pytest: How to test project-dependent directory creation?

Question

I'm developing a project that has the following architecture:

|-- weather
|   |-- __init__.py
|   |-- weather.py
|-- library
|   |-- data.json
|-- test
|   |-- __init__.py
|   |-- test_weather.py

I would like to test that performing a save creates the library directory if it does not already exist. test_weather.py contains:

import os
import shutil

from weather import weather

def test_library_creation():
    # Initialize.
    app = weather.WeatherApp()
    temp_data = app.get_data()
    # Library should not exist initially.
    assert not os.path.isdir('library')
    app.save(temp_data)
    # Library should exist after save.
    assert os.path.isdir('library')
    assert os.path.isfile(os.path.join('library', 'data.json'))
    # Cleanup.
    shutil.rmtree('library')

However, I may have some data saved in data.json that I don't want to delete as a result of running this test.

Does pytest have a solution for this case?

---

EDIT :

weather.py contains:

import os
import json

DEFAULT_SAVE_PATH = os.path.join('library', 'data.json')

class WeatherApp:
    def get_data(self):
        return dict(temperature=25, humidity=0.5)

    def save(self, data, save_path=DEFAULT_SAVE_PATH):
        with open('data.json', 'w') as jf:
            json.dump(data, jf)
        os.renames('data.json', save_path)

Answer 1

Since you have already thought about providing a custom save path, you don't even need to mock anything if you don't want to; just pass the custom path derived from the tmp_path fixture:

def test_library_creation(tmp_path):
    app = weather.WeatherApp()
    temp_data = app.get_data()
    lib_dir = tmp_path / 'library'
    # this check is somewhat redundant because pytest will ensure
    # the temp dir is created from scratch and is empty
    assert not lib_dir.is_dir()
    app.save(temp_data, save_path=str(lib_dir / 'data.json'))
    # Library should exist after save.
    assert lib_dir.is_dir()
    assert (lib_dir / 'data.json').is_file()
    shutil.rmtree(str(lib_dir))

Some notes:

1. The string conversion of path-like objects is required only on Python versions older than 3.6; if you use 3.6 or 3.7, you can work directly with path-like objects, e.g.

   app.save(temp_data, save_path=lib_dir / 'data.json')
   

   or

   shutil.rmtree(lib_dir)
   

2. Beware that os.rename/os.renames are not error-prone against changing filesystems, e.g. you write data.json on a local ext4 partition and save_path points to a CIFS share, and here comes the error.

3. Maybe the renaming operation is redundant? You can write the data directly to save_path. You just need to ensure the target dir exists first, e.g. with os.makedirs(os.path.dirname(save_path), exist_ok=True).

4. If an assertion fails in the test, the line shutil.rmtree(str(lib_dir)) won't be executed; this is no big deal since the tmp_path is created on tmpfs and will be removed after the next reboot anyway. However, if you want to handle the deletion yourself, I would do this in the test teardown using a custom fixture:

   import os
   import shutil
   import pytest
   
   from weather import weather
   
   
   @pytest.fixture
   def lib_dir(tmp_path):
       d = tmp_path / 'library'
       yield d
       shutil.rmtree(str(d))
   
   
   def test_library_creation(lib_dir):
       app = weather.WeatherApp()
       temp_data = app.get_data()
       assert not lib_dir.is_dir()
       app.save(temp_data, save_path=str(lib_dir))
       # Library should exist after save.
       assert lib_dir.is_dir()
       assert (lib_dir / 'data.json').is_file()
   

   Now the lib_dir will be removed regardless whether the test passes or not.

5. If you want to test the default arguments (the app.save(temp_data) case), you will need to monkeypatch the DEFAULT_SAVE_PATH constant in the weather module. It's pretty easy using the monkeypatch fixture:

   def test_library_creation(monkeypatch, lib_dir):
       app = weather.WeatherApp()
       temp_data = app.get_data()
       assert not lib_dir.is_dir()
       with monkeypatch.context() as m:
           m.setattr(weather, 'DEFAULT_SAVE_PATH', os.path.join(lib_dir, 'data.json'))
           app.save(temp_data)
       # Library should exist after save.
       assert lib_dir.is_dir()
       assert (lib_dir / 'data.json').is_file()