Python: how to implement __getattr__()?

Question

My class has a dict, for example:

class MyClass(object):
    def __init__(self):
        self.data = {\'a\': \'v1\', \'b\': \'v2\'}

Then I

Answer 1

Late to the party, but found two really good resources that explain this better (IMHO).

As explained here, you should use self.__dict__ to access fields from within __getattr__, in order to avoid infinite recursion. The example provided is:

def __getattr__(self, attrName):
  if not self.__dict__.has_key(attrName):
     value = self.fetchAttr(attrName)    # computes the value
     self.__dict__[attrName] = value
  return self.__dict__[attrName]

Note: in the second line (above), a more Pythonic way would be (has_key apparently was even removed in Python 3):

if attrName not in self.__dict__:

The other resource explains that the __getattr__ is invoked only when the attribute is not found in the object, and that hasattr always returns True if there is an implementation for __getattr__. It provides the following example, to demonstrate:

class Test(object):
    def __init__(self):
        self.a = 'a'
        self.b = 'b'

    def __getattr__(self, name):
        return 123456

t = Test()
print 'object variables: %r' % t.__dict__.keys()
#=> object variables: ['a', 'b']
print t.a
#=> a
print t.b
#=> b
print t.c
#=> 123456
print getattr(t, 'd')
#=> 123456
print hasattr(t, 'x')
#=> True