Thanks, Peter. I realize this is getting sort of academic now, as I know how
to do exactly what I want, but I'm still confused. Is __getattr__ a special
case then, even for classic classes?
class Adder(): # python 2.7, classic class
def __init__(self, x):
self.x = x
self.__add__= lambda other: Adder(self.x+other.x)
self.__getattr__ = lambda name: self.test(name)
def __str__(self):
return str(self.x)
def test(self, name):
print("Hello from test")
raise AttributeError
x = Adder(3)
y = Adder(4)
print(x+y)
x.junk()
7
Traceback (most recent call last):
File "C:\Users\Saul\Documents\PythonProjects\test.py", line 18
AttributeError: Adder instance has no attribute 'junk'
Why does this work for __add__ and not for __getattr__?
Of course, they both work if I write instead
def __add__self, other):
return Adder(self.x+other.x)
def __getattr__(self, name):
print(name)
raise AttributeError
like a sensible person.
Saul
On Monday, February 4, 2013 8:15:47 AM UTC-6, Peter Otten wrote:
> Saul Spatz wrote:
>
>
>
> > Now I have another question. If dunder methods are looked up only in the
>
> > class, not the instance, why did defining __nonzero__ the way I did work?
>
> > Shouldn't I have had to define it with a def? Is __nonzero__ a special
>
> > case?
>
>
>
> Unfortunately the situation is a bit more complex. Classic classes (like
>
> Tkinter.Frame) behave differently from newstyle classes (subclasses of
>
> object):
>
>
>
> >>> def nz():
>
> ... print "nonzero"
>
> ... return 0
>
> ...
>
> >>> class Classic: pass
>
> ...
>
> >>> c = Classic()
>
> >>> c.__nonzero__ = nz
>
> >>> not c
>
> nonzero
>
> True
>
> >>> class New(object): pass
>
> ...
>
> >>> n = New()
>
> >>> n.__nonzero__ = nz
>
> >>> not n
>
> False
>
>
>
> So Steven is wrong here.
>
>
>
> > Shouldn't I have had to define it with a def?
>
>
>
> If you mean as opposed to a lambda, there is no difference between
>
>
>
> f = lambda ...
>
>
>
> and
>
>
>
> def f(...): ...
>
>
>
> other than that the last one gives you a nice name in a traceback.
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