Question

I’ve been mucking around with class special methods to overload numeric operators via object.__add__(self,other) etc.

Is there a generic way to find out which binary operator was called though?

For unary operators you can use __getattr__, e.g:

class Test():
  def __getattr__(self, name):
    print name
t = Test()
~t  # prints __invert__

but calling binary operators e.g. t+t or t+5 always calls __coerce__ which looses the info about what the operator is.

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Solution based on Jochen Ritzel’s answer below: the easiest way is to add the following to the above class:

def __coerce__(self, other):
  self.other = other    # store 'other' somewhere we can access it
  return None

Then __getattr__ (which gets called after __coerce__) has got access to both the operands – as self and self.other– and the operation – as name.

HOWEVER it turns out trying to use dynamic dispatch via __getattr__ is almost certainly a really bad idea compared to just defining the special methods you actually need, as for operators (as opposed to attributes) Python will actually call whatever __getattr__ returns.

(Hope updating my question is ok, didn’t seem to fit in the comment box)

Answer 1

t+t calls __coerce__ and then __add__. __coerce__ is supposed to convert (numeric) types to a common class, for example 1.0 + 2 first coerces both numbers to floats.

If you want to do something special you can return proxy objects from __coerce__ that see the following operator.