python - Define custom error messages on invalid attribute call -

how define custom error messages specific invalid attribute calls in python?

i wrote class thats attribute assignment dependent on input on instance creation , return more descriptive error message if unassigned attribute called:

class test:     def __init__(self, input):         if input == 'foo':              self.type = 'foo'             self.a = 'foo'         if input == 'bar':              self.type = 'bar'             self.b = 'bar'  class_a = test('foo')  print class_a.a print class_a.b 

on execution error-message

attributeerror: test instance has no attribute 'b' 

instead of i'd like

attributeerror: test instance of type 'foo' , therefore has no b-attribute 

override getattr in class.

class test(object):     def __init__(self, input):         if input == 'foo':              self.type = 'foo'             self.a = 'foo'         if input == 'bar':              self.type = 'bar'             self.b = 'bar'      def __getattr__(self, attr):         raise attributeerror("'test' object of type '{}' , therefore has no {}-attribute.".format(self.type, attr)) 

getattr called when python can't find attribute normally. essentially, it's "except" clause when class raises attributeerror.