Why does Python use the same object for property X in all instances of a class when the default for property X is an empty dict?

Question

I know there are a few questions out there touching on similar issues but what I want to understand is WHY the following behavior is observed: when I use an empty dictionary {} as the default value for a property, THE SAME EMPTY DICTIONARY is set to be the default for that property for ALL the objects that I create. Which of course means that if I subsequently add to that dictionary in Object X, the dictionary will also change in Object Y. I find this annoying and I wonder why Python doesn't initialize separate empty dictionaries when I'm initializing separate objects. Here's a more concrete example:

class Insect(object):
    _description={}

    @property
    def description(self):
        return self._description
    @description.setter
    def description(self,value):
        self._description=value

    def __init__(self,description={}):
        self.description=description

def createsweirdness():
    x=Insect()
    y=Insect()
    x.description["color"]="blue"
    #I originally thought that changing x should not affect y
    print "y color is "+y.description["color"]
    #BUT IT DOES AFFECT Y!!! if you run this you see that "y color is blue"

class InsectNoError(object):
    """The only difference between Insect and InsectNoError is that in
    InsectNoError __init__(), description does NOT have a default value of {}"""
    _description={}

    @property
    def description(self):
        return self._description
    @description.setter
    def description(self,value):
        self._description=value

    def __init__(self,description):
        self.description=description

def noweirdnessanymore():
    x=InsectNoError({}) #have to provide {} now, since no default in __init__
    y=InsectNoError({})
    x.description["color"]="blue"
    print "y color is "+y.description["color"]
    #again I assume that changing x should not affect y, in which case calling this function
    #will lead to a KeyError...and it does

I just can't understand why you would want all your instances of a class to share exactly the same underlying dictionary for some property, BY DEFAULT...why doesn't Python give each new Insect object a different empty dictionary for the "description" property? Is this behavior a deliberate design decision, or is it a bug in Python?