[issue18652] Add itertools.coalesce
Nick Coghlan added the comment:
(Updated the issue title to reflect the currently proposed name and location
for the functionality)
While I'm a fan of explicit iteration as well (inside every reduce is a loop
trying to get out), I think the fact Martin's explicit loop is buggy (it will
never match re2 as it always bails on the first iteration) helps make the case
for coalesce. The correct explicit loop looks something like this:
for r in (re1, re2):
m = r.match('abc')
if m is None:
continue
if r is re1:
print('re1', m.group(1))
elif r is re2:
print('re1', m.group(1))
break # Matched something
else:
print('No match')
(Or the equivalent that indents the loop body further and avoids the continue
statement)
The coalesce version has a definite advantage in not needing a loop else clause
to handle the nothing matched case:
m = coalesce(regexp.match('abc') for regexp in [re1, re2])
if m is None:
print('no match!')
elif m.re is re1:
print('re1', m.group(1))
elif m.re is re2:
print('re2', m.group(1))
--
title: Add a “first”-like function to the stdlib - Add itertools.coalesce
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[issue18652] Add itertools.coalesce
Martin v. Löwis added the comment: Simplicity is in the eye of the beholder, yet... you need to remember the break when *writing* the code, so the loop might be more difficult to write (but then, I need to remember/lookup the function name and parameters for coalesce)... when reading the code, the break is already there, and easy to notice. With Nick's remark, it's even more obvious that it is difficult to write :-) If an unknown (to the reader) function is used, reading the code becomes actually difficult, since the reader either needs to guess what the function does, or look it up. Note that I'm not objecting the addition of the function (I'm neutral), just the claim that there are no simple alternatives. I'm neutral because it falls under the not every two-line function needs to go into the standard library rule; but then, if it is really popular, it helps readability if it is in the library (rather than duplicated by users). -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Mark Dickinson added the comment: [Nick] Regarding the key parameter name, I believe this is closer to itertools.groupby ... Isn't this mostly about the return type? `pred` is an indication that a boolean is being returned (or that the return value will be interpreted in a boolean context), while `key` is used for more general transformations. In that sense, `pred` makes more sense here. -- nosy: +mark.dickinson ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Nick Coghlan added the comment: Mark's rationale makes sense to me. I believe that would make the latest version of the proposed API (in the itertools module): def coalesce(iterable, default=None, pred=None): ... -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Serhiy Storchaka added the comment: def coalesce(iterable, default=None, pred=None): return next(filter(pred, iterable), default) Are you sure you want add this one-line function to the itertools module rather then to recipes? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Hynek Schlawack added the comment: def coalesce(iterable, default=None, pred=None): return next(filter(pred, iterable), default) Are you sure you want add this one-line function to the itertools module rather then to recipes? Well, for many – including me – it would mean to have this one-line function in every other project or a PyPI dependency. I’m certain there are other short but useful functions in the stdlib. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
R. David Murray added the comment: I'm not going to object to the name, since I see that it is used elsewhere in programming for the proposed meaning. But allow me to rant about the corruption of the English language here. To me, coalesce should involve a computation based on all of the elements of a list, not just picking out the first non-false value. -- nosy: +r.david.murray ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Serhiy Storchaka added the comment: Well, for many – including me – it would mean to have this one-line function in every other project or a PyPI dependency. But why you want to have a separate function instead of just use two builtins? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Tim Peters added the comment: FWIW, I like this. It would be a nice addition to the itertools module. +1 The `key` argument should be renamed to `pred`, as others have said. As to the name, I like first_true. Does what it says. Plain first is misleading, and coalesce is both inscrutable and nearly impossible to spell ;-) -- nosy: +tim_one ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Hynek Schlawack added the comment: But why you want to have a separate function instead of just use two builtins? This question has been answered twice now, once from Nick – please refer above. It's a clunky and error-prone solution to a common problem. Maybe you can't emphasize because it's not a common problem to you but that doesn't make it less useful. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Nick Coghlan added the comment: A wild timbot appears! :) Tim Peters added the comment: As to the name, I like first_true. Does what it says. Plain first is misleading, and coalesce is both inscrutable and nearly impossible to spell ;-) A fair point, although skipping the underscore (firsttrue or nexttrue) would arguably be more consistent with other itertools names. I guess it's over to Raymond now as module maintainer for a yes/docs recipe/no response, and if one of the first two, the exact name used. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue18652] Add itertools.coalesce
Tim Peters added the comment: skipping the underscore (firsttrue or nexttrue) would arguably be more consistent with other itertools names. Like izip_longest and combinations_with_replacement? ;-) I care about readability here more than consistency with the bulk of itertools names. first ends with t and true begins with t, which makes firsttrue an eyesore. first_true strikes me as very easy to read, spell, remember and understand. Offhand I don't know what nexttrue is supposed to mean (like if that's supposed to mean the first true iterate, why didn't they say 'first' instead of 'next'?). -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue18652 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
