Add a test helper pytest fixture. (#88)

* Add a test helper pytest fixture.

A way to have convenience helper functions for developing tests.
@sfmig prefers a fixture.
https://stackoverflow.com/a/42156088/1444054

* Suggestions from PR review.

Co-authored-by: sfmig <33267254+sfmig@users.noreply.github.com>

* Rename 'name' to be more helpful.

---------

Co-authored-by: sfmig <33267254+sfmig@users.noreply.github.com>

.gitignore

@@ -45,6 +45,7 @@ coverage.xml
 *,cover
 .hypothesis/
 .pytest_cache/
+.test_helper_screenshots/
 
 # mypy
 .mypy_cache/

tests/test_integration/conftest.py

@@ -1,6 +1,51 @@
+from pathlib import Path
+
+import pytest
+from dash.testing.composite import DashComposite
 from selenium.webdriver.chrome.options import Options
 
 
+class Helpers:
+    """A class to group helpful things when writing tests.
+
+    Added as a pytest fixture.
+
+    Example
+    -------
+    ```
+    def test_my_thing(helpers)
+        helpers.debug_screenshot()
+    ```
+    """
+
+    @staticmethod
+    def debug_screenshot(dash_duo: DashComposite, subdir: str = "") -> None:
+        """Save a screenshot of the current state of the testing server.
+
+        Useful for debugging and developing tests.
+        Saves to .test_debug_screenshots or
+        .test_debug_screenshots/<subdir> if `subdir` is provided.
+
+        Parameters
+        ----------
+            dash_duo : DashComposite
+                The current dash_duo fixture.
+            subdir: str, optional
+                A subdirectory name (to help you find the screenshots.)
+        """
+        screenshots_dirname = Path(".test_helper_screenshots")
+        if subdir:
+            screenshots_dirname /= subdir
+        screenshots_dirname.mkdir(exist_ok=True)  # mkdir -p
+        filename = screenshots_dirname / f"test-{dash_duo.session_id}.png"
+        dash_duo.driver.save_screenshot(filename.as_posix())
+
+
+@pytest.fixture
+def helpers():
+    return Helpers
+
+
 def pytest_setup_options():
     options = Options()
     options.add_argument("--headless")