Mocking the filesystem

The previous lesson established that you should test your code, not the OS. That means file-writing functions need a real filesystem for their I/O (to confirm the bytes are correct) but not the production filesystem. pytest's tmp_path fixture provides exactly that: a fresh, isolated directory for each test, automatically removed when the test finishes.

Testing a CSV writer with tmp_path

The demo below defines a function that writes a list of records to a CSV file, then shows how to test it. The key insight: pass the output path as a parameter so the test can supply a tmp_path-based location rather than a hardcoded one.

Notice that the test reads the file back and checks individual lines rather than comparing the whole string — this makes failures more informative.

Patching builtins.open

Sometimes you cannot inject the path as a parameter — the function has it hardcoded or reads it from config. unittest.mock.patch replaces the real open with a mock for the duration of the test:

from unittest.mock import patch, mock_open
import json

def load_config():
    """Reads config from a hardcoded location — not injectable."""
    with open("/etc/myapp/config.json") as f:
        return json.load(f)

def test_load_config():
    fake_config = json.dumps({"api_key": "test-key", "retries": 3})
    with patch("builtins.open", mock_open(read_data=fake_config)):
        result = load_config()
    assert result["api_key"] == "test-key"
    assert result["retries"] == 3

mock_open is a factory that produces a mock file object whose .read() returns read_data. For JSON, that is enough; for CSV-by-line reading you may need to configure mock_open(read_data=...).return_value.__iter__ manually, at which point switching to tmp_path is usually cleaner.

Checking that a function deletes the right file

Deletions are just as testable. Create the file in tmp_path, call the function, assert the file is gone:

def test_cleanup_removes_temp_files(tmp_path):
    temp = tmp_path / "run.tmp"
    temp.write_text("leftover")

    cleanup(tmp_path)   # function under test

    assert not temp.exists()

Where to go next

Next: mocking HTTP — why you should not hit real APIs in tests, and the responses library that lets you intercept requests calls at the transport layer.