[Python-Dev] list += string??

[Tim Peters]

[David Abrahams]
> I just discovered the following behavior:
>
>  C:\Documents and Settings\dave>python
>  Python 2.3 (#46, Aug 25 2003, 18:37:29) [MSC v.1200 32 bit (Intel)] on win32
>  Type "help", "copyright", "credits" or "license" for more information.
>  >>> prefix = ['foo', 'bar', 'baz']
>  >>> prefix += 'bomb'  # meant to write prefix += [ 'bomb' ]
>  >>> prefix
>  ['foo', 'bar', 'baz', 'b', 'o', 'm', 'b']
>  >>>
>
> Is it new and/or intentional?  I would have expected an error from the
> +=.  I was very surprised when my error "passed silently".

It was new when the iterator protocol was introduced.  It wasn't
explicitly intended that you get surprises for strings specifically,
but it was explicitly intended that

    list += whatever

work like

    list.extend(whatever)

and that whatever can be any iterable object.

>>> prefix = ['foo', 'bar', 'baz']
>>> def g():
...     for ch in "bomb":
...         yield ch
...
>>> prefix += g()
>>> prefix
['foo', 'bar', 'baz', 'b', 'o', 'm', 'b']
>>>

Since strings are iterable objects, it's no more or less surprising
than that 'for ch in "bomb"' works too <wink>.