使用__getattr__防止方法被调用

Question

I am trying to prevent an instance from throwing an exception if a method that does not exist for the instance is called. I have tried the following:

class myClass(object):
    def __init__(self):
        pass
    def __getattr__(self,name):
        print "Warning - method {0} does not exist for this instance".format(name)

o = myClass()
var = o.someNonExistantFunction()

The problem is that I get the following error:

TypeError: 'NoneType' object is not callable

The two things I want to make sure of doing is:

1. Return None as my code can deal with variables being set to None
2. Perform a function (printing a warning message)

What is the cleanest way to do this?

Answer 1

return a function that does nothing?

Couple of things: first you might want to use __getattr__ rather than __getattribute__ .
__getattr__ gets called when the runtime doesn't find anything by that name in the hierarchy, __getattribute__ gets called every time.

class Test(object):
    def __getattr__(self,key):
        def placeholder(*args, **kwargs):
            print "Warning - method '%s' does not exist for this instance"%key
        return placeholder

Answer 2

Your __getattr__ is the same as:

def __getattr__(self, name):
    print "Warning ..."
    return None

So, when you do var = o.someNonExistantFunction() , this is logically the same as:

var = o.someNonExistantFunction # == o.__getattr__('someNonExistantFunction') == None
var() # same as (None)()

Which is why you're getting the NoneType not callable error. Obtuse definitely has the rest of the answer, which is to return a callable function. However, you might think about other structural issues, and if it's truly a good idea to have a catchall function generator.