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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: surprising += for lists
Date: Sun, 04 Nov 2012 12:57:11 -0500
References: <afn3i8Fn4j1U1@mid.uni-berlin.de> <50966379.3070801@davea.name>
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Xref: csiph.com comp.lang.python:32738

On 11/4/2012 7:45 AM, Dave Angel wrote:

> What I wonder about is why list's __add__  is so fussy.

Guido's reason is that it is not clear what the types of [1,2] + (3,4), 
(1,2) + [3,4], [] + range(4), range(2) + [3,4], etcetera should be. Such 
mixtures may be bugs. Seq.__add__ exists to implement '+'.

[].extend() will clearly be a list, and accepts any iterable.

I believe the same logic is used for set operations.

-- 
Terry Jan Reedy