Re: 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

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