inconsistency with += between different types ?
Andreas.Leitgeb at siemens.at
Thu Aug 8 13:34:47 CEST 2002
Donn Cave <donn at u.washington.edu> wrote:
> class Currency:
> def __iadd__(self, v):
> return self + v
> def __add__(self, v):
> d+=2; # does not change c
While being effectively correct, this code-snippet kind of demonstrates,
that you, too, have a wrong understanding of how += leads to calling
__iadd__ or __and__.
Note: You don't need to define __iadd__ to be able to do a += .
In your example, you could just remove your definition
of __iadd__, as it yields nothing, but costs a little performance
and adds complexity.
In your example, "d+=2" causes "d.__iadd(...)__" to be called,
which then calls (through "+") d.__add(...)__. In the end, the result
is re-assigned to d, making d point to a new object (as you intended,
but as you could have done easier)
Without __iadd__ in your class, "d+=2" would detect that there is no
__iadd__, and as a consequence call __add__ instead, and then re-assign
the new object (returned by __add__) to d.
One actual call saved. One possible niche for bugs diminished.
My suggestion would push python-programmers into understanding what
__iadd__ is really good for and what it is not necessary for.
This thread got started, because I had failed to remember the
concept of immutable built-in types - my fault, because it was
clearly described even in my small book.
Now, I'm glad that this happened, because I've learnt from the answers,
that __iadd__ works quite different from what I'd have called intuitive.
(and this, the book omitted to explain, likely for brevity's sake)
Because += already has this fallback to __add__ builtin, __iadd__
shouldn't care about the immutable case. It need not even exist for
that case. Now for the mutable case, it should be __iadd__'s only
job to get the mutation done on the object.
Because there might be code out there, like the one you wrote, which
wouldn't work afterwards (as the __add__ is hidden by a non-mutating
__iadd__), I added, that there would have to be warnings issued for
helping transition of wrongly designed code.
Newsflash: Sproingy made it to the ground !
read more ... <http://avl.enemy.org/sproingy/>
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