Tests should be isolated from the production environment to prevent side effects and ensure reproducibility. This principle has two main applications: 1. **Use proper temporary files**: Instead of writing test files to the current directory where they might overwrite real files, use the `tempfile` module to create isolated test files. 2. **Establish correct directory context**: Ensure tests run in the appropriate directory context to prevent location-dependent failures. Example: ```python # Good: Use temporary files instead of hardcoded paths import tempfile message_file_path = tempfile.mktemp() main(["--message-file", message_file_path], input=DummyInput(), output=DummyOutput()) # Good: Explicitly set the directory context for tests with change_dir(testdir): # Test code runs in the correct context # ... ``` Following these practices prevents tests from accidentally modifying real files and ensures they'll run consistently regardless of the current working directory.