How named tuples are implemented internally in pyt

问题:

Named tuples are easy to create, lightweight object types. namedtuple instances can be referenced using object-like variable deferencing or the standard tuple syntax. If these data structures can be accessed both by object deferencing & indexes, how are they implemented internally? Is it via hash tables?

回答1:

Actually, it's very easy to find out how a given namedtuple is implemented: if you pass the keyword argument verbose=True when creating it, its class definition is printed:

>>> Point = namedtuple('Point', "x y", verbose=True)
from builtins import property as _property, tuple as _tuple
from operator import itemgetter as _itemgetter
from collections import OrderedDict

class Point(tuple):
    'Point(x, y)'

    __slots__ = ()

    _fields = ('x', 'y')

    def __new__(_cls, x, y):
        'Create new instance of Point(x, y)'
        return _tuple.__new__(_cls, (x, y))

    @classmethod
    def _make(cls, iterable, new=tuple.__new__, len=len):
        'Make a new Point object from a sequence or iterable'
        result = new(cls, iterable)
        if len(result) != 2:
            raise TypeError('Expected 2 arguments, got %d' % len(result))
        return result

    def _replace(_self, **kwds):
        'Return a new Point object replacing specified fields with new values'
        result = _self._make(map(kwds.pop, ('x', 'y'), _self))
        if kwds:
            raise ValueError('Got unexpected field names: %r' % list(kwds))
        return result

    def __repr__(self):
        'Return a nicely formatted representation string'
        return self.__class__.__name__ + '(x=%r, y=%r)' % self

    @property
    def __dict__(self):
        'A new OrderedDict mapping field names to their values'
        return OrderedDict(zip(self._fields, self))

    def _asdict(self):
        '''Return a new OrderedDict which maps field names to their values.
           This method is obsolete.  Use vars(nt) or nt.__dict__ instead.
        '''
        return self.__dict__

    def __getnewargs__(self):
        'Return self as a plain tuple.  Used by copy and pickle.'
        return tuple(self)

    def __getstate__(self):
        'Exclude the OrderedDict from pickling'
        return None

    x = _property(_itemgetter(0), doc='Alias for field number 0')

    y = _property(_itemgetter(1), doc='Alias for field number 1')

So, it's a subclass of tuple with some extra methods to give it the required behaviour, a _fields class-level constant containing the field names, and property methods for attribute access to the tuple's members.

As for the code behind actually building this class definition, that's deep magic.