On 6/10/2011 11:34 PM, Steven D'Aprano wrote:
I have a metaclass in Python 3.1:
class MC1(type):
@staticmethod
def get_mro(bases):
print('get_mro called')
return type('K', bases, {}).__mro__[1:]
The call to type figures out the proper metaclass from bases and
forwards the call to that (or to its __new__ method). See
Objects/typeobject.c in the source, or read the docs on metaclasses
carefully. If the proper metaclass is MC1, ...
def __new__(cls, name, bases, dict):
mro = None
docstring = dict.get('__doc__')
if docstring == 'ham':
mro = cls.get_mro(bases)
and you unconditionally call get_mro again, to call this again...
dict['__doc__'] = "spam spam spam"
# Create the class we want, and return it.
K = super().__new__(cls, name, bases, dict)
if mro:
assert K.__mro__ == (K,) + mro
return K
you are in an endless loop.
Since uou do not pass dict to get_mro. it passes {} to type and MC1 and
the test for docstring fails and the loop is broken and the empty class
is discarded after getting its mro.
--
Terry Jan Reedy
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