How to add __iter__ to dynamic type?

Question

Source

def flags(*opts):
    keys = [t[0] for t in opts]
    words = [t[1] for t in opts]
    nums = [2**i for i in range(len(opts))]
    attrs = dict(zip(keys,nums))
    choices = iter(zip(nums,words))
    return type('Flags', (), dict(attrs))

Abilities = flags(
    ('FLY', 'Can fly'),
    ('FIREBALL', 'Can shoot fireballs'),
    ('INVISIBLE', 'Can turn invisible'),
)

Question

How can I add an __iter__ method to Abilities so that I can iterate over choices?

Why?

This way I can use things like

hero.abilities = Abilities.FLY | Abilities.FIREBALL

if hero.abilities & Abilities.FIREBALL:

for k, v in Abilities:
    print k, v

in my code without having to use any magic numbers or strings, and I can also save the set of flags to the DB as a single int, or display the list in a readable format.

Other improvements are welcome.

Answer 1

There's no need to use a dynamic type here; I'd restructure this as a simple class, for example:

class flags(object):
    def __init__(self, *opts):
        keys = [t[0] for t in opts]
        words = [t[1] for t in opts]
        nums = [2**i for i in range(len(opts))]
        self.attrs = dict(zip(keys,nums))
        self.choices = zip(nums,words)

    def __getattr__(self, a):
        return self.attrs[a]

    def __iter__(self):
        return iter(self.choices)

Abilities = flags(
    ('FLY', 'Can fly'),
    ('FIREBALL', 'Can shoot fireballs'),
    ('INVISIBLE', 'Can turn invisible'),
)

print Abilities.FLY
for k, v in Abilities:
    print k, v

Answer 2

Why are you doing it the hard way? If you want a dict with __getattr__ overriding why not start with one:

class Flags(dict):
    def __init__(self, *args):
        dict.__init__(self, args)
    def __getattr__(self, name):
         return self[name]
...

This also has the Advantage of Least Surprise, since dict.__iter__() generates keys and dict.iteritems() yields tuples.

Answer 3

It would be a more Pythonic solution if you implement your own __getattr__ method for accessing dynamic fields instead of dealing with metaclasses through type.

Edit: It's not clear to me what do you mean by choices, but here is an example:

class Abilities(object):
    def __init__(self, abilities):
        self.abilities = abilities

    def __getattr__(self, name):
        a = [x for x in self.abilities if x[0] == name]
        if len(a) != 1:
            raise AttributeError('attribute {0} not found'.format(name))
        title, id, help = a[0]
        return id

    def __iter__(self):
        return (id, help for title, id, help in self.abilities)

SPEC = [
    ('FLY', 10, 'Can fly'),
    ('FIREBALL', 13, 'Can shoot fireballs'),
    ('INVISIBLE', 14, 'Can turn invisible'),
]

abitilies = Abilities(SPEC)

hero.abilities = abilities.FLY | abilities.FIREBALL
for k, v in abilities:
    print k, v

Answer 4

You need two key changes -- the last lines of flags should be:

choices = iter(zip(nums,words))
attrs['__iter__'] = lambda self: choices
return type('Flags', (), dict(attrs))()

Note that I've added a line setting __iter__, and a trailing () in the return to instantiate the type (to loop on a type you'd have to use a custom metaclass -- way overkill, no need).

The last line of flags should actually be:

return type('Flags', (), attrs)()

as there's no reason to make a copy of attrs, which is already a dict (but that's an innocuous redundancy, not a killer mistake;-).

Answer 5

Based on your guys' suggestions, I came up with this:

Source

class enumerable(object):
    def __init__(self, func, *opts):
        keys = func(len(opts))
        self.attrs = dict(zip([t[0] for t in opts], keys))
        self.opts = zip(keys, [t[1] for t in opts])
    def __getattr__(self, a):
        return self.attrs[a] 
    def __len__(self):
        return len(self.opts)
    def __iter__(self):
        return iter(self.opts)
    def __deepcopy__(self, memo):
        return self

class enum(enumerable):
    def __init__(self, *opts):
        return super(enum, self).__init__(range, *opts)

class flags(enumerable):
    def __init__(self, *opts):
        return super(flags, self).__init__(lambda l: [1<<i for i in range(l)], *opts)

### tests:

Abilities = enum(
    ('FLY', 'Can fly'),
    ('FIREBALL', 'Can shoot fireballs'),
    ('INVISIBLE', 'Can turn invisible'),
    ('FROST_NOVA', 'Can call down an ice storm'),
    ('BLINK', 'Can teleport short distances'),
)

print 'Fireball = %d' % Abilities.FIREBALL
print 'Number of options = %d' % len(Abilities) 
for k, v in Abilities:
    print '%d: %s' % (k, v)

Output

Fireball = 1
Number of options = 5
0: Can fly
1: Can shoot fireballs
2: Can turn invisible
3: Can call down an ice storm
4: Can teleport short distances

For whatever reason, my particular application needs __deepcopy__ to be implemented. Since these classes are for building "constants", none of their attributes should ever be changed after creation; thus I hope it's safe just to return self.