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?

`getattr(myobj, "d")['a']` would do it but I guess that is not what you want. — Felix Kling, Feb 01 '12 at 18:21
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. — user394430, Feb 02 '12 at 02:09
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... — Luiz C., Feb 02 '12 at 20:13

kindall · Answer 1 · 2012-02-02T22:45:14.540

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")

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? — user394430, Feb 02 '12 at 02:11

score 3 · Answer 2 · answered Feb 01 '12 at 18:21

3

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

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

That'll set val to 1.

answered Feb 01 '12 at 18:21

NPE

486,780
108
951
1,012

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. – user394430 Feb 02 '12 at 02:09

score 2 · Answer 3 · answered Feb 01 '12 at 18:53

2

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')

answered Feb 01 '12 at 18:53

Daniel Roseman

588,541
66
880
895

I would even go a step farther and do `getattr(myobj, 'd', {}).get('a')` – jdi Feb 02 '12 at 00:00

score 1 · Answer 4 · answered Feb 05 '12 at 18:59

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")

Dynamically get dict elements via getattr?

4 Answers4

Linked