Andrea Griffini <ag****@tin.it> wrote in
news:7f********************************@4ax.com:
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 ?
+= 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.
