metaclass example of looping through classes of password validation in python

metaclass_password.py

class PluginMount(type):
	"""
	A Meta Class plugin mount location for a list of plugin classes.
	By creating this, on all subclasses of PluginMount, python will
	instantiate a plugin attribute list and maintain it for the Class 
	using this Meta Class.
	"""
	def __init__(cls, name, bases, attrs):
		if not hasattr(cls, 'plugins'):
			cls.plugins = []
		else:
			cls.plugins.append(cls)


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

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


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


def get_password_errors(password):
	"""
	Returns 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 as 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 at least one special character.')


def run():
	"""
	Call run() function to demonstrate metaclass password usage
	"""
	for password in ('pass', 'password', 'p@ssword!'):
		print(('Checking %r...' % password), end=' ')
		if is_valid_password(password):
			print('valid!')
		else:
			print() # Force new line
			for error in get_password_errors(password):
				print(' %s ' % error)