in place functions from operator module

Raymond Hettinger python at rcn.com
Mon Aug 30 08:41:42 CEST 2010


On Aug 29, 8:33 am, Arnaud Delobelle <arno... at googlemail.com> wrote:
> ernest <nfdi... at gmail.com> writes:
> > Hi,
>
> > The operator module provides separate functions for
> > "in place" operations, such as iadd(), isub(), etc.
> > However, it appears that these functions don't really
> > do the operation in place:
>
> > In [34]: a = 4
>
> > In [35]: operator.iadd(a, 3)
> > Out[35]: 7
>
> > In [36]: a
> > Out[36]: 4
>
> > So, what's the point? If you have to make the
> > assignment yourself... I don't understand.
>
> > Cheers,
> > Ernest
>
> That's because
>
>    a += b
>
> is executed as:
>
>    a = a.__iadd__(b)
>
> For immutable objects, (such as integers), a.__iadd__(b) returns a + b
> *and then* this value is assigned to a (or rather 'a' is bound to the
> value).  So for immutables objects, iadd(a, b) is the same as a + b
>
> For mutable objects (such as lists), a.__iadd__(b) mutates the object
> *and then* returns self so that when the assignement is executed, 'a'
> will still be bound the the same object.  E.g. if a = [1, 2] then
>
>     a += [3]
>
> will first append 3 to the list and then reassign the list to 'a' (it is
> unnecessary in this case but if this step was omitted, the "in place"
> operators wouldn't work on immutables types).

This is an excellent explanation.
Perhaps, you can submit a documentation
patch for the operator module so this
doesn't get lost.


Raymond





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