new decumulate() function in itertools module

[carloverre]

BPO	13252
Nosy	@rhettinger, @ncoghlan, @ezio-melotti, @akheron
Files	decumulate.py

^{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 2012-08-15.05:40:23.091>
created_at = <Date 2011-10-23.22:32:47.616>
labels = ['extension-modules', 'type-feature']
title = 'new decumulate() function in itertools module'
updated_at = <Date 2012-08-15.05:40:23.090>
user = 'https://bugs.python.org/carloverre'

bugs.python.org fields:

activity = <Date 2012-08-15.05:40:23.090>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2012-08-15.05:40:23.091>
closer = 'rhettinger'
components = ['Extension Modules']
creation = <Date 2011-10-23.22:32:47.616>
creator = 'carlo.verre'
dependencies = []
files = ['23507']
hgrepos = []
issue_num = 13252
keywords = []
message_count = 4.0
messages = ['146263', '146290', '166167', '168260']
nosy_count = 5.0
nosy_names = ['rhettinger', 'ncoghlan', 'ezio.melotti', 'petri.lehtinen', 'carlo.verre']
pr_nums = []
priority = 'low'
resolution = 'rejected'
stage = 'needs patch'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue13252'
versions = ['Python 3.4']

[carloverre]

After in Python 3.2 the accumulate() function has been added to itertools module, for self-evident reasons of completeness and symmetry we could add in 3.3 the inverse decumulate() function, which, given the iterable argument p, yields p[0], p[1] - p[0], p[2] - p[1]... Its equivalent python form is in the attached file decumulate.py, below.

[ncoghlan]

OK, looking at the code I realised what you're trying to get at is the idea of reporting the differences between values in a series, such that:

x = list(accumulate(seq))
assert x == list(accumulate(differences(x)))

I don't think the use cases are there to justify the addition (not every iterator we can think of should be a building block in itertools), but if such a thing were to be added, "differences" would convey the intention significantly better than the invented term "decumulate".

[ezio-melotti]

I don't think this should go in itertools. It could go in the recipes at the bottom of the itertools doc, if you can provide some valid use cases (and possibly a patch too).

[rhettinger]

I also agree that the itertools module doesn't need this. (Reasons of "symmetry" are insufficient to make the module fatter that it already is). In addition, there are already a number of simple ways to do this.