diff --git a/doc/api/next_api_changes/2018-11-24-AL.rst b/doc/api/next_api_changes/2018-11-24-AL.rst
new file mode 100644
index 000000000000..ed5f50ff3c26
--- /dev/null
+++ b/doc/api/next_api_changes/2018-11-24-AL.rst
@@ -0,0 +1,10 @@
+API changes
+```````````
+The arguments of `matplotlib.testing.compare.calculate_rms` have been renamed
+from ``expectedImage, actualImage``, to ``expected_image, actual_image``.
+
+Deprecations
+````````````
+The ``matplotlib.testing.decorators.switch_backend`` decorator is deprecated.
+Test functions should use ``pytest.mark.backend(...)``, and the mark will be
+picked up by the ``matplotlib.testing.conftest.mpl_test_settings`` fixture.
diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
index 42092dafd7b5..9bab7e02c4d6 100644
--- a/lib/matplotlib/testing/compare.py
+++ b/lib/matplotlib/testing/compare.py
@@ -352,14 +352,14 @@ def crop_to_same(actual_path, actual_image, expected_path, expected_image):
     return actual_image, expected_image
 
 
-def calculate_rms(expectedImage, actualImage):
+def calculate_rms(expected_image, actual_image):
     "Calculate the per-pixel errors, then compute the root mean square error."
-    if expectedImage.shape != actualImage.shape:
+    if expected_image.shape != actual_image.shape:
         raise ImageComparisonFailure(
             "Image sizes do not match expected size: {} "
-            "actual size {}".format(expectedImage.shape, actualImage.shape))
+            "actual size {}".format(expected_image.shape, actual_image.shape))
     # Convert to float to avoid overflowing finite integer types.
-    return np.sqrt(((expectedImage - actualImage).astype(float) ** 2).mean())
+    return np.sqrt(((expected_image - actual_image).astype(float) ** 2).mean())
 
 
 def compare_images(expected, actual, tol, in_decorator=False):
@@ -428,26 +428,26 @@ def compare_images(expected, actual, tol, in_decorator=False):
         expected = convert(expected, True)
 
     # open the image files and remove the alpha channel (if it exists)
-    expectedImage = _png.read_png_int(expected)
-    actualImage = _png.read_png_int(actual)
-    expectedImage = expectedImage[:, :, :3]
-    actualImage = actualImage[:, :, :3]
+    expected_image = _png.read_png_int(expected)
+    actual_image = _png.read_png_int(actual)
+    expected_image = expected_image[:, :, :3]
+    actual_image = actual_image[:, :, :3]
 
-    actualImage, expectedImage = crop_to_same(
-        actual, actualImage, expected, expectedImage)
+    actual_image, expected_image = crop_to_same(
+        actual, actual_image, expected, expected_image)
 
     diff_image = make_test_filename(actual, 'failed-diff')
 
     if tol <= 0:
-        if np.array_equal(expectedImage, actualImage):
+        if np.array_equal(expected_image, actual_image):
             return None
 
     # convert to signed integers, so that the images can be subtracted without
     # overflow
-    expectedImage = expectedImage.astype(np.int16)
-    actualImage = actualImage.astype(np.int16)
+    expected_image = expected_image.astype(np.int16)
+    actual_image = actual_image.astype(np.int16)
 
-    rms = calculate_rms(expectedImage, actualImage)
+    rms = calculate_rms(expected_image, actual_image)
 
     if rms <= tol:
         return None
@@ -481,21 +481,21 @@ def save_diff_image(expected, actual, output):
         File path to save difference image to.
     '''
     # Drop alpha channels, similarly to compare_images.
-    expectedImage = _png.read_png(expected)[..., :3]
-    actualImage = _png.read_png(actual)[..., :3]
-    actualImage, expectedImage = crop_to_same(
-        actual, actualImage, expected, expectedImage)
-    expectedImage = np.array(expectedImage).astype(float)
-    actualImage = np.array(actualImage).astype(float)
-    if expectedImage.shape != actualImage.shape:
+    expected_image = _png.read_png(expected)[..., :3]
+    actual_image = _png.read_png(actual)[..., :3]
+    actual_image, expected_image = crop_to_same(
+        actual, actual_image, expected, expected_image)
+    expected_image = np.array(expected_image).astype(float)
+    actual_image = np.array(actual_image).astype(float)
+    if expected_image.shape != actual_image.shape:
         raise ImageComparisonFailure(
             "Image sizes do not match expected size: {} "
-            "actual size {}".format(expectedImage.shape, actualImage.shape))
-    absDiffImage = np.abs(expectedImage - actualImage)
+            "actual size {}".format(expected_image.shape, actual_image.shape))
+    abs_diff_image = np.abs(expected_image - actual_image)
 
     # expand differences in luminance domain
-    absDiffImage *= 255 * 10
-    save_image_np = np.clip(absDiffImage, 0, 255).astype(np.uint8)
+    abs_diff_image *= 255 * 10
+    save_image_np = np.clip(abs_diff_image, 0, 255).astype(np.uint8)
     height, width, depth = save_image_np.shape
 
     # The PDF renderer doesn't produce an alpha channel, but the
diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
index 3d2cfb8f664f..989cc566bf5f 100644
--- a/lib/matplotlib/testing/decorators.py
+++ b/lib/matplotlib/testing/decorators.py
@@ -487,6 +487,7 @@ def _image_directories(func):
     return str(baseline_dir), str(result_dir)
 
 
+@cbook.deprecated("3.1", alternative="pytest.mark.backend")
 def switch_backend(backend):
 
     def switch_backend_decorator(func):