Remove assertion so that @image_comparison can be used outside a 'tes…

…ts' module, but issue warning because the code logic seems a bit fragile and may fail.
matplotlib · WeatherGod · Jan 13, 2016 · Jan 13, 2016 · Jan 13, 2016 · Jan 13, 2016
commit 90b8f113b77e1ff833ea9ea5db659e945c14dabe
diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
@@ -359,7 +359,13 @@ def _image_directories(func):
            # namespace package pip installed and run via the nose
            # multiprocess plugin or as a specific test this may be
            # missing. See https://github.com/matplotlib/matplotlib/issues/3314
-        assert mods.pop(0) == 'tests'
+        if mods.pop(0) != 'tests':
+            warnings.warn(("Module '%s' does not live in a parent module "
+                "named 'tests'. This is probably ok, but we may not be able "
+                "to guess the correct subdirectory containing the baseline "
+                "images. If things go wrong please make sure that there is "
+                "a parent directory named 'tests' and that it contains a "
+                "__init__.py file (can be empty).") % module_name)
        subdir = os.path.join(*mods)

        import imp