Skipping code in python if in a unittest

The unittest package has extensive support for "mocking" functions and methods. Encapsulate the call to an external program in a simple function that your unit tests can override ("mock out") without modifying the structure of your program. Example:

Here is part of your program, in the module realcode.py

def internal_function_calling_exec(arg1):
    """The real thing"""
    print("I am executing an external program")


def bigger_function_being_tested(arg1, arg2):
    """
    A complex function with one or more calls to `internal_function_calling_exec`
    """
    print("I will now call `internal_function_calling_exec()`")
    internal_function_calling_exec(42)

Your unit test can then look like this:

import unittest
from unittest.mock import patch
import realcode

class MyTest(unittest.TestCase):

    @patch("realcode.internal_function_calling_exec")
    def test_something(self, mocked_func):
        realcode.bigger_function_being_tested(1, 2)
        mocked_func.assert_called_with(42)

This will never call the original internal_function_calling_exec(). Instead, this will trigger a call to the mock object; your test can then query the object to confirm that it was called properly.

There are ways to mock class methods etc., so you could mock subprocess.call instead, for example. But I think the above is the better pattern.