Merge pull request #2187 from plotly/testing-import-error

Improve error message when using fixtures but testing not installed.

Author: T4rk1n

Diff for: CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ### Fixed
 
 - [#2152](https://github.com/plotly/dash/pull/2152) Fix bug [#2128](https://github.com/plotly/dash/issues/2128) preventing rendering of multiple components inside a dictionary.
+- [#2187](https://github.com/plotly/dash/pull/2187) Fix confusing error message when trying to use pytest fixtures but `dash[testing]` is not installed.
 
 ## [2.6.1] - 2022-08-01
 

Diff for: dash/testing/plugin.py

@@ -1,10 +1,17 @@
 # pylint: disable=missing-docstring,redefined-outer-name
-from typing import Any
-
 import pytest
 from .consts import SELENIUM_GRID_DEFAULT
 
 
+# pylint: disable=too-few-public-methods
+class MissingDashTesting:
+    def __init__(self, **kwargs):
+        raise Exception(
+            "dash[testing] was not installed. "
+            "Please install to use the dash testing fixtures."
+        )
+
+
 try:
     from dash.testing.application_runners import (
         ThreadedRunner,
@@ -17,15 +24,15 @@
     from dash.testing.composite import DashComposite, DashRComposite, DashJuliaComposite
 except ImportError:
     # Running pytest without dash[testing] installed.
-    ThreadedRunner = Any
-    ProcessRunner = Any
-    MultiProcessRunner = Any
-    RRunner = Any
-    JuliaRunner = Any
-    Browser = Any
-    DashComposite = Any
-    DashRComposite = Any
-    DashJuliaComposite = Any
+    ThreadedRunner = MissingDashTesting
+    ProcessRunner = MissingDashTesting
+    MultiProcessRunner = MissingDashTesting
+    RRunner = MissingDashTesting
+    JuliaRunner = MissingDashTesting
+    Browser = MissingDashTesting
+    DashComposite = MissingDashTesting
+    DashRComposite = MissingDashTesting
+    DashJuliaComposite = MissingDashTesting
 
 
 def pytest_addoption(parser):