Error when an item in pykov.Chain is None

[dvircohen]

Hey,
first of all this is my first time using github and also i'm very new to python so be gentle with me..
I have a pykov.Chain where one of the items is None and when i'm trying to use the function chain.move(None) i get the following error:
AttributeError: 'OrderedDict' object has no attribute 'choose'
to fix it i did the following changes in the succ function (i only use positive numbers in my project):
from:

def succ(self, key=None):
        try:
            if key is not None:
                return self._succ[key]
            else:
                return self._succ
        except AttributeError:
            self._succ = OrderedDict([(state, Vector()) for state in self.states()])
            for link, probability in six.iteritems(self):
                self._succ[link[0]][link[1]] = probability
            if key is not None:
                return self._succ[key]
            else:
                return self._succ

to:

def succ(self, key=-1): 
        try:
            if key is not -1:
                return self._succ[key]
            else:
                return self._succ
        except AttributeError:
            self._succ = OrderedDict([(state, Vector()) for state in self.states()])
            for link, probability in six.iteritems(self):
                self._succ[link[0]][link[1]] = probability
            if key is not -1:
                return self._succ[key]
            else:
                return self._succ

My solution works for me, but is there a better way?

[riccardoscalco]

Hi Dvir, your solution works for you, but it is not suitable for a generic use case.
Think about this case for example:

T = pykov.Chain({(1,-1): .3, (1,1): .7, (-1,1): 1.})

My suggestion is to replace None with a defined value, maybe 'None'. What do you think?