Nikita Sobolev sobolevn.me

Make tests a part of your app  ↦

Here’s a pretty useful idea for library authors and their users: there are better ways to test your code!

I give three examples of how user projects can be self-tested without actually writing any real test cases by the end-user. One is hypothetical about django and two examples are real and working: featuring deal and dry-python/returns. A brief example with deal:

import deal

@deal.pre(lambda a, b: a >= 0 and b >= 0)
@deal.raises(ZeroDivisionError)  # this function can raise if `b=0`, it is ok
def div(a: int, b: int) -> float:
    if a > 50:  # Custom, in real life this would be a bug in our logic:
        raise Exception('Oh no! Bug happened!')
    return a / b

This bug can be automatically found by writing a single line of test code: test_div = deal.cases(div). As easy as it gets! From this article you will learn:

  • How to use property-based testing on the next level
  • How a simple decorator @deal.pre(lambda a, b: a >= 0 and b >= 0) can help you to generate hundreds of test cases with almost no effort
  • What “Monad laws as values” is all about and how dry-python/returns helps its users to build their own monads

I really like this idea! And I would appreciate your feedback on it.


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