Add Look Inside Pytest tmp_path as a Python TIL

python/look-inside-pytest-tmp-path.md

@@ -0,0 +1,41 @@
+# Look Inside Pytest tmp_path
+
+In [Isolate and Debug File Side-Effects with Pytest
+`tmp_path`](https://www.visualmode.dev/isolate-and-debug-file-side-effects-with-pytest-tmp-path),
+I wrote about how I use
+[`tmp_path`](https://docs.pytest.org/en/stable/reference/reference.html#std-fixture-tmp_path)
+in a Pytest fixture to test [my `py-vmt` CLI](https://github.com/jbranchaud/py-vmt). During testing of the CLI interface
+via [`click`'s testing utilities](https://click.palletsprojects.com/en/stable/testing/), `vmt` creates,
+modifies, and reads from files. Isolating that behavior with the `tmp_path`
+fixture is useful because it prevents individual test cases from conflicting
+with one another.
+
+Here is what the fixture looks like at the top of my test file:
+
+```python
+# auto fixture for all test cases that monkeypatches the platform dirs to a tmp
+# path so that test side-effects don't persist between runs
+@pytest.fixture(autouse=True)
+def use_tmp_platform_dirs(tmp_path, monkeypatch):
+    data_dir = tmp_path / "data"
+    config_dir = tmp_path / "config"
+    data_dir.mkdir()
+    config_dir.mkdir()
+    monkeypatch.setattr(CliContext, "get_data_dir", staticmethod(lambda: data_dir))
+    monkeypatch.setattr(CliContext, "get_config_dir", staticmethod(lambda: config_dir))
+```
+
+The root of the temp directory is located at `tempfile.gettempdir()` and the
+directories from there are organized with this structure:
+
+```
+{temproot}/pytest-of-{user}/pytest-{num}/{testname}/
+```
+
+So, in the case of `vmt`, I can find the `config` and `data` dirs for a specific
+test run here:
+
+```bash
+❯ ls /var/folders/zc/q6gnvbgx6kq77828jn38716r0000gn/T/pytest-of-lastword/pytest-2/test_start_status_stop_flow0
+config data
+```