python: detecting if my method is called with 0 or 1 args

Question

I want to write a suite of methods that act as getters when passed zero arguments and as setters when passed a single argument. I have two two reasonable implementations, shown below. Is one better than the other, or is there perhaps an even better way?

class Thing(object):

    def __init__(self):
        self._size = 0

    def sizeA(self, *args):
        if not args:
            return self._size
        self._size = args[0]
        return self

    UNIQUE_ID = []

    def sizeB(self, arg=UNIQUE_ID):
        if arg is self.UNIQUE_ID:
            return self._size
        self._size = arg
        return self

A couple of comments:

sizeA() benchmarks faster than sizeB(), but doesn't give me argument count checking. sizeB() works, but it depends on UNIQUE_ID being invariant, and I wonder if there are circumstances (e.g. pickling) that would cause it to break.

P.S.: Though I'm currently developing in python-2.7, I'm open to python-3.x specific answers as appropriate.

Answer 1

A sentinel and a defaulted arg is a good way to do it. I would use an arbitrary object though, not a list (preference, and this way someone won't go about fiddling with it).

class A(object):
    _DEFAULT = object()

    def __init__(self):
        self._size = 0

    def sizeC(self, arg=_DEFAULT):
        if arg is not DEFAULT:
            self._size = arg
        return self._size

And pickle will handle it fine (it also handles it fine with a list, change _DEAFULT to be [] and change it to print id(y._DEFAULT)).

a = A()
a.sizeC(10)

x = pickle.dumps(a)
y = pickle.loads(x)

print y._DEFAULT
print y.sizeC()
print y.sizeC(20)
print y.sizeC()
print y._DEFAULT
print a._DEFAULT

gives

andy@batman:~$ p tmp.py 
<object object at 0x7ff952223090>
10
20
20
<object object at 0x7ff952223090>
<object object at 0x7ff952223090>

I can't think of a situation where this would cause issue, and is an accepted pattern as far as I'm aware (I don't have any citations to hand.)

Out of curiosity, what is the difference in the benchmark between the two options?