Wrapper class in Python

Question

I want to have a wrapper class that behaves exactly like the object it wraps except that it adds or overwrites a few select methods.

My code currently looks like this:

# Create a wrapper class that equips instances with specified functions
def equipWith(**methods):

  class Wrapper(object):
    def __init__(self, instance):
      object.__setattr__(self, 'instance',instance)

    def __setattr__(self, name, value):
      object.__setattr__(object.__getattribute__(self,'instance'), name, value)

    def __getattribute__(self, name):
      instance = object.__getattribute__(self, 'instance')

      # If this is a wrapped method, return a bound method
      if name in methods: return (lambda *args, **kargs: methods[name](self,*args,**kargs))

      # Otherwise, just return attribute of instance
      return instance.__getattribute__(name)

  return Wrapper

In order to test this out I wrote:

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

a = A(10)
W = equipWith(__add__ = (lambda self, other: self.a + other.a))
b = W(a)
b.a = 12
print(a.a)
print(b.__add__(b))
print(b + b)

When on the last line, my interpreter complains:

Traceback (most recent call last):
  File "metax.py", line 39, in <module>
    print(b + b)
TypeError: unsupported operand type(s) for +: 'Wrapper' and 'Wrapper'

Why is this? How can I get my wrapper class to behave the way I want it to?

http://stackoverflow.com/questions/972/adding-a-method-to-an-existing-object I think this might also be part of the problem. — tacaswell, Feb 08 '13 at 05:32
This problem looks like one typically solved by metaclasses. — Barney Szabolcs, May 22 '20 at 03:52

tacaswell · Accepted Answer · 2017-08-10T21:08:19.620

7

It looks like what you want can only be done with new-style object with extraordinary measures. see https://stackoverflow.com/a/9059858/380231, this blog post and the documentation.

Basically, the 'special' functions short-circuit the look-up procedure on new-style objects.

edited Aug 10 '17 at 21:08

answered Feb 08 '13 at 05:45

tacaswell

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What do you think is wrong with the top answer on https://stackoverflow.com/questions/9057669/how-can-i-intercept-calls-to-pythons-magic-methods-in-new-style-classes – Dave Kielpinski Aug 10 '17 at 21:04
1

updated with that link. Definitely did not know meta-classes when I answered this! – tacaswell Aug 10 '17 at 21:09

Wrapper class in Python

1 Answers1