Data Validation Patterns

Error Collection

Collecting all validation errors lets a caller see every problem in one pass.

error collection Instead of stopping at the first invalid field, collect each error message into a list.

Error Collection

price
errors.py
Replay: real traced execution (multi-file project)
def validate_order(quantity, price):
    errors = []

    if quantity <= 0:
        errors.append("quantity")
    if price <= 0:
        errors.append("price")

    return errors


price = 10
errors = validate_order(2, price)

print("errors=" + ",".join(errors) if errors else "errors=none")

def validate_order(quantity, price):
    errors = []

    if quantity <= 0:
        errors.append("quantity")
    if price <= 0:
        errors.append("price")

    return errors


price = 0
errors = validate_order(2, price)

print("errors=" + ",".join(errors) if errors else "errors=none")

  1. price ← 10

    12price→ 10 = 10  #@price=013errors = validate_order(2, price10)

  2. errors ← []

    1def validate_order(quantity2, price10):2    errors→ [] = []34    if quantity <= 0:5        errors.append("quantity")6    if price <= 0:7        errors.append("price")89    return errors[]

  3. errors ← []

    12price = 10  #@price=013errors→ [] = validate_order(2, price10)1415print("errors=" + ",".join(errors[]) if errors else "errors=none")
    outputerrors=none

  1. price ← 0

    12price→ 0 = 013errors = validate_order(2, price0)

  2. errors ← []

    1def validate_order(quantity2, price0):2    errors→ [] = []

  3. errors ← ['price']

    5    errors.append("quantity")6if price0 <= 0:7    errors→ ['price'].append("price")

  4. return errors

    9return errors['price']

  5. errors ← ['price']

    12price = 013errors→ ['price'] = validate_order(2, price0)1415print("errors=" + ",".join(errors['price']) if errors else "errors=none")
    outputerrors=price