When is a subclass not right?

[Chaz Ginger]

I was writing some code that used someone else class as a subclass. He 
wrote me to tell me that using his class as a subclass was incorrect. I 
am wondering under what conditions, if ever, does a class using a 
subclass not work.

Here is an example. For instance the original class might look like:

class A :
  def __init__(self,arg) :
	self.foo = arg
  def bar(self) :
	return self.foo


And I defined a class B1 which looked like:


class B1(A);
  def __init__(self,a1,a2) :
	self.c = a1
	A.__init__(self,ag)


He said I should use it this way:

class B2:
  def __init__(self,a1,a2):
	self.c = a1
	self.t = A(a2)

  def bar(self) :
	self.t.bar()


Other than the obvious difference of B2 having an attribute 't', I can't 
see any other obvious differences. Is there something I am missing?

TIA
Chaz