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'

Mike Frysinger · Accepted Answer · 2019-07-03 22:36:46Z

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.

Thanks for this. It got my favourite plugin system working in python 3.
It would be possible to leave the existing __metaclass__ syntax in place. python.org/dev/peps/pep-3115

Collectives™ on Stack Overflow

metaclass in Python 3

1 Answer 1

3 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Linked

Related