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Flatten nested event payloads · Python scenario · D8LooP

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Flatten nested event payloads

The warehouse loader wants one flat column per field, but the producer sends arbitrarily nested JSON: objects inside objects, and lists of objects. Write flatten_payload(payload). It takes one payload and returns a single flat dict whose keys are the path to each value, joined with dots.

Function to write

flatten_payload(payload: dict) -> dict

A flat dict whose keys are dotted paths and whose values are the leaves.

How to approach it

Decide what counts as a leaf first — then the recursion writes itself.

Sample cases

+ 2 held back until you submit

a nested event

Every rule at once: nested dicts, a list of dicts, a None leaf, and two empty containers.

Input

Argument 1

{
  'event': 'checkout',
  'user': {
    'id': 'u1',
    'geo': {
      'country': 'IN',
      'city': None
    }
  },
  'items': [
    {
      'sku': 'A1',
      'qty': 2
    },
    {
      'sku': 'B2',
      'qty': 1
    }
  ],
  'flags': {},
  'tags': []
}

Returns

{
  'event': 'checkout',
  'user.id': 'u1',
  'user.geo.country': 'IN',
  'user.geo.city': None,
  'items.0.sku': 'A1',
  'items.0.qty': 2,
  'items.1.sku': 'B2',
  'items.1.qty': 1,
  'flags': {},
  'tags': []
}

already flat

Nothing to descend into — the payload comes back unchanged.

Input

Argument 1

{
  'id': 'e1',
  'amount': 12.5,
  'ok': True
}

Returns

{
  'id': 'e1',
  'amount': 12.5,
  'ok': True
}

a list of scalars

Lists are indexed by position, so three scores become three keys.

Input

Argument 1

{
  'scores': [
    10,
    20,
    30
  ],
  'note': None
}

Returns

{
  'scores.0': 10,
  'scores.1': 20,
  'scores.2': 30,
  'note': None
}

Constraints

1A nested dict contributes its key to the path: {'user': {'id': 'u1'}} becomes {'user.id': 'u1'}.
2A list contributes the index as a path segment: {'scores': [10, 20]} becomes {'scores.0': 10, 'scores.1': 20}.
3An empty dict or list has nothing below it, so it is a leaf and survives as its own value — {'flags': {}} stays {'flags': {}}.
4None is a value, not an absence. Keep it.
5An empty payload returns an empty dict, not a dict with an empty-string key.
6Do not modify the payload you were given.

Worked example

{'user': {'geo': {'country': 'IN', 'city': None}}} flattens to {'user.geo.country': 'IN', 'user.geo.city': None} — the None is kept, because a field that was explicitly null is different from a field that was never sent. The rule that catches most people is 'flags': {}. The obvious recursion descends into it, finds no keys, adds nothing, and the column disappears from the output entirely — silently, with no error to notice.

What this tests

Recursion over a heterogeneous structure, and the discipline of deciding what a leaf is before writing the walk. Empty containers are the case that separates a flattener that works from one that quietly loses columns.

PYTHONflatten_payload(payload: dict) -> dict

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