Unflatten Dot Keys to Nested Dict in Python

Convert a flat dictionary with dot-separated keys into a nested dictionary structure using recursive setdefault loops.

Medium Python 3.9+ Aug 9, 2026 Dictionaries & sets 14 views 0 copies

dictionaries nested flatten setdefault

Python code

21 lines

Python 3.9+

def unflatten_dot_keys(flat_dict):
    result = {}
    for flat_key, value in flat_dict.items():
        parts = flat_key.split(".")
        current = result
        for part in parts[:-1]:
            current = current.setdefault(part, {})
        current[parts[-1]] = value
    return result


if __name__ == "__main__":
    flat = {
        "name": "Alice",
        "address.city": "Paris",
        "address.zip": "75001",
        "employer.name": "ACME",
        "employer.location.city": "London",
    }
    nested = unflatten_dot_keys(flat)
    print(nested)

Output

stdout

{'name': 'Alice', 'address': {'city': 'Paris', 'zip': '75001'}, 'employer': {'name': 'ACME', 'location': {'city': 'London'}}}

How it works

The function splits each flat key into parts using split('.'), then walks through all parts except the last, setting any missing intermediate keys to a new empty dictionary using setdefault. The final part is assigned the value. This transforms flat mappings like translation dictionaries, config files, or ORM results into nested structures for easier access. The pattern preserves the original insertion order but creates nested dicts on demand, which is safe for overlapping prefixes such as 'employer' and 'employer.location'.

Common mistakes

Mutating the original dictionary instead of returning a new one.
Using `dict.get` instead of `setdefault`, which fails to create missing intermediate keys.
Assuming keys with the same prefix always share the same nesting level.

Variations

Use recursion to unflatten when keys can contain dots as literal characters.
Use `collections.defaultdict` with a factory for building nested dicts automatically.

Real-world use cases

Parsing flattened form submissions (e.g., 'user[name]') into nested request payloads.
Converting object properties stored as dot-separated keys in a flat database table into JSON documents.
Restructuring nested configuration trees from environment variables where keys use dots as separators.

Unflatten Dot Keys to Nested Dict in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

More from Dictionaries & sets

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