AttributeError: ClassA instance has no attribute '__len__'

[Michael Spencer]

MackS wrote:
> I'm new to Python. In general I manage to understand what is happening
> when things go wrong. However, the small program I am writing now fails
> with the following message:
> 
In general you are more likely to get helpful responses from this group if you 
post the actual code that has the problem and include the actual traceback. 
However...

> AttributeError: ClassA instance has no attribute '__len__'
> 
> Following the traceback,I see that the offending line is
> 
> self.x = arg1 + len(self.y) + 1

len calls the object's __len__ method.  self.y is bound to something (an 
instance of ClassA) that apparently has no __len__ method

> 
> Why should this call to the built-in len() fail? In a small test
> program it works with no problems:
> 
> class foo:
>   def __init__(self):
>     self.x = 0
>     self.y = 'y'
> 
>   def fun(self, arg1):
>       self.x = arg1 + len(self.y) + 1
> 
> 
>>>>a = foo()
>>>>a.fun(2)
>>>>
> 
In this case self.y is bound to something different i.e., 'y' :an object of type 
str, which has a __len__ method:
  >>> 'y'.__len__()
  1

Michael