[issue46379] itertools.product reference implementation creates temporaries

[Markus Wallerberger]

Markus Wallerberger  added the comment:

> To a person well versed in recursion and in generator chains it makes sense 
> but not so much for anyone else.

There I pretty much fundamentally disagree.  I find the version in the docs 
much more magical in the sense that it builds up "laterally", i.e., 
level-by-level, rather than element-by-element.

Also, I think from a functional programming perspective, which, let's face it, 
is what these iteration/generator tools are really modelling, a recursive 
version is much more natural.  It also generalizes nicely to other problems 
which people may be having -- so it has the added benefit of explaining the 
code and teaching people useful patterns.

Take the itertools.permutation as an example:  writing that as it was in the 
reference implementation the code is IMHO pretty opaque and hard to reason 
about.  Write it in a recursive style and both its working and correctness is 
immediately obvious.

>  Plus it is hard to step through by hand to see what it is doing.

This I agree with.

Anyway, thanks for taking the time to explain the rejection.

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[issue46379] itertools.product reference implementation creates temporaries

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Please do keep looking for improvements.  Suggestions are always welcome.

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[issue46379] itertools.product reference implementation creates temporaries

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Markus, thank you for the suggestion but I'm going to decline.  When this rough 
equivalent was first created, we looked at several recipes and chose this one 
as being one of the least magical.  Intentionally, we did not use the variant 
you've proposed.  To a person well versed in recursion and in generator chains 
it makes sense but not so much for anyone else.  Plus it is hard to step 
through by hand to see what it is doing.

In general, the rough equivalents were intended to a way to understand what 
output is going to be generated.  That is why they are mostly simple rather 
than being faithful to the actual implementations (otherwise, we would use 
classes rather than generators for all the equivalents).  Viewed in this light, 
we place almost zero weight to making the recipe memory efficient with respect 
to temporary variables.

--
resolution:  -> rejected
stage: patch review -> resolved
status: open -> closed

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[issue46379] itertools.product reference implementation creates temporaries

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee: docs@python -> rhettinger

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[issue46379] itertools.product reference implementation creates temporaries

[Alex Waygood]

Alex Waygood  added the comment:

(I'm removing 3.6 and 3.7 from the "versions" field, since those two branches 
are now only accepting patches if it relates to security.)

--
nosy: +AlexWaygood, rhettinger
versions:  -Python 3.7, Python 3.8

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[issue46379] itertools.product reference implementation creates temporaries

[Markus Wallerberger]

Change by Markus Wallerberger :


--
keywords: +patch
pull_requests: +28804
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/30605

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[issue46379] itertools.product reference implementation creates temporaries

[Markus Wallerberger]

New submission from Markus Wallerberger :

The reference implementation of itertools.product creates large temporaries, 
which we need to remind people of at the top of the code block.

However, using generator magic, we don't need to do this and can even simplify 
the code in the process!  Basically,we iterate over a generator of 
product(*seq[:-1]), and extend each of the values by every value in seq[-1].

--
assignee: docs@python
components: Documentation
messages: 410573
nosy: docs@python, mwallerb
priority: normal
severity: normal
status: open
title: itertools.product reference implementation creates temporaries
type: enhancement
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9

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