__metaclass__ in Python 3

Question

In Python2.7 this code can work very well, __getattr__ in MetaTable will run. But in Python 3 it doesn't work.

class MetaTable(type):     def __getattr__(cls, key):         temp = key.split("__")         name = temp[0]         alias = None          if len(temp) > 1:             alias = temp[1]          return cls(name, alias)   class Table(object):     __metaclass__ = MetaTable      def __init__(self, name, alias=None):         self._name = name         self._alias = alias   d = Table d.student__s 

But in Python 3.5 I get an attribute error instead:

Traceback (most recent call last):   File "/Users/wyx/project/python3/sql/dd.py", line 31, in <module>     d.student__s AttributeError: type object 'Table' has no attribute 'student__s' 

Answer 1

Python 3 changed how you specify a metaclass, __metaclass__ is no longer checked.

Use metaclass=... in the class signature:

class Table(object, metaclass=MetaTable): 

Demo:

>>> class MetaTable(type): ...     def __getattr__(cls, key): ...         temp = key.split("__") ...         name = temp[0] ...         alias = None ...         if len(temp) > 1: ...             alias = temp[1] ...         return cls(name, alias) ... >>> class Table(object, metaclass=MetaTable): ...     def __init__(self, name, alias=None): ...         self._name = name ...         self._alias = alias ... >>> d = Table >>> d.student__s <__main__.Table object at 0x10d7b56a0> 

If you need to provide support for both Python 2 and 3 in your codebase, you can use the six.with_metaclass() baseclass generator or the @six.add_metaclass() class decorator to specify the metaclass.