Python max(): Select a Record with key, Ties and Empty Input

Start with a small, explicit task

Select a fictional proposal by score. Two proposals share the highest score, so state whether first input order or another property should break the tie. Compare the selected record with a separate calculation of the highest numeric score.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_max.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

proposals = [
    {"name": "Pine", "score": 8},
    {"name": "Cedar", "score": 11},
    {"name": "Elm", "score": 11},
]
chosen = max(proposals, key=lambda item: item["score"])
highest = max(item["score"] for item in proposals)
empty = max([], key=lambda item: item["score"], default=None)
assert chosen is proposals[1]
assert chosen["name"] == "Cedar" and highest == 11
assert empty is None
alphabetical_tie = max(proposals, key=lambda item: (item["score"], item["name"]))
assert alphabetical_tie["name"] == "Elm"
print(chosen)
print("highest score:", highest)
print("empty:", empty)
print("explicit name tie-break:", alphabetical_tie["name"])

Explain the result

The score-only key selects Cedar because it appears before Elm among equal maxima. max returns that dictionary itself, confirmed by identity, rather than returning just the score. The generator expression computes the separate numeric maximum when that is all a caller needs.

The tuple key adds name as a secondary comparison, selecting Elm under the explicitly stated lexicographic rule. That is not a universal fair tie-break; it is simply a reproducible alternative. The empty call returns None because a default is supplied, allowing the caller to handle no proposal separately from a legitimate record.

Avoid these mistakes

Do not compare dictionaries directly and expect Python to infer a score field. Define a key that expresses the task. Also avoid sorting an entire collection solely to retrieve one maximum unless the sorted order is needed for another reason.