Code Smell 15 - Missed Preconditions

Code Smell 15 - Missed Preconditions

Maxi Contieri
·Nov 3, 2020·

2 min read

Subscribe to my newsletter and never miss my upcoming articles

Play this article

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.

Sample Code


class Date:
  def __init__(self, day, month, year): = day
    self.month = month
    self.year = year

  def setMonth(self, month):
    self.month = month

startDate = Date(3, 11, 2020)

startDate = Date(31, 11, 2020)
#Should fail

#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)

startDate = Date(31, 11, 2020)

#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.

More info



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

This article is part of the CodeSmell Series.

Last update: 2021/06/23

Share this