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


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steveo at syslang.net
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