Assertions, Preconditions, Postconditions and invariants are our allies to avoid invalid objects. Avoiding them leads to hard-to-find errors.
TL;DR: If you turn off your assertions just in production your phone will ring at late hours.
- Consistency
- 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 failclass 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.
- Consistency
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.
Code Smell 189 - Not Sanitized Input
Photo by Jonathan Chng on Unsplash
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.
Bertrand Meyer
Software Engineering Great Quotes
This article is part of the CodeSmell Series.