Calling a method with getattr in Python

Question

How to call a method using getattr? I want to create a metaclass, which can call non-existing methods of some other class that start with the word 'oposite_'. The method should have the same number of arguments, but to return the opposite result.

def oposite(func):
    return lambda s, *args, **kw:  not oposite(s, *args, **kw)


class Negate(type):
    def __getattr__(self, name):
        if name.startswith('oposite_'):
            return oposite(self.__getattr__(name[8:]))        
    def __init__(self,*args,**kwargs):
        self.__getattr__ = Negate.__getattr__


class P(metaclass=Negate):
    def yep(self):
        return True
    def maybe(self, sth):
        return sth

But the problem is that

self.__getattr__(sth)

returns a NoneType object.

>>> p = P()
>>> p.oposite_yep()    #should be False
Traceback (most recent call last):
  File "<pyshell#115>", line 1, in <module>
    p.oposite_yep()
TypeError: <lambda>() takes at least 1 positional argument (0 given)
>>> p.oposite_maybe(False)    #should be true

How to deal with this?

For the record, "oposite" is spelled with two p's. But, as a colleague of mine used to say, "Programmers don't need to spell correctly, they need to spell consistently." — Seth
– Seth, Commented May 27, 2010 at 0:05

unutbu · Accepted Answer · 2010-05-27 11:23:10Z

2

class Negate(type):
    def __getattr__(self, name):
        if name.startswith('oposite_'):
            def oposite(*args,**kwargs):
                return not getattr(self,name[8:])(*args,**kwargs)
            return oposite
        else:
            raise AttributeError("%r object has no attribute %r" %
                                 (type(self).__name__, name))

    def __init__(self,*args,**kwargs):
        self.__getattr__ = Negate.__getattr__

class P(metaclass=Negate):
    def yep(self):
        return True
    def same(self,x,y):
        return x==y

p=P()
print(p.oposite_yep())
# False
print(p.oposite_same(1,2))
# True
print(p.nope())

By the way, you can also do this with a class decorator instead of a metaclass:

def Negater(cls):
    def __getattr__(self, name):
        if name.startswith('oposite_'):
            def oposite(*args,**kwargs):
                return not getattr(self,name[8:])(*args,**kwargs)
            return oposite
        else:
            raise AttributeError("%r object has no attribute %r" %
                                 (type(self).__name__, name))
    setattr(cls,'__getattr__',__getattr__)
    return cls

@Negater
class P():
    ....

edited May 27, 2010 at 11:23

answered May 26, 2010 at 23:52

unutbu

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1 Comment

brain_damage Over a year ago

ok, but that is if yep has no args except of self. If yep takes another argument, it doesn't work.

Ignacio Vazquez-Abrams · Accepted Answer · 2010-05-26 23:46:15Z

0

You forgot to handle the case where the attribute name doesn't start with 'oposite_'.

answered May 26, 2010 at 23:46

Ignacio Vazquez-Abrams

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1 Comment

brain_damage Over a year ago

It's just one return statement more. But even with it, the problem remains.

Bite code · Accepted Answer · 2011-12-22 10:29:16Z

0

The error is "TypeError: <lambda>() takes at least 1 positional argument (0 given) "

This has nothing to do with you metaclass magic, it's because you don't pass any argument to the lambda function.

When you do this:

p.oposite_yep

It call __getattr__, which returns the result of opposite(), which is a lambda function.

Your lamda function is returned, it is created on the fly, and never bouned to the instance. It won't receive 'self' as a first argument: it's just an anonymous function returned on the fly.

So when you do this:

p.oposite_yep()

You basically call the lambda, without any argument, which cause the error.

answered Dec 22, 2011 at 10:29

Bite code

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