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

[Duncan Booth]

Andrea Griffini <agriff at tin.it> wrote in 
news:7ff790taojrrqka0trq9ltm82g0ac95ria at 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?threadm=3CFCF3E7.AE891EBE%40noaa.gov 
for a (long) discussion of this topic.