Code examples for blog post: http://benno.id.au/blog/2014/11/30/a-better-namedtuple

namedtuple_blog.py

from timeit import timeit
from collections import namedtuple
from operator import itemgetter


RUNS = 1000000


Foo = namedtuple('Foo', 'bar baz')
foo = Foo(5, 10)
assert foo.bar == 5
assert foo.baz == 10

def foo_bar_times_baz(foo):
    return foo.bar * foo.baz

assert foo_bar_times_baz(foo) == 50


class Foo(namedtuple('Foo', 'bar baz')):
    def bar_times_baz(self):
        return self.bar * self.baz


foo = Foo(5, 10)
assert foo.bar_times_baz() == 50


Foo = namedtuple('Foo', 'bar baz')
Foo.bar_times_baz = lambda self: self.bar * self.baz
foo = Foo(5, 10)
assert foo.bar_times_baz() == 50

class NamedTuple(tuple):
    __slots__ = ()

    _fields = None  # Subclass must provide this

    def __new__(_cls, *args, **kwargs):
        if len(args) > len(_cls._fields):
            raise TypeError("__new__ takes {} positional arguments but {} were given".format(len(_cls._fields) + 1, len(args) + 1))

        missing_args = tuple(fld for fld in _cls._fields[len(args):] if fld not in kwargs)
        if len(missing_args):
            raise TypeError("__new__ missing {} required positional arguments".format(len(missing_args)))
        extra_args = tuple(kwargs.pop(fld) for fld in _cls._fields[len(args):] if fld in kwargs)
        if len(kwargs) > 0:
            raise TypeError("__new__ got an unexpected keyword argument '{}'".format(list(kwargs.keys())[0]))

        return tuple.__new__(_cls, tuple(args + extra_args))

    def _make(self, iterable, new=tuple.__new__, len=len):
        'Make a new Bar object from a sequence or iterable'
        cls = self.__class__
        result = new(cls, iterable)
        if len(result) != len(cls._fields):
            raise TypeError('Expected {} arguments, got {}'.format(len(self._fields), len(result)))
        return result

    def __repr__(self):
        'Return a nicely formatted representation string'
        fmt = '(' + ', '.join('%s=%%r' % x for x in self._fields) + ')'
        return self.__class__.__name__ + fmt % self

    def _asdict(self):
        'Return a new OrderedDict which maps field names to their values'
        return OrderedDict(zip(self._fields, self))

    __dict__ = property(_asdict)

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

    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

    def __getattr__(self, field):
        try:
            idx = self._fields.index(field)
        except ValueError:
            raise AttributeError("'{}' NamedTuple has no attribute '{}'".format(self.__class__.__name__, field))

        return self[idx]


class FooSubClass(NamedTuple):
    _fields = ('bar', 'baz')

    def bar_times_baz(self):
        return self.bar * self.baz

foo_sub_class = FooSubClass(5, 10)
assert foo_sub_class.bar_times_baz() == 50
assert foo_sub_class == (5, 10)

# This isn't great because we can add attributes
foo_sub_class.blah = 29



class FooSubClass(NamedTuple):
    __slots__ = ()
    _fields = ('bar', 'baz')

    def bar_times_baz(self):
        return self.bar * self.baz

foo_sub_class = FooSubClass(5, 10)
assert foo_sub_class.bar_times_baz() == 50
# Check we can't assign other attributes
try:
    foo_sub_class.blah = 29
    raise Exception("Should fail with attribute error.")
except AttributeError:
    pass
assert foo_sub_class == (5, 10)


direct_idx_time = timeit('foo[0]', setup='from __main__ import foo', number=RUNS)
direct_attr_time = timeit('foo.bar', setup='from __main__ import foo', number=RUNS)
sub_class_idx_time = timeit('foo[0]', setup='from __main__ import foo_sub_class as foo', number=RUNS)
sub_class_attr_time = timeit('foo.bar', setup='from __main__ import foo_sub_class as foo', number=RUNS)

print("namedtuple   (idx time) ", direct_idx_time)
print("namedtuple   (attr time)", direct_attr_time)
print("subclass     (idx time) ", sub_class_idx_time)
print("sublcass     (attr time)", sub_class_attr_time)


def optimize(cls):
    for idx, fld in enumerate(cls._fields):
        setattr(cls, fld, property(itemgetter(idx), doc='Alias for field number {}'.format(idx)))
    setattr(cls, "blah", 37)
    return cls

@optimize
class FooSubClassOptimized(NamedTuple):
    __slots__ = ()
    _fields = ('bar', 'baz')
    blahX = 42

    def bar_times_baz(self):
        return self.bar * self.baz


foo_sub_class_optimized = FooSubClassOptimized(5, 10)
assert foo_sub_class_optimized.bar_times_baz() == 50
try:
    foo_sub_class.blah = 29
    raise Exception("Should fail with attribute error.")
except AttributeError:
    pass
assert foo_sub_class_optimized == (5, 10)

sub_class_optimized_idx_time = timeit('foo[0]', setup='from __main__ import foo_sub_class_optimized as foo', number=RUNS)
sub_class_optimized_attr_time = timeit('foo.bar', setup='from __main__ import foo_sub_class_optimized as foo', number=RUNS)

print("subclass-opt (idx time) ", sub_class_optimized_idx_time)
print("subclass-opt (attr time)", sub_class_optimized_attr_time)


def namedfields(*fields):
    def inner(cls):
        if not issubclass(cls, tuple):
            raise TypeError("namefields decorated classes must be subclass of tuple")

        attrs = {
            '__slots__': (),
        }

        methods = ['__new__', '_make', '__repr__', '_asdict',
                   '__dict__', '_replace', '__getnewargs__',
                   '__getstate__']

        attrs.update({attr: getattr(NamedTuple, attr) for attr in methods})

        attrs['_fields'] = fields

        attrs.update({fld: property(itemgetter(idx), doc='Alias for field number {}'.format(idx))
                      for idx, fld in enumerate(fields)})

        attrs.update({key: val for key, val in cls.__dict__.items()
                      if key not in ('__weakref__', '__dict__')})

        return type(cls.__name__, cls.__bases__, attrs)

    return inner

@namedfields('bar', 'baz')
class FooDecorate(tuple):
    def bar_times_baz(self):
        return self.bar * self.baz


foo_decorate = FooDecorate(5, 10)
assert foo_decorate.bar_times_baz() == 50
decorate_idx_time = timeit('foo[0]', setup='from __main__ import foo_decorate as foo', number=RUNS)
decorate_attr_time = timeit('foo.bar', setup='from __main__ import foo_decorate as foo', number=RUNS)

print("decorator    (idx time) ", decorate_idx_time)
print("decorator    (attr time)", decorate_attr_time)


Foo = namedfields('bar', 'baz')(type('Foo', (tuple, ), {}))
foo = Foo(5, 10)
assert foo == (5, 10)
print(timeit('foo.bar', setup='from __main__ import foo', number=RUNS))