[Python-Dev] [Python-checkins] cpython (3.3): #17973: Add FAQ entry for ([], )[0] += [1] both extending and raising.

[Nick Coghlan]

On Tue, May 21, 2013 at 12:35 AM, r.david.murray
<python-checkins at python.org> wrote:

Yay for having this in the FAQ, but...

> +If you wrote::
> +
> +   >>> a_tuple = (1, 2)
> +   >>> a_tuple[0] += 1
> +   Traceback (most recent call last):
> +      ...
> +   TypeError: 'tuple' object does not support item assignment
> +
> +The reason for the exception should be immediately clear: ``1`` is added to the
> +object ``a_tuple[0]`` points to (``1``), producing the result object, ``2``,
> +but when we attempt to assign the result of the computation, ``2``, to element
> +``0`` of the tuple, we get an error because we can't change what an element of
> +a tuple points to.
> +
> +Under the covers, what this augmented assignment statement is doing is
> +approximately this::
> +
> +   >>> result = a_tuple[0].__iadd__(1)
> +   >>> a_tuple[0] = result
> +   Traceback (most recent call last):
> +     ...
> +   TypeError: 'tuple' object does not support item assignment

For the immutable case, this expansion is incorrect:

>>> hasattr(0, "__iadd__")
False

With immutable objects, += almost always expands to:

>>> result = a_tuple[0] + 1
>>> a_tuple[0] = result

(For containers that support binary operators, the presence of the
relevant __i*__ methods is actually a reasonable heuristic for
distinguishing the mutable and immutable versions)

Cheers,
Nick.

--
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia