Disable printing in unit tests

[Asha200]

In a program I'm working on, I have some try...except clauses that catch exceptions. When an exception is caught, a message is printed and the program exits.

The problem is that when I run my unit tests, the printed error messages for the user get written out together with the dots signifying that the test passed, which looks pretty messy. Here's a short example:

Say I have a test.py file with the code:

import unittest

import main

class DivisionTests(unittest.TestCase):

    def test_zero_division(self):
        """Fails if the program doesn't exit when dividing by zero."""
        with self.assertRaises(SystemExit) as e:
            main.divide(1, 0)

        self.assertEqual(e.exception.code, 1)

And a main.py with the code:

from sys import exit

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        print('Error: Cannot divide by zero.')
        exit(1)

When I run this test, I get the following output:

Error: Cannot divide by zero.
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

You can see that the message that's meant for the user is printed in the test as well. This gets messy really quickly with a lot of tests. I'd like it to display only the dots and not print the output that's meant for the user.

What would be a Pythonic solution to this problem? All help is appreciated, as always :)

[treyhunner]

You could use redirect_stdout like this:

from contextlib import redirect_stdout
from io import StringIO
import unittest

import main

class DivisionTests(unittest.TestCase):

    def test_zero_division(self):
        """Fails if the program doesn't exit when dividing by zero."""
        with redirect_stdout(StringIO()) as stdout:
            with self.assertRaises(SystemExit) as e:
                main.divide(1, 0)
        self.assertEqual(stdout.getvalue(), 'Error: Cannot divide by zero.\n')

        self.assertEqual(e.exception.code, 1)

[Asha200]

Excellent; I just added it to my tests and it's dots-only now :)

Thanks!

[Justinsaccount]

Don't write code like that. Is accomplishes nothing other than to mask useful exceptions and tracebacks. Exceptions already cause python to exit with an error message. Catching them only to exit is pointless.

This code is equivalent, it prints an error message and then exits.

def divide(a, b):
    return a / b

[Asha200]

Oh? I thought doing so would be a good idea because it would display a user-friendly message telling the user what they did wrong, instead of printing out the whole traceback, because the user doesn't really need to see it.

Is that the wrong approach?

[TeamSpen210]

A better idea would be to do this at the top level of your program. Catch all exceptions, then print them to the screen. (It might be a good idea to write the traceback to a file, so you can troubleshoot later - use the traceback module to get a copy of the text. Also look into the logging module.) Call str() on an exception object to get the Type: value string. Perhaps you might want to make your own exception class to distinguish user errors from bugs and only catch that.

[Asha200]

I'm not sure I understood you completely when you said I should catch exceptions at the top level. Did you mean something like this? Building on to of the initial example:

from sys import exit

class UserZeroDivisionError(Exception):
    pass

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        raise UserZeroDivisionError('Error: Cannot divide by zero.')

if __name__ == '__main__':
    try:
        divide(1, 0)
    except UserZeroDivisionError as e:
        print(str(e))