How to unittest with command line arguments

Question

I am using the python unittest module for testing a file that takes a command line argument. The argument is a file name which is then passed into a function like so:

file_name = str(sys.argv[1])
file = open(file_name)
result = main_loop(file)
print(result)

My test is set up like so:

class testMainFile(unittest.TestCase):
 def test_main_loop(self):
 file = open('file_name.json')
 result = main_file.main_loop(file)
 self.assertEqual(result, 'Expected Result')
 
if __name__ == 'main':
 unittest.main()

When I run the test I get an "IndexError: list index out of range".

I tried passing the argument when running the test but to no avail. How do I run my test without error?

• chepner: You can assign directly to sys.argv, but it's not clear how you are importing your script for testing. There's a lot of apparent duplication between your script and the test code, and no mention of where main_file is defined.
• bendowlingtech: Can you explain further on how to do that?

Answer 1

I think you have couple of options here. Firstly go to documentation and checkout patch because i think you can get away with

from unittest.mock import patch
@patch('sys.argv', ['mock.py', 'test-value'])
def test_main_loop(self):

---

Options for fun:

One would be simply to override the sys.argv next to your call

def test_main_loop(self):
 file = open('file_name.json')
+ orginal_argv = sys.argv
+ sys.argv = ['mock argv', 'my-test-value'] 
 result = main_file.main_loop(file)
+ sys.argv = orginal_argv 
 self.assertEqual(result, 'Expected Result')

Second would be to create a simple wrapper for your function

def set_sys_argv(func: Callable):
 sys.argv = ['mock.py', 'my_test_value']
 def wrapper(*args, **kwargs):
 func()
 return wrapper

and use it with test function

@set_sys_argv
def test_main_loop(self):

We can improve it slightly and make it more generic making a decorator that accepts the values to mock

def set_sys_argv(*argv):
 sys.argv = argv
 def _decorator(func: Callable):
 def wrapper(*args, **kwargs):
 func()
 return wrapper
 return _decorator

and use it similarly to patch

@set_sys_argv('mock.py', 'test-value')
def test_main_loop(self):

Third would be to create a context manager, likewise:

class ReplaceSysArgv(list):
 def __enter__(self):
 self._argv = sys.argv
 sys.argv = ['mock', 'my-test-value']
 return self
 def __exit__(self, *args):
 sys.argv = self._argv

and use it with your code

 def test_main_loop(self):
 file = open('file_name.json')
 with ReplaceSysArgv():
 result = main_file.main_loop(file)
 self.assertEqual(result, 'Expected Result')

Answer 2

you have to push the arguments onto sys.argv before retrieving them (if your code is pulling from command-line arguments - it's unclear to me where in your test you're using the command-line arguments but I digress)

so something like first doing

import sys
sys.argv = ['mock_filename.py', 'json_file.json']
#... continue with rest of program / test.

• bendowlingtech: Sorry I am a little unclear what I do with the sys.argv list after defining it.