Is there a library to follow a sequence of indexing and `getattr` operations?

Question

I have a bunch of objects, they have members, their members have members, ..., somewhere I need to do indexing, and then access members ...

So, basically, I want to get obj.member1.member2[3].member4 and I also want to assign obj.member1[2].member3.member4 = new_value. I want to generate these "paths" which describe when to use getattr and when to use indexing dynamically. Is there a library for this? I imagine interface like

get_obj_path(obj, (("member1", "a"), ("member2", "a"), (3, "i"), ("member4", "a")))

and

assign_obj_path(obj, (("member1", "a"), (2, "i"), ("member3", "a"), ("member4", "a")), new_value)

Reinventing [`jq`](https://stedolan.github.io/jq/)? – wim Jul 05 '19 at 12:31 — wim, Jul 05 '19 at 12:31
It should not be hard to implement this. – tobias_k Jul 05 '19 at 12:31 — tobias_k, Jul 05 '19 at 12:31
@CrabMan ok, post some sample `obj` – RomanPerekhrest Jul 05 '19 at 12:38 — RomanPerekhrest, Jul 05 '19 at 12:38

jdehesa · Accepted Answer · 2019-07-05T12:39:40.043

1

You can implement those functions yourself:

def get_obj_path(obj, path):
    if not path:
        return obj
    (key, kind), *path = path
    if kind == "a":
        obj = getattr(obj, key)
    elif kind == "i":
        obj = obj[key]
    else:
        raise ValueError(f"invalid kind '{kind}'.")
    return get_obj_path(obj, path)

def assign_obj_path(obj, path, value):
    (key, kind), *path = path
    if not path:
        if kind == "a":
            setattr(obj, key, value)
        elif kind == "i":
            obj[key] = value
        else:
            raise ValueError(f"invalid kind '{kind}'.")
    else:
        if kind == "a":
            obj = getattr(obj, key)
        elif kind == "i":
            obj = obj[key]
        else:
            raise ValueError(f"invalid kind '{kind}'.")
        assign_obj_path(obj, path, value)

# Test
class MyClass: pass
obj1 = MyClass()
obj1.i = 1
obj2 = MyClass()
obj2.lst = [obj1]
assign_obj_path(obj2, (("lst", "a"), (0, "i"), ("i", "a")), 2)
print(get_obj_path(obj2, (("lst", "a"), (0, "i"), ("i", "a"))))
# 2

edited Jul 05 '19 at 12:39

answered Jul 05 '19 at 12:35

jdehesa

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this is just a flood – user8426627 Jul 05 '19 at 12:39
@user8426627 Not sure what you mean. Is that something good or bad? – jdehesa Jul 05 '19 at 12:40
i am doing the same with 4 lines, you have a flood over >20 lines with unessesary parameter "a" and "i". Also you should not produce your own exception without nessesarity – user8426627 Jul 05 '19 at 12:41
1

@user8426627 Because that is what the OP asked for. There is no "own exception", it is an standard exception, and it makes sense to use it when given parameters are wrong. – jdehesa Jul 05 '19 at 12:42
you could at least map function to "a" and "i" – user8426627 Jul 05 '19 at 12:43
@user8426627 Not sure what you mean but feel free to implement your own approach in your answer. – jdehesa Jul 05 '19 at 12:44
Thanks. I have modified your solution to my taste a little bit but basically this is what I am gonna use. – CrabMan Jul 05 '19 at 13:08

Is there a library to follow a sequence of indexing and `getattr` operations?

1 Answers1