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Hugo Santos

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Preconditions, Postconditions and Invariants

Frequently when debugging software we usually spend lots of time figuring out where exactly is the bug that is creating the failure of the software. Maybe it’s where the failure occurs but probably it’s not. Bugs can infect parts of software and these parts can cause failure of other parts. Sometimes finding a bug is like finding a needle in a haystack.

These 3 topics belong to the notion of design-by-contract (DbC) programming. Using this approach to design software, programmers should define code contracts - conditions that hold true before a method is run (preconditions), or after the method is run (postconditions) or both (invariants). Eiffel programming language was built around this concept (among others).


Set of conditions that must assert to True before running the code in a method. These conditions can be, for example, variables that must have a positive value in order to the code in the method behave correctly.

def square_root(number):
    assert number >= 0


Set of conditions that must assert to True after the code in a method is run. These conditions verify the correct execution of the method code.

def square_root(number):
    # variable root holds the square root value of number
    assert root * root == number  # i'm ignoring precision for readability purposes
    return root


Set of conditions that must assert to True before and after the execution of a method. Invariants are composed by a set of conditions that are always valid. This is useful to ensure that the code is running according to the algorithm.