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'

Asked By: wyx

||

Source

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.

Answered By: Martijn Pieters

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