Python Dictionary Mutation & Scope Tracing

Python dictionaries maintain insertion order (Python 3.7+), are mutable, and use fast hash table lookups. Predicting their output in code traces requires understanding in-place updates, reference aliasing, and dictionary comprehensions.

Tracing In-Place Mutation with dict.update()

user = {'name': 'Alice', 'role': 'member'}
profile = user
profile.update({'role': 'admin', 'verified': True})
print(user['role'])
print(profile is user)

Because profile = user creates an alias pointing to the same dictionary, profile.update() modifies the original user dictionary. Output: admin followed by True.

Evaluating Dictionary Comprehensions with Conditions

scores = {'Alice': 85, 'Bob': 45, 'Carol': 92}
passed = {k: v for k, v in scores.items() if v >= 50}
print(passed)

Trace each item against the condition v >= 50. Alice (85) and Carol (92) pass, while Bob (45) is excluded. The output is {'Alice': 85, 'Carol': 92}.