Writing tests for Python

A coworker asked me about "tests" from a programming practice perspective, "what's the big deal?". For me, writing tests is a very small price to pay to comfortably daring to change code without the fear of breaking it. Every bug I fix, even during the design phase, gives rise to at least one test.

No library or script is too small to have tests, here is a silly example (let's call it rootof.py):

#!/usr/bin/env python

import argh

def root_of(number=1.0):
    return number**0.5

def test_root_of():
    assert root_of(4) == 2
    assert abs(root_of(2) - 1.4142) < 1e-4

if __name__ == '__main__':
    argh.dispatch_command(root_of)

The example is so short it's silly, but the point is that each time you edit rootof.py you should run

$ python -m pytest rootof.py

which should exit without errors. That's pretty much the gist of it. Happy testing!

P.S. if you don't have argh just install it by typing:

$ python -m pip install --user argh

P.P.S if you don't have pip: see this.