Python unittest call and unexpected program run

Question

Let's say i have a simple python program and a simple test file

iseven.py:

import math

def is_even(n):
    return n%2==0

print is_even(2)
print is_even(3)

and test_iseven.py:

import unittest
from iseven import is_even

class IsevenTests(unittest.TestCase):
    def test1(self):
        self.assertTrue(is_even(2))
        self.assertFalse(is_even(3))

if __name__ == '__main__':
    unittest.main()

Is there a difference in running the tests between

python test_iseven.py

and

python -m unittest test_iseven.py

? Because I've seen both in guides and tutorials, and output is identical. Also: The test is just for that one function, yet the whole program is executed when running the tests, so I get the program's output in the console. That is not supposed to happen, right?

Aaron Digulla · Accepted Answer · 2014-04-15 08:28:47Z

1

Is there a difference between ...?

There is no big difference when it comes to executing the tests.

The main difference is that in the latter case, you can omit the

if __name__ == '__main__':
    unittest.main()

but you have to type more on the command line every time you want to run the tests.

So I prefer the first solution.

Python executes the whole program.

The behavior is correct. To be able to import is_even from the module iseven, Python has to parse the whole module. It can't really just look at the function.

Since Python is a scripting language, parsing the module means that it has to execute all the commands in it. From Python's point of view, def is a command like print which creates a new function instance and adds it to the current scope.

Or to put it differently: If it wouldn't run the print, it also couldn't run the def.

This behavior is often used to do magic. For example, in my i18n module for Python, I use:

@i18n
def name(): pass

At runtime, I collect all functions decorated with @i18n and turn them into code which checks the current language, loads the correct text from the translation file and returns it. That means I can do later:

print name()

and it will do the right thing.

EDIT Now you may have code in your module that you want to execute only when the module is run as a "program" (i.e. not when it's imported from somewhere else). Here is how you can do that:

def is_even(n):
    return n%2==0

def main():
    print is_even(2)
    print is_even(3)

if __name__ == '__main__':
    main()

General recipe: Always move all code into functions; avoid keeping anything at the "root" level of the module.

edited Apr 15, 2014 at 8:28

answered Apr 15, 2014 at 8:07

Aaron Digulla

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1 Comment

Basti Over a year ago

Well ok, thanks for the great answer.The second part is still annoying since it displays existing prints and means long runtime for a simple unit test if the main program does some number crunching. Also this is never mentioned in the videos :x

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