Michael wrote:
While thinking about Steven D'Aprano's thread about automatically
generating arithmetic operations for a subclass, I stumbled upon
something confusing. Having defined the following class to do funky
addition,
class MyInt(int):
def __getattribute__(self, key):
if key == "__add__":
print("In __getattribute__('__add__')")
return lambda other: MyInt(int.__add__(self, other+100))
else:
return object.__getattribute__(self, key)
def __getattr__(self, key):
if key == "__add__":
print("In __getattr__('__add__')")
return lambda other: MyInt(int.__add__(self, other+100))
else:
return object.__getattr__(self, key)
I then do this:
a = MyInt(4)
a.__add__(2)
In __getattribute__('__add__')
106
a + 2
6
Why doesn't "a + 2" look up the __add__ attribute and use my lambda?
Answer 1: because it was not programmed that way ;-).
Answer 2: because __getattribute__/__getattr__ are for looking up
attributes of instances of the class, whereas special methods are
generally required to be attributes of the class. So *their* lookup
would use type(MyInt).__getxxx__. When that fails, the add code looks
for int.__radd__. (I *think* this your answer.)
> If I manually define __add__(self, other) then "a + 2" will of course
use that method.
because that is that type(MyInt).__getxxx__ will find.
tjr
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