Argparse Unit Test with IP Address Module - python-3.x

Question

I'd like to write unit tests for the following function:
#!/usr/bin/env python3
"""IPv4 validation using `ipaddress module` and argparse."""
import argparse
from ipaddress import ip_address
def parse_cli_args():
"""
Command line parser for subnet of interest.
Args:
--ip 0.0.0.0
Returns:
String, e.g. 0.0.0.0
"""
parser = argparse.ArgumentParser(description="IPv4 address of interest.")
parser.add_argument("--ip", action="store", type=ip_address,\
required=True,\
help="IP address of interest, e.g. 0.0.0.0")
args = parser.parse_args()
return args
if __name__ == '__main__':
args = parse_cli_args()
print(args.ip)
which works as expected, e.g.:
python3 test.py --ip 192.168.1.1
192.168.1.1
python3 test.py --ip derp
usage: test.py [-h] --ip IP
test.py: error: argument --ip: invalid ip_address value: 'derp'
python3 test.py --ip
usage: test.py [-h] --ip IP
test.py: error: argument --ip: expected one argument
How can I mock these three conditions in unit tests?
I tried a few variations of this:
import unittest
from unittest.mock import patch
class ParseCLIArgs(unittest.TestCase):
"""Unit tests."""
#patch('builtins.input', return_value='192.168.1.1')
def test_parse_cli_args_01(self, input):
"""Valid return value."""
self.assertIsInstance(parse_cli_args(), ipaddress.IPv4Address)
if __name__ == '__main__':
unittest.main()
without success. What am I doing wrong, and how can I fix that?
EDIT I got a bit further with this:
class ParseCLIArgs(unittest.TestCase):
def setUp(self):
self.parser = parse_cli_args()
def test_parser_cli_args(self):
parsed = self.parser.parse_args(['--ip', '192.168.1.1'])
self.assertIs(parsed.ip, '192.168.1.1')
if __name__ == '__main__':
unittest.main()
Which fails with: TypeError: isinstance() arg 2 must be a type or tuple of types. I believe this is because the function actually transforms user input.

Answer 1

To test a parser you need to either modify sys.argv or provide your own substitute.
When called with
args = parser.parse_args(argv)
if argv is None (or not provided) it parses sys.argv[1:]. This is the list that the shell/interpreter gives it. Otherwise it will parse the equivalent list that you provide.
test_argparse.py uses both ways to test a parser - constructing a custom sys.argv and call parse_args with a custom argv.
Another thing to watch out for is error trapping. Most parsing errors display usage, error message and then exits. Capturing that exit and the stderr message takes some work. test_argparse makes a subclass of ArgumentParser with a custom error method.
In sum, doing unittest on code that depends on sys.argv and does a system exit, may require more work than it's worth. That said, I'm not an expert on unit testing and mocking tools; I've just studied the test_argparse file.

Answer 2

Hopefully this can serve as good sample for someone with a similar question. This test does most (but not all) of what I want it to do. I'm still working out a unit test to check for Type.
#!/usr/bin/env python3
"""IPv4 validation using `ipaddress module` and argparse."""
import argparse
from ipaddress import ip_address
import unittest
def parse_cli_args():
"""
Command line parser for subnet of interest.
Args:
--ip 0.0.0.0
Returns:
String, e.g. 0.0.0.0
"""
parser = argparse.ArgumentParser(description="IPv4 address of interest.")
parser.add_argument("--ip", action="store",\
required=True,\
help="IP address of interest, e.g. 0.0.0.0")
return parser
class ParseCLIArgs(unittest.TestCase):
def setUp(self):
self.parser = parse_cli_args()
def test_parser_cli_args(self):
parsed = self.parser.parse_args(['--ip', '192.168.1.1'])
self.assertEqual(parsed.ip, '192.168.1.1')
if __name__ == '__main__':
unittest.main()