Filtering and Transforming

Add Derived Field

Enrich a list of records by computing a new field from existing ones — here, total = price × qty. The replay shows each record r alongside the computed total, one iteration at a time.

By hand

The Pythonic way

A list comprehension with the dict-spread idiom {**r, 'total': r['p'] * r['q']} produces each enriched record in one expression, without an explicit loop or accumulator.

naive.py 
records = [{'n': 'a', 'p': 2, 'q': 5}, {'n': 'b', 'p': 5, 'q': 6}, {'n': 'c', 'p': 3, 'q': 7}, {'n': 'd', 'p': 8, 'q': 3}, {'n': 'e', 'p': 4, 'q': 8}, {'n': 'f', 'p': 6, 'q': 9}]
# trace: ignore records, enriched
enriched = []
for r in records:
    total = r['p'] * r['q']
    new = {**r, 'total': total}
    enriched.append(new)
print('RESULT:', [d['total'] for d in enriched])
library.py 
records = [{'n': 'a', 'p': 2, 'q': 5}, {'n': 'b', 'p': 5, 'q': 6}, {'n': 'c', 'p': 3, 'q': 7}, {'n': 'd', 'p': 8, 'q': 3}, {'n': 'e', 'p': 4, 'q': 8}, {'n': 'f', 'p': 6, 'q': 9}]
enriched = [{**r, 'total': r['p'] * r['q']} for r in records]
print('RESULT:', [d['total'] for d in enriched])

RESULT: [10, 30, 21, 24, 32, 54]

Implementation notes

  • records and enriched are excluded from tracing (# trace: ignore) because even two full-dict records push the list repr past 80 characters. The per-step visible state is r (the input record, ~25 chars), total (the computed value), and new (the enriched record with the added field, ~40 chars) — exactly the three variables that teach "add a derived field".
  • {**r, 'total': total} is the standard idiom for non-destructive dict enrichment: it copies every key from r into a new dict and adds 'total'.
  • The enriched.append(new) events are zero-delta in the trace because enriched is ignored; the RESULT line confirms the accumulated output.