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?

Hydro

getattr(myobj, "d")['a'] would do it but I guess that is not what you want. – Aggrandize

No, it is not what I want. I want the technique to be generic so that I can ask for an attribute of an object or subattribute/subobject/subitem of the object with a dynamically constructed string. I can do this with "eval", but I'd rather not. – Hydro

You're gonna have to create some complex function that parses the string, which will essentially be a slimmed down version of "eval". Why do you need this btw? Sounds like there should be a better solution... – Divaricate

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")

Tenterhook

Thanks for the comment. You understand the issue exactly, but I want the technique to be generic so that I can ask for an attribute of an object or subattribute/subobject/subitem of the object with a dynamically constructed string. I can do this with "eval", but I'd rather not. Any other ideas? – Hydro

Answer 2

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

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

That'll set val to 1.

Earthbound

Thanks, but this is not what I want. (Copied from above) I want the technique to be generic so that I can ask for an attribute of an object or subattribute/subobject/subitem of the object with a dynamically constructed string. I can do this with "eval", but I'd rather not. – Hydro

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')

Sidney

I would even go a step farther and do getattr(myobj, 'd', {}).get('a') – Handstand

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")

Hydro