suggested
simplifaction slowed down the tuple case by most of a
percent (I benchmarked it but left it out of the comment).
Preternatural, I tell you.

[anthonybaxter]

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Hm. Making the code agree with the docs is a good plan.
Please make sure to include a very clear NEWS item.

[rhettinger]

This was fixed in r60013.