Bring back callable()

[pitrou]

BPO	10518
Nosy	@brettcannon, @birkenfeld, @pitrou, @benjaminp, @ezio-melotti, @merwok, @voidspace, @durban, @ambv
Files	callable.patch

^{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/birkenfeld'
closed_at = <Date 2010-11-27.22:00:39.566>
created_at = <Date 2010-11-24.13:05:16.647>
labels = ['interpreter-core', 'type-feature']
title = 'Bring back callable()'
updated_at = <Date 2010-11-27.23:00:39.816>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2010-11-27.23:00:39.816>
actor = 'pitrou'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2010-11-27.22:00:39.566>
closer = 'pitrou'
components = ['Interpreter Core']
creation = <Date 2010-11-24.13:05:16.647>
creator = 'pitrou'
dependencies = []
files = ['19795']
hgrepos = []
issue_num = 10518
keywords = ['patch']
message_count = 24.0
messages = ['122273', '122277', '122292', '122293', '122309', '122345', '122364', '122366', '122367', '122368', '122370', '122371', '122373', '122562', '122563', '122570', '122572', '122574', '122576', '122577', '122579', '122581', '122582', '122584']
nosy_count = 10.0
nosy_names = ['brett.cannon', 'georg.brandl', 'pitrou', 'benjamin.peterson', 'ezio.melotti', 'eric.araujo', 'michael.foord', 'SilentGhost', 'daniel.urban', 'lukasz.langa']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue10518'
versions = ['Python 3.2']

[pitrou]

It was just resting. Here is a patch to wake it up for 3.2 (or 3.3 pending moratorium interpretation).

As for the py3k warning in 2.x (and the 2to3 fixer), it's not obvious what we should do: callable() clearly doesn't exist in 3.0 and 3.1.

[benjaminp]

I think we should lift the py3k warning after 3.2 is released, but leave the 2to3 fixer in for the time being.

[merwok]

Does the plan include deprecating collections.Callable?

[SilentGhost]

What is the reason for this? Why do we need it?

[ezio-melotti]

I'm not sure it's worth bringing callable() back at this point.
It would have made more sense for 3.1, but now we already have 2 callable()-less versions of Python if we do it for 3.2 (what about the moratorium though?) or 3 if we do it for 3.3. Also if isinstance(obj, collections.Callable) is correct in what it does, it's trivial to do something like:

def callable(obj):
  return isinstance(obj, collections.Callable)

If it goes in the iscallable name suggested on python-dev might be better (IMHO). At least people will realize that it's something "new" and don't assume that the old callable() has been there all the time (it also clear that it returns a boolean and reads better in e.g. if iscallable(obj): ...).
(The patch should also include an entry in the table at the top of functions.rst now that I committed bpo-10299 in r86732.)

[ambv]

I was originally surprised to find that callable() was gone. I pointed it out at Europython and got a very informative explanation from Brett. The isinstance(obj, collections.Callable) was introduced and it works well.

I'm with Ezio here. I'm -1 for callable(), +0 for iscallable() (there should be preferably one obvious way to do it and using isinstance() seems to be that way at this point).

[pitrou]

I'm with Ezio here. I'm -1 for callable(), +0 for iscallable() (there
should be preferably one obvious way to do it and using isinstance()
seems to be that way at this point).

The thing is, "isisinstance(x, collections.Callable)" is hardly obvious
to anybody, while callable() is.
As for the naming, it's the same as in 2.x, which is a good enough
reason for me. Nobody stops you from calling your callables "callable"
anyway, or your maxima "max" and your sums "sum".

[voidspace]

Bringing back callable but with a different name is horrible. Just bring it back for goodness sake.

[SilentGhost]

Just for the reference: "What's new in Python 3.0" page says:

Removed callable(). Instead of callable(f) you can use isinstance(f, collections.Callable). The operator.isCallable() function is also gone.

There doesn't seem to be any indication than in two-versions time anyone would feel a need to resurrect it.

It might be not "obvious", but it's consistent with the check for other attributes. And I doubt that it's any more non-obvious than the need to use it in a first place.

[voidspace]

Well, Guido has already approved its return - so further debate is relatively pointless. (Not that that usually stops us...)

[SilentGhost]

surely, such a relevant bit of information is worth linking to!

[voidspace]

It was on python-ideas in the recent thread about bringing back callable. Feel free to post a link here for the record.