def _convert(self, value):
"""Returns a PasswordHash from the given string.
PasswordHash instances or None values will return unchanged.
Strings will be hashed and the resulting PasswordHash returned.
Any other input will result in a TypeError.
"""
if isinstance(value, PasswordHash):
return value
elif isinstance(value, basestring):
return PasswordHash.new(value, self.rounds)
elif value is not None:
raise TypeError(
'Cannot convert {} to a PasswordHash'.format(type(value)))
