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Python Collections Gotchas: defaultdict Creating Keys on Read, Counter Subtraction, deque maxlen and Shallow dict Copies

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Self-Hosting & Infra beginner 5 min read $0.01 Updated 2026-10-20

Executed on Python 3.10: defaultdict key creation by read, get versus index, Counter most_common and missing keys, subtraction removing negatives, deque(maxlen=3), dict.fromkeys ordering, setdefault, KeyError text, shallow copy mutation and merge operators.

Reading a defaultdict inserts the key, Counter drops non-positive counts, a deque silently discards old items and dict(b) shares nested objects. Each behavior shown with real output and the safe alternative.

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#python#collections#defaultdict#counter#dictionaries

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