plugin.py

class PluginMount(type):
    def __init__(cls, name, bases, attrs):
        if not hasattr(cls, 'plugins'):
            # This branch only executes when processing the mount point itself.
            # So, since this is a new plugin type, not an implementation, this
            # class shouldn't be registered as a plugin. Instead, it sets up a
            # list where plugins can be registered later.
            cls.plugins = []
        else:
            # This must be a plugin implementation, which should be registered.
            # Simply appending it to the list is all that's needed to keep
            # track of it later.

            cls.plugins.append(cls)

class PasswordValidator(object):
    """
    Plugins extending this class will be used to validate passwords.
    Valid plugins must provide the following method.

    validate(self, password)
        Receiveds a password to test, and either finished silently or raises a 
        ValueError if the password was invalid. The exception may be displayed
        to the user, so make sure it adequately describes waht's wrong.
    """
    __metaclass__ = PluginMount

def is_valid_password(pasword):
    """
    Return True if the password was fine, False if where was a problem.
    """
    for plugin in PasswordValidator.plugins:
        try: 
            plugin().validate(password)
        except ValueError:
            return False
    return True

def get_password_errors(password):
    """
    Return a list of messages indicating any problems that were found 
    with the password. If it was fine, this returns an empty list.
    """
    errors = []
    for plugin in PasswordValidator.plugins:
        try:
            plugin().validate(password)
        except ValueError, e:
            errors.append(str(e))
    return errors


class MinimumLength(PasswordValidator):
    def validate(self, password):
        "Raises ValueError if the password is too short."
        if len(password) < 6:
            raise ValueError('Passwords must be at least 6 characters.')

class SpecialCharacters(PasswordValidator):
    def validate(self, password):
        "Raises ValueError if the password doesn't contain any special characters."
        if password.isalnum():
            raise ValueError('Passwords must contain on special character.')

for password in ('pass', 'password', 'p@ssword!'):
    print ('Checking %r ...' % password),
    if is_valid_password(password):
        print 'valid!'
    else:
        print # Force a new line
        for error in get_password_errors(password):
            print ' %s' % error