How to Group a List of Dictionaries by Key in Python

Group a list of dictionaries by a specified key field using dict.setdefault to build a dictionary of lists.

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

dictionaries grouping setdefault data-structures

Python code

17 lines

Python 3.9+

def group_by_key(records, key):
    grouped = {}
    for record in records:
        grouped.setdefault(record[key], []).append(record)
    return grouped

if __name__ == "__main__":
    data = [
        {"name": "Alice", "dept": "engineering"},
        {"name": "Bob", "dept": "sales"},
        {"name": "Carol", "dept": "engineering"},
        {"name": "Dave", "dept": "sales"},
        {"name": "Eve", "dept": "engineering"},
    ]
    result = group_by_key(data, "dept")
    for dept, people in result.items():
        print(f"{dept}: {[p['name'] for p in people]}")

Output

stdout

engineering: ['Alice', 'Carol', 'Eve']
sales: ['Bob', 'Dave']

How it works

The group_by_key function iterates over each record and uses setdefault on the grouping key. setdefault(key, []) returns the existing list if the key is present, otherwise inserts an empty list and returns it — so records always append to a valid list. This avoids the common pattern of checking if key in grouped and explicitly initializing the list. The result is a dictionary where each key maps to a list of all dictionaries sharing that key value. This approach is efficient because setdefault performs a single lookup instead of separate get/set operations.

Common mistakes

Using `grouped[record[key]].append(record)` without initializing the list first, which raises KeyError
Forgetting that the grouping key must exist in every record — missing keys will raise a KeyError
Mutating the grouped dictionary while iterating over the source list, which can cause confusing behavior

Variations

Use `collections.defaultdict(list)` to automatically create lists for new keys
Use a dictionary comprehension with `itertools.groupby` if the input is already sorted by the grouping key

Real-world use cases

Partitioning API response records by status code to handle successes and failures separately.
Grouping log entries by severity level before writing them to distinct output streams.
Organizing customer orders by region in a reporting pipeline for per-region analytics.

How to Group a List of Dictionaries by Key in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Dictionaries & sets

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