Roy Smith wrote:
In article <j5797e$s57$1...@speranza.aioe.org>,
Henrik Faber <hfa...@invalid.net> wrote:
On 19.09.2011 13:23, Paul Rudin wrote:
Henrik Faber <hfa...@invalid.net> writes:
How can I make this commutative?
Incidentally - this isn't really about commutativity at all - the
question is how can you define both left and right versions of add,
irrespective of whether they yield the same result.
Right. The operator+ in my case just happens to be commutative and I
wanted a language way to express this.
I think __radd__ is what you're after.
It is, thank you very much - I knew there was some way to get this done
nicely. Perfect! :-)
__radd__() only solves the problem if the left-hand operand has no
__add__() method itself.
Only true if the left-hand operand is so ill-behaved it doesn't check to
see if it makes sense to add itself to the right-hand operand. If it
doesn't know how, it should `return NotImplemented` -- Python will then
try __radd__ on the left-hand operand.
Also, if the right-hand operand is a subclass of the left-hand operand
then Python will try right-hand_operand.__radd__ first.
Now, if the left-hand operand *does* know how (or thinks it does, which
could be another matter entirely), and the right-hand operand is *not* a
subclass of the left-hand operand, then you are correct -- the
right-hand operand wil not be called.
~Ethan~
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