Group by Key - Python DSA

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Python DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.py

pairs = [("a", 1), ("b", 2), ("a", 3), ("c", 4), ("b", 5)]
groups = {}
order = []
for key, value in pairs:
    if key not in groups:
        groups[key] = []
        order.append(key)
    groups[key].append(value)
parts = [f"{key}: {groups[key]}" for key in order]
print("{" + ", ".join(parts) + "}")

Complexity

Time: O(n) average
Space: O(k + n) for buckets and values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.