Helper for creating more robust immutable objects.

README.rst

The goal is to make a namedtuple subclass with helper functions (which obviously are read-only, because it's a namedtuple). Simply creating a namedtuple and subclassing it doesn't work, as the subclass gets __dict__ and other mutable things that ruin the __slots__ awesomeness of namedtuple. So instead, this uses ActuallyCalls (from actually.py, https://gist.github.com/dplepage/5095902) to create a new namedtuple class and add your functions to it.

Obvious caveats:

• If you subclass one of these classes, you'll have the same problem as with subclassing namedtuple directly
• Don't specify methods like __new__ that will conflict with namedtuple's members

example.py

class Point3(NamedTuple('x', 'y', 'z')):
    def radius(self):
        return (self.x**2 + self.y**2 + self.z**2)**.5

    def manhattan_radius(self):
        return self.x+self.y+self.z

p = Point3(1, 2, 3)
assert str(p) == 'Point3(x=1, y=2, z=3)'
assert p.manhattan_radius() == 6
# Point3's attributes can't be set:
try:
    p.x = 12
except AttributeError:
    pass
else:
    raise Exception("That shouldn't have worked.")

# And you can't add new ones:
try:
    p.w = 12
except AttributeError:
    pass
else:
    raise Exception("That shouldn't have worked.")

named_tuple.py

from collections import namedtuple

from actually import ActuallyCalls

def make_named_tuple(field_names, name='Struct', **kwargs):
    t = namedtuple(name, field_names)
    for k, val in kwargs.items():
        setattr(t, k, val)
    return t

def NamedTuple(*field_names):
    return ActuallyCalls(
        lambda __clsname, **kwargs:
            make_named_tuple(field_names, __clsname, **kwargs),
        namearg='__clsname')

nt_motivation.py

# In case you're curious, here's what's wrong with subclassing namedtuple directly:

from collections import namedtuple

class Point3(namedtuple('Point3', 'x y z')):
    def manhattan(self): return self.x+self.y+self.z

p = Point3(1,2,3)
# This works because subclassing gave you a __dict__, instead of keeping __slots__ around.
p.w = 12
assert p.w == 12


# To be fair, you can get the same behavior by passing in empty slots, without the magic of the above:
class Point3(namedtuple('Point3', 'x y z')):
    __slots__ = ()
    def manhattan(self): return self.x+self.y+self.z

p = Point3(1,2,3)

try:
    p.w = 12
except AttributeError:
    pass
else:
    raise Exception("That shouldn't have worked.")