Why doesn't the namedtuple module use a metacl

问题:

I spent some time investigating the collections.namedtuple module a few weeks ago. The module uses a factory function which populates the dynamic data (the name of the new namedtuple class, and the class attribute names) into a very large string. Then exec is executed with the string (which represents the code) as the argument, and the new class is returned.

Does anyone know why it was done this way, when there is a specific tool for this kind of thing readily available, i.e. the metaclass? I haven't tried to do it myself, but it seems like everything that is happening in the namedtuple module could have been easily accomplished using a namedtuple metaclass, like so:

class namedtuple(type):

etc etc.

回答1:

There are some hints in the issue 3974. The author proposed a new way to create named tuples, which was rejected with the following comments:

It seems the benefit of the original version is that it's faster, thanks to hardcoding critical methods. - Antoine Pitrou

There is nothing unholy about using exec. Earlier versions used other approaches and they proved unnecessarily complex and had unexpected problems. It is a key feature for named tuples that they are exactly equivalent to a hand-written class. - Raymond Hettinger

Additionally, here is the part of the description of the original namedtuple recipe:

... the recipe has evolved to its current exec-style where we get all of Python's high-speed builtin argument checking for free. The new style of building and exec-ing a template made both the __new__ and __repr__ functions faster and cleaner than in previous versions of this recipe.

If you're looking for some alternative implementations:

• abstract base class + mix-in for named tuples recipe by Jan Kaliszewski

• metaclass-based implementation by Aaron Iles (see his blog post)

回答2:

As a sidenote: The other objection I see most often against using exec is that some locations (read companies) disable it for security reasons.

Besides an advanced Enum and NamedConstant, the aenum library* also has NamedTuple which is metaclass-based.

---

* aenum is written by the author of enum and the enum34 backport.

回答3:

Here is another approach.

""" Subclass of tuple with named fields """
from operator import itemgetter
from inspect import signature

class MetaTuple(type):
    """ metaclass for NamedTuple """

    def __new__(mcs, name, bases, namespace):
        cls = type.__new__(mcs, name, bases, namespace)
        names = signature(cls._signature).parameters.keys()
        for i, key in enumerate(names):
            setattr(cls, key, property(itemgetter(i)))
        return cls

class NamedTuple(tuple, metaclass=MetaTuple):
    """ Subclass of tuple with named fields """

    @staticmethod
    def _signature():
        " Override in subclass "

    def __new__(cls, *args):
        new = super().__new__(cls, *args)
        if len(new) == len(signature(cls._signature).parameters):
            return new
        return new._signature(*new)

if __name__ == '__main__':
    class Point(NamedTuple):
        " Simple test "
        @staticmethod
        def _signature(x, y, z): # pylint: disable=arguments-differ
            " Three coordinates "
    print(Point((1, 2, 4)))

If this approach has any virtue at all, it's the simplicity. It would be simpler yet without NamedTuple.__new__, which serves only the purpose of enforcing the element count. Without that, it happily allows additional anonymous elements past the named ones, and the primary effect of omitting elements is the IndexError on omitted elements when accessing them by name (with a little work that could be translated to an AttributeError). The error message for an incorrect element count is a bit strange, but it gets the point across. I wouldn't expect this to work with Python 2.

There is room for further complication, such as a __repr__ method. I have no idea how the performance compares to other implementations (caching the signature length might help), but I much prefer the calling convention as compared to the native namedtuple implementation.