Shouldn't __metaclass__ force the use of a metaclass in Python?
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03-07-2019 - |
Question
I've been trying to learn about metaclasses in Python. I get the main idea, but I can't seem to activate the mechanism. As I understand it, you can specify M to be as the metaclass when constructing a class K by setting __metaclass__
to M at the global or class level. To test this out, I wrote the following program:
p = print
class M(type):
def __init__(*args):
type.__init__(*args)
print("The rain in Spain")
p(1)
class ClassMeta:
__metaclass__ = M
p(2)
__metaclass__ = M
class GlobalMeta: pass
p(3)
M('NotMeta2', (), {})
p(4)
However, when I run it, I get the following output:
C:\Documents and Settings\Daniel Wong\Desktop>python --version Python 3.0.1 C:\Documents and Settings\Daniel Wong\Desktop>python meta.py 1 2 3 The rain in Spain 4
Shouldn't I see "The rain in Spain" after 1 and 2? What's going on here?
Solution
In Python 3 (which you are using) metaclasses are specified by a keyword parameter in the class definition:
class ClassMeta(metaclass=M):
pass
Specifying a __metaclass__
class property or global variable is old syntax from Python 2.x and not longer supported. See also "What's new in Python 3" and PEP 2115.
OTHER TIPS
This works as you expect in Python 2.6 (and earlier), but in 3.0 metaclasses are specified differently:
class ArgMeta(metaclass=M): ...
The syntax of metaclasses has changed in Python 3.0. The __metaclass__
attribute is no longer special at either the class nor the module level. To do what you're trying to do, you need to specify metaclass
as a keyword argument to the class
statement:
p = print
class M(type):
def __init__(*args):
type.__init__(*args)
print("The rain in Spain")
p(1)
class ClassMeta(metaclass=M): pass
Yields:
1
The rain in Spain
As you'd expect.