Python properties as instance attributes

Question

I am trying to write a class with dynamic properties. Consider the following class with two read-only properties:

class Monster(object):
    def __init__(self,color,has_fur):
        self._color = color
        self._has_fur = has_fur

    @property
    def color(self): return self._color

    @property
    def has_fur(self): return self._has_fur

I want to generalize this so that __init__ can take an arbitrary dictionary and create read-only properties from each item in the dictionary. I could do that like this:

class Monster2(object):
    def __init__(self,traits):
        self._traits = traits

        for key,value in traits.iteritems():
            setattr(self.__class__,key,property(lambda self,key=key: self._traits[key]))

However, this has a serious drawback: every time I create a new instance of Monster, I am actually modifying the Monster class. Instead of creating properties for my new Monster instance, I am effectively adding properties to all instances of Monster. To see this:

>>> hasattr(Monster2,"height")
False
>>> hasattr(Monster2,"has_claws")
False
>>> blue_monster = Monster2({"height":4.3,"color":"blue"})
>>> hasattr(Monster2,"height")
True
>>> hasattr(Monster2,"has_claws")
False
>>> red_monster = Monster2({"color":"red","has_claws":True})
>>> hasattr(Monster2,"height")
True
>>> hasattr(Monster2,"has_claws")
True

This of course makes sense, since I explicitly added the properties as class attributes with setattr(self.__class__,key,property(lambda self,key=key: self._traits[key])). What I need here instead are properties that can be added to the instance. (i.e. "instance properties"). Unfortunately, according to everything I have read and tried, properties are always class attributes, not instance attributes. For example, this doesn't work:

class Monster3(object):
    def __init__(self,traits):
        self._traits = traits

        for key,value in traits.iteritems():
            self.__dict__[key] = property(lambda self,key=key: self._traits[key])

>>> green_monster = Monster3({"color":"green"})
>>> green_monster.color
<property object at 0x028FDAB0>

So my question is this: do "instance properties" exist? If not, what is the reason? I have been able to find lots about how properties are used in Python, but precious little about how they are implemented. If "instance properties" don't make sense, I would like to understand why.

Answer 1

No, there is no such thing as per-instance properties; like all descriptors, properties are always looked up on the class. See the descriptor HOWTO for exactly how that works.

You can implement dynamic attributes using a __getattr__ hook instead, which can check for instance attributes dynamically:

class Monster(object):
    def __init__(self, traits):
        self._traits = traits

    def __getattr__(self, name):
        if name in self._traits:
            return self._traits[name]
        raise AttributeError(name)

These attributes are not really dynamic though; you could just set these directly on the instance:

class Monster(object):
    def __init__(self, traits):
        self.__dict__.update(traits)