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.

Problems

• Consistency
• Contract breaking
• Hard to debug
• Bad cohesion

Solutions

• Create strong preconditions
• Raise exceptions
• Fail Fast
• Defensive Programming

Examples

• Constructors are an excellent first line of defense

• Anemic Objects lack these rules.

Sample Code

Wrong

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

Right

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

Detection

• It's difficult to find missing preconditions, as long with assertions and invariants.

Tags

• Consistency

Conclusion

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.

More info

• Object-Oriented Software Construction (by Bertrand Meyer)

Relations

Credits

Photo by Jonathan Chng on Unsplash

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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

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This article is part of the CodeSmell Series.

Last update: 2021/06/23