On Wednesday, June 24, 2015 at 8:17:08 PM UTC-4, Chris Angelico wrote:
> On Thu, Jun 25, 2015 at 9:52 AM, fl <rxjgmail.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
I have revisit the past post. In the example code snippet:
type(tup[1])
Out[162]: list
'list' is mutable. Why does the following line have errors?
In practical Python code, error is not acceptable. Then, what purpose is
for the following code here to show?
Thanks,
>>> tup[1] += [6,7]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
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