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Date: Thu, 25 Jun 2015 10:16:47 +1000
Subject: Re: Could you explain this rebinding (or some other action) on "nums = nums"?
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Thu, Jun 25, 2015 at 9:52 AM, fl <rxjwg98@gmail.com> wrote:
> The reason is that list implements __iadd__ like this (except in C, not Python):
>
> class List:
>     def __iadd__(self, other):
>         self.extend(other)
>         return self
> When you execute "nums += more", you're getting the same effect as:
>
> nums = nums.__iadd__(more)
> which, because of the implementation of __iadd__, acts like this:
>
> nums.extend(more)
> nums = nums
> So there is a rebinding operation here, but first, there's a mutating operation, and the rebinding operation is a no-op.

It's not a complete no-op, as can be demonstrated if you use something
other than a simple name:

>>> tup = ("spam", [1, 2, 3], "ham")
>>> tup[1]
[1, 2, 3]
>>> tup[1].extend([4,5])
>>> tup[1] = tup[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> tup
('spam', [1, 2, 3, 4, 5], 'ham')
>>> tup[1] += [6,7]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> tup
('spam', [1, 2, 3, 4, 5, 6, 7], 'ham')

The reason for the rebinding is that += can do two completely
different things: with mutable objects, like lists, it changes them in
place, but with immutables, it returns a new one:

>>> msg = "Hello"
>>> msg += ", world!"
>>> msg
'Hello, world!'

This didn't change the string "Hello", because you can't do that.
Instead, it rebound msg to "Hello, world!". For consistency, the +=
operator will *always* rebind, but in situations where that's not
necessary, it rebinds to the exact same object.

Does that answer the question?

ChrisA