Trying to learn about metaclasses

[Steven W. Orr]

I have been doing a lot of reading. I'm starting to get it. I think it's 
really cool as well as dangerous, but I plan on being respectful of the 
construct. I found a web page that I found quite readable.

http://cleverdevil.org/computing/78/

So, I tried to run the example code (below), and I get AttributeError. If 
someone can get me over this hump, I'd be grateful. It complains that Person 
has no _fields attribute, but the print hello I have in EnforcerMeta doesn't 
happen either. Am I doing something wrong?

I'm running python 2.6.2

Here's the error:

585 > ./meta1.py
Traceback (most recent call last):
   File "./meta1.py", line 38, in <module>
     swo.name = 'swo'
   File "./meta1.py", line 27, in __setattr__
     if key in self._fields:
AttributeError: 'Person' object has no attribute '_fields'


And here's the code:

#! /usr/bin/python
# http://cleverdevil.org/computing/78/
class Field(object):
     def __init__(self, ftype):
         self.ftype = ftype

     def is_valid(self, value):
         return isinstance(value, self.ftype)

class EnforcerMeta(type):
     def __init(cls, name, bases, ns):
         # Store the field definitions on the class as a dictionary
         # mapping the field name to the Field instance.
         print 'Hello'
         cls._fields = {}

         # loop through the namespace looking for Field instances.
         for key, value in ns.items():
             if isinstance(value, Field):
                 cls._fields[key] = value

class Enforcer(object):
     # attach the metaclass
     __metaclass__ = EnforcerMeta

     def __setattr__(self, key, value):
         if key in self._fields:
             if not self._fields[key].is_valid(value):
                 raise TypeError('Invalid type for field.')
         super(Enforcer, self).__setattr__(key, value)

class Person(Enforcer):
     name = Field(str)
     age = Field(int)

if __name__ == '__main__':
     swo = Person()
     swo.name = 'swo'
     print 'swo:', swo


-- 
Time flies like the wind. Fruit flies like a banana. Stranger things have  .0.
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Donor?Black holes are where God divided by zero. Listen to me! We are all- 000
individuals! What if this weren't a hypothetical question?
steveo at syslang.net