Establishing why an object can't be pickled

Question

I\'m receiving an object, t, from an api of type Object. I am unable to pickle it, getting the error:

  File \"p.py\", line 55, in          


        

Answer 1

Here's an extension of Alastair's solution, in Python 3.

It:

• is recursive, to deal with complex objects where the problem might be many layers deep.

  The output is in the form .x[i].y.z.... to allow you to see which members were called to get to the problem. With dict it just prints [key/val type=...] instead, since either keys or values can be the problem, making it harder (but not impossible) to reference a specific key or value in the dict.

• accounts for more types, specifically list, tuple and dict, which need to be handled separately, since they don't have __dict__ attributes.

• returns all problems, rather than just the first one.

def get_unpicklable(instance, exception=None, string='', first_only=True):
    """
    Recursively go through all attributes of instance and return a list of whatever
    can't be pickled.

    Set first_only to only print the first problematic element in a list, tuple or
    dict (otherwise there could be lots of duplication).
    """
    problems = []
    if isinstance(instance, tuple) or isinstance(instance, list):
        for k, v in enumerate(instance):
            try:
                pickle.dumps(v)
            except BaseException as e:
                problems.extend(get_unpicklable(v, e, string + f'[{k}]'))
                if first_only:
                    break
    elif isinstance(instance, dict):
        for k in instance:
            try:
                pickle.dumps(k)
            except BaseException as e:
                problems.extend(get_unpicklable(
                    k, e, string + f'[key type={type(k).__name__}]'
                ))
                if first_only:
                    break
        for v in instance.values():
            try:
                pickle.dumps(v)
            except BaseException as e:
                problems.extend(get_unpicklable(
                    v, e, string + f'[val type={type(v).__name__}]'
                ))
                if first_only:
                    break
    else:
        for k, v in instance.__dict__.items():
            try:
                pickle.dumps(v)
            except BaseException as e:
                problems.extend(get_unpicklable(v, e, string + '.' + k))

    # if we get here, it means pickling instance caused an exception (string is not
    # empty), yet no member was a problem (problems is empty), thus instance itself
    # is the problem.
    if string != '' and not problems:
        problems.append(
            string + f" (Type '{type(instance).__name__}' caused: {exception})"
        )

    return problems