Recursively convert python object graph to dictionary

Question

I'm trying to convert the data from a simple object graph into a dictionary. I don't need type information or methods and I don't need to be able to convert it back to an object again.

I found this question about creating a dictionary from an object's fields, but it doesn't do it recursively.

Being relatively new to python, I'm concerned that my solution may be ugly, or unpythonic, or broken in some obscure way, or just plain old NIH.

My first attempt appeared to work until I tried it with lists and dictionaries, and it seemed easier just to check if the object passed had an internal dictionary, and if not, to just treat it as a value (rather than doing all that isinstance checking). My previous attempts also didn't recurse into lists of objects:

def todict(obj):
    if hasattr(obj, "__iter__"):
        return [todict(v) for v in obj]
    elif hasattr(obj, "__dict__"):
        return dict([(key, todict(value)) 
            for key, value in obj.__dict__.iteritems() 
            if not callable(value) and not key.startswith('_')])
    else:
        return obj

This seems to work better and doesn't require exceptions, but again I'm still not sure if there are cases here I'm not aware of where it falls down.

Any suggestions would be much appreciated.

Answer 1

I don't know what is the purpose of checking for basestring or object is? also dict will not contain any callables unless you have attributes pointing to such callables, but in that case isn't that part of object?

so instead of checking for various types and values, let todict convert the object and if it raises the exception, user the orginal value.

todict will only raise exception if obj doesn't have dict e.g.

class A(object):
    def __init__(self):
        self.a1 = 1

class B(object):
    def __init__(self):
        self.b1 = 1
        self.b2 = 2
        self.o1 = A()

    def func1(self):
        pass

def todict(obj):
    data = {}
    for key, value in obj.__dict__.iteritems():
        try:
            data[key] = todict(value)
        except AttributeError:
            data[key] = value
    return data

b = B()
print todict(b)

it prints {'b1': 1, 'b2': 2, 'o1': {'a1': 1}} there may be some other cases to consider, but it may be a good start

special cases if a object uses slots then you will not be able to get dict e.g.

class A(object):
    __slots__ = ["a1"]
    def __init__(self):
        self.a1 = 1

fix for the slots cases can be to use dir() instead of directly using the dict

Answer 2

In Python there are many ways of making objects behave slightly differently, like metaclasses and whatnot, and it can override getattr and thereby have "magical" attributes you can't see through dict, etc. In short, it's unlikely that you are going to get a 100% complete picture in the generic case with whatever method you use.

Therefore, the answer is: If it works for you in the use case you have now, then the code is correct. ;-)

To make somewhat more generic code you could do something like this:

import types
def todict(obj):
    # Functions, methods and None have no further info of interest.
    if obj is None or isinstance(subobj, (types.FunctionType, types.MethodType))
        return obj

    try: # If it's an iterable, return all the contents
        return [todict(x) for x in iter(obj)]
    except TypeError:
        pass

    try: # If it's a dictionary, recurse over it:
        result = {}
        for key in obj:
            result[key] = todict(obj)
        return result
    except TypeError:
        pass

    # It's neither a list nor a dict, so it's a normal object.
    # Get everything from dir and __dict__. That should be most things we can get hold of.
    attrs = set(dir(obj))
    try:
        attrs.update(obj.__dict__.keys())
    except AttributeError:
        pass

    result = {}
    for attr in attrs:
        result[attr] = todict(getattr(obj, attr, None))
    return result

Something like that. That code is untested, though. This still doesn't cover the case when you override getattr, and I'm sure there are many more cases that it doens't cover and may not be coverable. :)

Answer 3

An amalgamation of my own attempt and clues derived from Anurag Uniyal and Lennart Regebro's answers works best for me:

def todict(obj, classkey=None):
    if isinstance(obj, dict):
        for k in obj.keys():
            obj[k] = todict(obj[k], classkey)
        return obj
    elif hasattr(obj, "__iter__"):
        return [todict(v, classkey) for v in obj]
    elif hasattr(obj, "__dict__"):
        data = dict([(key, todict(value, classkey)) 
            for key, value in obj.__dict__.iteritems() 
            if not callable(value) and not key.startswith('_')])
        if classkey is not None and hasattr(obj, "__class__"):
            data[classkey] = obj.__class__.__name__
        return data
    else:
        return obj