Question

   class test:
      a="hi"

      def msg(self):
          print the variable for which the object is referring to

   t= test()
    print t.b

From the above code is there any way to tell that the object is referring to a variable b which does not exist

Answer 1

Use the __getattr__ method, see documentation:

In [1]: class test(object):
   ...:     a = 'hi'
   ...:     def __getattr__(self, val):
   ...:         print 'you are accessing ', val
   ...:         
   ...:         

In [2]: t = test()

In [3]: t.b
you are accessing  b

In [4]: t.c
you are accessing  c

In [5]: t.a
Out[5]: 'hi'

EDIT:

class test(object):
    a = 'hi'

    def msg(self, var, default='undefined'):
        setattr(self, var, default)
        return default

    def __getattr__(self, val):
        print 'attribute %s not found, setting..' % val
        return self.msg(val)


>>> t = test()
>>> print t.a
'hi'
>>> print t.b
'attribute b not found, setting..'
'undefined'
>>> t.b = 'this is black magic'
>>> # notice no message is printed here about attribute not found
>>> print t.b
'this is black magic'

EDIT2:

>>> d = {'a': '1'}
>>> d.setdefault('b', 'b')
'b'
>>> d
{'a': '1', 'b': 'b'}
>>> d.setdefault('a', 'b')
'1'
>>> d
{'a': '1', 'b': 'b'}