I have some code which can preprocess (using a metaclass) or postprocess
(using a decorator) a class:
@process
class K:
pass
class K(metaclass=process):
pass
Both should give the same result, but I have found that the results are
slightly different because the dict passed to process as a metaclass is
different from the the class dict seen by the decorator. I can demonstrate
the difference in Python 3.6 by using this metaclass/decorator:
class process(type):
def __new__(meta, *args):
if len(args) == 1:
# decorator usage
cls = args[0]
d = cls.__dict__
elif len(args) == 3:
# metaclass usage
name, bases, d = args
cls = super().__new__(meta, *args)
print(sorted(d.keys()))
return cls
If I then try it against two identical (apart from their names) classes, I
get these results:
py> @process
... class K:
... x = 1
...
['__dict__', '__doc__', '__module__', '__weakref__', 'x']
py> class Q(metaclass=process):
... x = 1
...
['__module__', '__qualname__', 'x']
Now if I check the newly created Q, I see the same keys K has:
py> sorted(Q.__dict__.keys())
['__dict__', '__doc__', '__module__', '__weakref__', 'x']
Is there any documentation for exactly what keys are added to classes when?
--
Steven
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.
--
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