Python Dictionary Tracing: How to Predict Dict Output in Interviews

Python dictionaries are ordered (Python 3.7+), mutable, and use hash-based key lookups. Predicting their output in code traces requires understanding key collision, in-place mutation, and nested reference behavior. Practice dict tracing at /practice/dictionaries-basic/.

Tracing Dict.update() vs Direct Assignment

d = {'a': 1, 'b': 2}
e = d
e.update({'b': 99, 'c': 3})
print(d)
print(e is d)

Since `e = d` creates an alias (not a copy), `e.update(...)` mutates the original dict `d` in place. Output: {'a': 1, 'b': 99, 'c': 3} then True.

Tracing Dict Comprehensions with Conditions

scores = {'Alice': 88, 'Bob': 42, 'Carol': 95}
passing = {k: v for k, v in scores.items() if v >= 50}
print(passing)

Trace each key-value pair against the condition v >= 50: Alice 88 passes, Bob 42 fails, Carol 95 passes. Output: {'Alice': 88, 'Carol': 95}.