Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed2a.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Date: Wed, 4 Feb 2015 14:38:31 +0000 (UTC)
From: Albert-Jan Roskam <fomcl@yahoo.com>
To: Chris Angelico <rosuav@gmail.com>
In-Reply-To: <CAPTjJmpqx1Cmm89Lg6-XL4-xdM+xD6=GHKvWb2zYTe4=11ZPPg@mail.gmail.com>
References: <CAPTjJmpqx1Cmm89Lg6-XL4-xdM+xD6=GHKvWb2zYTe4=11ZPPg@mail.gmail.com>
Subject: Re: meaning of: line, =
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Cc: "python-list@python.org" <python-list@python.org>
Precedence: list
Reply-To: Albert-Jan Roskam <fomcl@yahoo.com>
Newsgroups: comp.lang.python
Message-ID: <mailman.18464.1423061056.18130.python-list@python.org>
Lines: 62
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:85216



----- Original Message -----

> From: Chris Angelico <rosuav@gmail.com>
> To: 
> Cc: "python-list@python.org" <python-list@python.org>
> Sent: Wednesday, February 4, 2015 3:24 PM
> Subject: Re: meaning of: line, =
> 
> On Thu, Feb 5, 2015 at 1:08 AM, ast <nomail@invalid.com> wrote:
>>  I dont understand why there is a comma just after line in the following
>>  command:
>> 
>>  line, = plt.plot(x, np.sin(x), '--', linewidth=2)
>> 
>> 
>>  I never saw that before
>> 
>>  Found here:
>> 
> http://matplotlib.org/examples/lines_bars_and_markers/line_demo_dash_control.html
>> 
> 
> That's a slightly unusual form of unpacking. Compare:
> 
> def get_values():
>     return 5, 7, 2
> 
> x, y, z = get_values()
> 
> This is like "x = 5; y = 7; z = 2", because it unpacks the 
> function's
> return value into those three targets.
> 
> What you have is exactly the same, except that it has only one target.
> So it's expecting plt.plot() to return an iterable with exactly one
> thing in it, and it'll unpack it and put that thing into line:
> 
> def get_packaged_value():
>     return [42]
> 
> x, = get_packaged_value()
> 
> This is equivalent to "x = 42". I don't know matplotlib, so I 
> don't
> know what it's returning or why, but as long as it's iterable and
> yields exactly one thing, this will work.



I have also never seen this before, but perhaps this:

>>> f = lambda: [42]
>>> result, = f()
>>> result
42

... is slightly cleaner than this:
>>> result = f()[0]
>>> result
42