Code Smell 15 - Missed Preconditions
Assertions, Preconditions, Postconditions and invariants are our allies to avoid invalid objects. Avoiding them leads to hard-to-find errors.
TD;DR: If you turn off your assertions just in production your phone will ring at late hours.
- Contract breaking
- Hard to debug
- Bad cohesion
- Create strong preconditions
- Raise exceptions
- Fail Fast
- Defensive Programming
Constructors are an excellent first line of defense
Anemic Objects lack these rules.
class Date: def __init__(self, day, month, year): self.day = day self.month = month self.year = year def setMonth(self, month): self.month = month startDate = Date(3, 11, 2020) #OK startDate = Date(31, 11, 2020) #Should fail startDate.setMonth(13) #Should fail
class Date: def __init__(self, day, month, year): if month > 12: raise Exception("Month should not exceed 12") # # etc ... self._day = day self._month = month self._year = year startDate = Date(3, 11, 2020) #OK startDate = Date(31, 11, 2020) #fails startDate.setMonth(13) #fails since invariant makes object immutable
- It's difficult to find missing preconditions, as long with assertions and invariants.
Always be explicit on object integrity.
Turn on production assertions.
Even if it brings performance penalties.
Data and object corruption is harder to find.
Fail fast is a blessing.
Writing a class without its contract would be similar to producing an engineering component (electrical circuit, VLSI (Very Large Scale Integration) chip, bridge, engine...) without a spec. No professional engineer would even consider the idea.
This article is part of the CodeSmell Series.
Last update: 2021/06/23