Dynamically get dict elements via getattr?

Question

I want to dynamically query which objects from a class I would like to retrieve. getattr seems like what I want, and it performs fine for top-level objects in the class. However, I'd like to also specify sub-elements.

class MyObj(object):
    def __init__(self):
        self.d = {'a':1, 'b':2}
        self.c = 3

myobj = MyObj()
val = getattr(myobj, "c")
print val # Correctly prints 3
val = getattr(myobj, "d['a']") # Seemingly incorrectly formatted query
print val # Throws an AttributeError

How can I get the object's dictionary elements via a string?

Answer 1

The reason you're getting an error is that getattr(myobj, "d['a']") looks for an attribute named d['a'] on the object, and there isn't one. Your attribute is named d and it's a dictionary. Once you have a reference to the dictionary, then you can access items in it.

mydict = getattr(myobj, "d")
val    = mydict["a"]

Or as others have shown, you can combine this in one step (I showed it as two to better illustrate what is actually happening):

val = getattr(myobj, "d")["a"]

Your question implies that you think that items of a dictionary in an object are "sub-elements" of the object. An item in a dictionary, however, is a different thing from an attribute of an object. (getattr() wouldn't work with something like o.a either, though; it just gets one attribute of one object. If that's an object too and you want to get one of its attributes, that's another getattr().)

You can pretty easily write a function that walks an attribute path (given in a string) and attempts to resolve each name either as a dictionary key or an attribute:

def resolve(obj, attrspec):
    for attr in attrspec.split("."):
        try:
            obj = obj[attr]
        except (TypeError, KeyError):
            obj = getattr(obj, attr)
    return obj

The basic idea here is that you take a path and for each component of the path, try to find either an item in a dictionary-like container or an attribute on an object. When you get to the end of the path, return what you've got. Your example would be resolve(myobj, "d.a")

Answer 2

You simply use square brackets to get the dictionary's element:

val = getattr(myobj, "d")["a"]

That'll set val to 1.

Answer 3

If you need the dictionary item to be dynamic as well, you'll need to call get on the result of getattr:

value = getattr(myobj, 'd').get('a')

Answer 4

Thanks to Kindall's answer, I found the following works well for dict keys that are stings.

class Obj2(object):
    def __init__(self):
        self.d = {'a':'A', 'b':'B', 'c': {'three': 3, 'twothree': (2,3)}}
        self.c = 4

class MyObj(object):
    def __init__(self):
        self.d = {'a':1, 'b':2, 'c': {'two': 2, 'onetwo': (1,2)}}
        self.c = 3
        self.obj2 = Obj2()

    def resolve(self, obj, attrspec):
        attrssplit = attrspec.split(".")
        attr = attrssplit[0]
        try:
            obj = obj[attr]
        except (TypeError, KeyError):
            obj = getattr(obj, attr)
        if len(attrssplit) > 1:
            attrspec = attrspec.partition(".")[2] # right part of the string.
            return self.resolve(obj, attrspec) # Recurse
        return obj

    def __getattr__(self, name):
        return self.resolve(self, name)

# Test  
myobj = MyObj()
print getattr(myobj, "c")
print getattr(myobj, "d.a")
print getattr(myobj, "d.c.two")
print getattr(myobj, "obj2.d.a")
print getattr(myobj, "obj2.d.c.twothree")