Cross-posting (but then adapting further) from https://stackoverflow.com/a/62186053/4355695 : In this repo: https://github.com/ScriptSmith/socialreaper/blob/master/socialreaper/tools.py#L8 , I found an implementation of the list-inclusion comment by @roneo to the answer posted by @Imran.

I've added checks to it for catching empty lists and empty dicts. And also added print lines that will help one understand precisely how this function works. You can turn on those print statements by passing crumbs=True in the function's args.

from collections.abc import MutableMapping
def flatten(dictionary, parent_key=False, separator='.', crumbs=False):
    """
    Turn a nested dictionary into a flattened dictionary
    :param dictionary: The dictionary to flatten
    :param parent_key: The string to prepend to dictionary's keys
    :param separator: The string used to separate flattened keys
    :return: A flattened dictionary
    """

    items = []
    for key, value in dictionary.items():
        if crumbs: print('checking:',key)
        new_key = str(parent_key) + separator + key if parent_key else key
        if isinstance(value, MutableMapping):
            if crumbs: print(new_key,': dict found')
            if not value.items():
                if crumbs: print('Adding key-value pair:',new_key,None)
                items.append((new_key,None))
            else:
                items.extend(flatten(value, new_key, separator).items())
        elif isinstance(value, list):
            if crumbs: print(new_key,': list found')
            if len(value):
                for k, v in enumerate(value):
                    items.extend(flatten({str(k): v}, new_key, separator).items())
            else:
                if crumbs: print('Adding key-value pair:',new_key,None)
                items.append((new_key,None))
        else:
            if crumbs: print('Adding key-value pair:',new_key,value)
            items.append((new_key, value))
    return dict(items)

Test it:

ans = flatten({'a': 1, 'c': {'a': 2, 'b': {'x': 5, 'y' : 10}}, 'd': [1, 2, 3], 'e':{'f':[], 'g':{}} })
print('\nflattened:',ans)

Output:

checking: a
Adding key-value pair: a 1
checking: c
c : dict found
checking: a
Adding key-value pair: c.a 2
checking: b
c.b : dict found
checking: x
Adding key-value pair: c.b.x 5
checking: y
Adding key-value pair: c.b.y 10
checking: d
d : list found
checking: 0
Adding key-value pair: d.0 1
checking: 1
Adding key-value pair: d.1 2
checking: 2
Adding key-value pair: d.2 3
checking: e
e : dict found
checking: f
e.f : list found
Adding key-value pair: e.f None
checking: g
e.g : dict found
Adding key-value pair: e.g None

flattened: {'a': 1, 'c.a': 2, 'c.b.x': 5, 'c.b.y': 10, 'd.0': 1, 'd.1': 2, 'd.2': 3, 'e.f': None, 'e.g': None}

Annd that does the job I need done: I throw any complicated json at this and it flattens it out for me. I added a check to the original code to handle empty lists too

Credits to https://github.com/ScriptSmith whose repo I found the intial flatten function in.

Testing OP's sample json, here's the output:

{'count': 13,
 'virtualmachine.0.id': '1082e2ed-ff66-40b1-a41b-26061afd4a0b',
 'virtualmachine.0.name': 'test-2',
 'virtualmachine.0.displayname': 'test-2',
 'virtualmachine.0.securitygroup.0.id': '9e649fbc-3e64-4395-9629-5e1215b34e58',
 'virtualmachine.0.securitygroup.0.name': 'test',
 'virtualmachine.0.securitygroup.0.tags': None,
 'virtualmachine.0.nic.0.id': '79568b14-b377-4d4f-b024-87dc22492b8e',
 'virtualmachine.0.nic.0.networkid': '05c0e278-7ab4-4a6d-aa9c-3158620b6471',
 'virtualmachine.0.nic.1.id': '3d7f2818-1f19-46e7-aa98-956526c5b1ad',
 'virtualmachine.0.nic.1.networkid': 'b4648cfd-0795-43fc-9e50-6ee9ddefc5bd',
 'virtualmachine.0.nic.1.traffictype': 'Guest',
 'virtualmachine.0.hypervisor': 'KVM',
 'virtualmachine.0.affinitygroup': None,
 'virtualmachine.0.isdynamicallyscalable': False}

So you'll see that 'tags' and 'affinitygroup' keys are also handled and added to output. Original code was omitting them.

2021-05-30 : Updated: collections.MutableMapping is changed to collections.abc.MutableMapping

2023-01-11 : edited, added separator arg in second items.extend() call as advised by @MHebes

2024-02-20 : how did that .abc go missing from the import statement?

2025-07-21 : moved crumbs param into function's args