AttributeError: ClassA instance has no attribute '__len__'

[vincent wehren]

"MackS" <mackstevenson at hotmail.com> schrieb im Newsbeitrag 
news:1112209152.554037.194030 at f14g2000cwb.googlegroups.com...
| 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:
|
| AttributeError: ClassA instance has no attribute '__len__'
|
| Following the traceback,I see that the offending line is
|
| self.x = arg1 + len(self.y) + 1
|
| 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)
| >>>
|
| No problems; can you help me make some sense of what is happening?

In your program, self.y is an instance of ClassA. The traceback tells you 
that ClassA has no __len__ attribute (i.e.
it is an object that has no no "special" method called __len__, which is 
what gets called
when you do len(obj).

In your test program, self.y is "y", a string, which has a __len__ method by 
design:
(see dir("y"), which gives you:

['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', 
'__ge__', '__getattribute__', '__getitem__', '__getnewargs__', 
'__getslice__', '__gt__', '__hash__', '__init__', '__le__', '__len__', 
'__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', 
'__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', 
'__str__', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 
'expandtabs', 'find', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 
'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 
'replace', 'rfind', 'rindex', 'rjust', 'rsplit', 'rstrip', 'split', 
'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 
'upper', 'zfill']

If you want len(self.y) to work, self.y must be an object that implements a 
__len__ method. In other words, your "ClassA" needs a __len__ method.

A trivial example:

class ClassA:
    def __init__(self, text):
       self.text = text
    def __len__(self):
       #return something useful
       return len(self.text)

y = ClassA("Hello")
print len(y) # prints 5


Regards,

--
Vincent Wehren


|
| Thanks in advance
|
| Mack
|