a=([],); a[0]+=[1]

[Andrea Griffini]

When reading about python "warts" i stepped in this
example I find quite puzzling:

a = ( [] , )
a[0] += [1]

this gives an error, because a tuple cannot be
modified. But if you print the content of "a"
it will show it has indeed been modified.

How can this happen ? if "+=" for list was called
then no modification to the tuple was requested and
so no error should have been raised; if instead "+"
and "=" was executed then after a failing assignment
the original list is safe... (or at least I thought
so; if you type "a[0] = [2]" you get the error and
"a" is not modified).

Is there an explanation simpler than investigating
on python sources ?

Andrea

[Duncan Booth]

Re: a=([],); a[0]+=[1]

Andrea Griffini <agriff@tin.i t> wrote in
news:7ff790taoj rrqka0trq9ltm82 g0ac95ria@4ax.c om:
[color=blue]
> When reading about python "warts" i stepped in this
> example I find quite puzzling:
>
> a = ( [] , )
> a[0] += [1]
>
> this gives an error, because a tuple cannot be
> modified. But if you print the content of "a"
> it will show it has indeed been modified.
>
> How can this happen ? if "+=" for list was called
> then no modification to the tuple was requested and
> so no error should have been raised; if instead "+"
> and "=" was executed then after a failing assignment
> the original list is safe... (or at least I thought
> so; if you type "a[0] = [2]" you get the error and
> "a" is not modified).
>
> Is there an explanation simpler than investigating
> on python sources ?
>[/color]

+= is implemented by calling the __iadd__ method (if it exists, otherwise
__add__ is called). In either case the result of the method is then
assigned to the destination.

So what is happening here is that list.__iadd__ modifies the list in place,
but then the attempt to assign the result into the tuple fails.

Although __iadd__ extends the list in place this need not be true for all
types. An object could have an implementation for inplace addition that
actually creates a new object, so Python cannot optimise out the store.

See http://groups.google.co.uk/groups?th...EBE%40noaa.gov
for a (long) discussion of this topic.