How do I set sys.argv so I can unit test it?

Question

I would like to set

sys.argv 

so I can unit test passing in different combinations. The following doesn't work:

#!/usr/bin/env python import argparse, sys def test_parse_args():     global sys.argv     sys.argv = ["prog", "-f", "/home/fenton/project/setup.py"]     setup = get_setup_file()     assert setup == "/home/fenton/project/setup.py" def get_setup_file():     parser = argparse.ArgumentParser()     parser.add_argument('-f')     args = parser.parse_args()     return args.file if __name__ == '__main__':     test_parse_args() 

Then running the file:

pscripts % ./test.py                                                                                              File "./test.py", line 4     global sys.argv               ^ SyntaxError: invalid syntax pscripts %   

Answer 1

Changing sys.argv at runtime is a pretty fragile way of testing. You should use mock's patch functionality, which can be used as a context manager to substitute one object (or attribute, method, function, etc.) with another, within a given block of code.

The following example uses patch() to effectively "replace" sys.argv with the specified return value (testargs).

try:     # python 3.4+ should use builtin unittest.mock not mock package     from unittest.mock import patch except ImportError:     from mock import patch  def test_parse_args():     testargs = ["prog", "-f", "/home/fenton/project/setup.py"]     with patch.object(sys, 'argv', testargs):         setup = get_setup_file()         assert setup == "/home/fenton/project/setup.py" 

Answer 2

test_argparse.py, the official argparse unittest file, uses several means of setting/using argv:

parser.parse_args(args) 

where args is a list of 'words', e.g. ['--foo','test'] or --foo test'.split().

old_sys_argv = sys.argv sys.argv = [old_sys_argv[0]] + args try:     return parser.parse_args() finally:     sys.argv = old_sys_argv 

This pushes the args onto sys.argv.

I just came across a case (using mutually_exclusive_groups) where ['--foo','test'] produces different behavior than '--foo test'.split(). It's a subtle point involving the id of strings like test.