defaultdict

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defaultdict#

collections.defaultdict is a dict that auto-initialises missing keys with a factory. Saves the setdefault / get(...) or ... dance.

from collections import defaultdict

by_host = defaultdict(list)
for scan in scans:
    by_host[scan.host].append(scan)

counts = defaultdict(int)
for c in text:
    counts[c] += 1

The factory is any zero-arg callable: list, dict, set, int, lambda: 0, lambda: {"port": 80}. The factory runs only on first access to a missing key.

A defaultdict “is a” dict; pass it anywhere a dict is expected. The auto-create only kicks in on __getitem__; in particular defaultdict(list).get(k) returns None for a missing key without creating it.

References#

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