Is there an official way to add methods to an instance?

Bruno Desthuilliers bruno.42.desthuilliers at websiteburo.invalid
Fri Apr 4 13:50:45 CEST 2008


Peter Otten a écrit :
(snip)

> Anyway, here is one more option to add too the zoo:
> 
>>>> class A(object):
> ...     def __init__(self, f, x):
> ...         self._f = f
> ...         self.x = x
> ...     @property
> ...     def f(self):
> ...         return self._f.__get__(self)
> ...     def __del__(self):
> ...         print "deleting"
> ...

This is nice but requires that you know in advance how many methods 
you're going to add and how they will be named (which is not a bad thing 
in itself - on the contrary - but may not be what the OP is after), and 
that you can add these methods at instanciation time.

A variant could be:

class A(object):
     def __init__(self, x):
         self.x = x

     def __getattr__(self, name):
         target = '_' + name
         # avoids recursion
         if hasattr(self, target):
             func = getattr(self, target)
             if hasattr(func, '__get__'):
                 return func.__get__(self, type(self))

         # nothing found, bye...
         raise AttributeError(
                 "%s object has no attribute %s" % (self, name)
             )


a = A(21)
a._foo = lambda self: "answer is %s" % (self.x * 2)
print a.foo()



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