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Date: Sun, 01 Feb 2015 00:12:10 -0800
From: Ethan Furman <ethan@stoneleaf.us>
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To: python-list@python.org
Subject: Re: dunder-docs (was Python is DOOMED! Again!)
References: <54c07d04$0$13012$c3e8da3$5496439d@news.astraweb.com> <MPG.2f2b9739d70d7220989688@nntp.aioe.org> <35a40ec6-3763-448b-9ea4-4a233a04979b@googlegroups.com> <54c1ccc8$0$13005$c3e8da3$5496439d@news.astraweb.com> <mailman.18150.1422284487.18130.python-list@python.org> <54c6d7c2$0$12992$c3e8da3$5496439d@news.astraweb.com> <mailman.18178.1422380228.18130.python-list@python.org> <MPG.2f31f0482d3837cb98969f@nntp.aioe.org> <54c83ab4$0$12982$c3e8da3$5496439d@news.astraweb.com> <MPG.2f3318cb4d02fd059896b2@nntp.aioe.org> <mab931$qqu$1@ger.gmane.org> <mailman.18262.1422540523.18130.python-list@python.org> <54ca583e$0$13005$c3e8da3$5496439d@news.astraweb.com> <mailman.18276.1422555831.18130.python-list@python.org> <54ccc2fc$0$13009$c3e8da3$5496439d@news.astraweb.com> <mailman.18340.1422716000.18130.python-list@python.org> <54cd9a7a$0$12984$c3e8da3$5496439d@news.astraweb.com> <mailman.18350.1422765114.18130.python-list@python.org> <e4b6415e-f3f0-4a47-8f66-6f39609df0f6@googlegroups.com>
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On 01/31/2015 09:36 PM, Rustom Mody wrote:
>=20
> And a student asked me the diff between
> dir([])
> and
> [].__dir__()
>=20
> I didnt know what to say...
> Now surely the amount of python I dont know is significantly larger tha=
n what I know
> Still it would be nice to have surface-syntax =E2=86=90=E2=86=92 dunder=
-magic more
> systematically documented

I don't have a complete answer for you, but I can say this:

In simple cases (such as __len__) there is little difference between call=
ing the surface operator and the dunder version
(the error message differs in this case).

In more complex cases (such as __add__) using the surface syntax (+) buys=
 lots of extras:

  - if one operand is a subclass of the other, calling its __add__ or __r=
add__ method as appropriate
  - if the first operand returns NotImplemented, calling the other operan=
d's __radd__ to see if that works
  - possibly others that I don't recall at the moment

Basically, unless you're programming at the system (or class internals) l=
evel, don't call dunder methods directly.

--
~Ethan~


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