code explanation

Ian Kelly ian.g.kelly at gmail.com
Tue Jan 15 06:39:57 CET 2013


On Mon, Jan 14, 2013 at 9:51 PM, Rodrick Brown <rodrick.brown at gmail.com> wrote:
> import sys
>
> PY3K = sys.version_info >= (3,)
>
>
> methods = set([
>     "__iter__",
>     "__len__",
>     "__contains__",
>
>     "__lt__",
>     "__le__",
>     "__eq__",
>     "__ne__",
>     "__gt__",
>     "__ge__",
>
>     "__add__",
>     "__and__",
>     "__divmod__",
>     "__floordiv__",
>     "__lshift__",
>     "__mod__",
>     "__mul__",
>     "__or__",
>     "__pow__",
>     "__rshift__",
>     "__sub__",
>     "__truediv__",
>     "__xor__",
> ])
> if PY3K:
>     methods.add("__next__")
>     methods.add("__bool__")
> else:
>     methods.add("__div__")
>     methods.add("__nonzero__")
> MAGIC_METHODS = frozenset(methods)
> del methods
>
> def _build_magic_dispatcher(method):
>     def inner(self, *args, **kwargs):
>         return self.__dict__[method](*args, **kwargs)
>     inner.__name__ = method
>     return inner
>
>
> class stub(object):
>     _classes_cache = {}
>
>     def __new__(cls, **kwargs):
>         magic_methods_present = MAGIC_METHODS.intersection(kwargs)
>         if magic_methods_present not in cls._classes_cache:
>             attrs = dict(
>                 (method, _build_magic_dispatcher(method))
>                 for method in magic_methods_present
>             )
>             attrs["__module__"] = cls.__module__
>             cls._classes_cache[magic_methods_present] = type("stub", (cls,),
> attrs)
>         new_cls = cls._classes_cache[magic_methods_present]
>         return super(stub, new_cls).__new__(new_cls, **kwargs)
>
>     def __init__(self, **kwargs):
>         self.__dict__.update(kwargs)


The stub class is called with keyword arguments where the keys are the
names of Python "magic methods" and the values are functions.  When
called, it builds a new subclass of itself populated with a
corresponding set of methods, each of which is built by
_build_magic_dispatcher; these look up the method with the same name
in the instance dictionary and delegate the call to whatever they
find.  For some reason that eludes me, the generated methods appear to
discard the "self" argument in the process.  After the subclass is
generated, it constructs and returns an instance of the subclass, and
then when the __init__ method is called it simply populates the
instance dictionary with the functions that were passed in.

The purpose of this appears to be to construct objects with magic
methods that are defined on the object itself rather than on the
class.  Normally, when Python calls a magic method it doesn't look in
the instance dictionary at all and only looks in the class dictionary.
 The subclasses of "stub" side-step that by having the desired magic
methods on the class delegate calls to the instance.



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