[Tutor] Problem with calling class methods stored in a list

[Peter Otten]

Tobias M. wrote:

> Am 10.01.2013 13:48, schrieb Peter Otten:
>> If you adopt this approach you might omit the lookup dictionary and use
>> getattr():
>>
>> ...     @classmethod
>> ...     def handle_foo(cls): print "Hello from B.handle_foo()"
>> ...     @classmethod
>> ...     def get_handler(cls, packet_type):
>> ...             return getattr(cls, "handle_" + packet_type)
>> ...
> Well I think this is an elegant solution.
> 
> But due to my protocol a packet type is uniquely defined by a
> combination of two header fields (that contain integers). Building a
> verbose method name from these would complicate it again.

Technically, not much:

>>> class C(object):
...     @classmethod
...     def handle_1_42(self): print "Hi"
...     @classmethod
...     def get_handler(cls, packet_type):
...             return getattr(cls, "handle_%d_%d" % packet_type)
... 
>>> C.get_handler((1, 42))()
Hi

Of course handle_1_42() is not exactly the method name one would hope for. 
You could, again, strive for simplicity and add a lookup table that maps 
protocol tuples to function /names/ , but as simplicity doesn't seem to be 
your cup of tea:

class HandlersType(type):
    def __init__(cls, name, bases, classdict):
        cls.lookup_protocol = lookup = {}
        for k, v in classdict.items():
            if isinstance(v, protocol_method):
                lookup[v.protocol] = getattr(cls, k)

class protocol_method(classmethod):
    pass

def protocol(x, y):
    def attach_protocol(f):
        f = protocol_method(f)
        f.protocol = x, y
        return f
    return attach_protocol

class D:
    __metaclass__ = HandlersType

    @protocol(42, 17)
    def foo(cls):
        print "Hi"

D.lookup_protocol[42, 17]()

;)