widths param for percy_snapshot and visit_and_snapshot test methods

Author: alexcjohnson

dash/testing/browser.py

@@ -106,6 +106,7 @@ def visit_and_snapshot(
         convert_canvases=False,
         assert_check=True,
         stay_on_page=False,
+        widths=None,
     ):
         try:
             path = resource_path.lstrip("/")
@@ -119,6 +120,7 @@ def visit_and_snapshot(
                 path,
                 wait_for_callbacks=wait_for_callbacks,
                 convert_canvases=convert_canvases,
+                widths=widths,
             )
             if assert_check:
                 assert not self.driver.find_elements_by_css_selector(
@@ -130,10 +132,26 @@ def visit_and_snapshot(
             logger.exception("snapshot at resource %s error", path)
             raise e
 
-    def percy_snapshot(self, name="", wait_for_callbacks=False, convert_canvases=False):
+    def percy_snapshot(
+        self, name="", wait_for_callbacks=False, convert_canvases=False, widths=None
+    ):
         """percy_snapshot - visual test api shortcut to `percy_runner.snapshot`.
-        It also combines the snapshot `name` with the Python version.
+        It also combines the snapshot `name` with the Python version,
+        args:
+        - name: combined with the python version to give the final snapshot name
+        - wait_for_callbacks: default False, whether to wait for Dash callbacks,
+            after an extra second to ensure that any relevant callbacks have
+            been initiated
+        - convert_canvases: default False, whether to convert all canvas elements
+            in the DOM into static images for percy to see. They will be restored
+            after the snapshot is complete.
+        - widths: a list of pixel widths for percy to render the page with. Note
+            that this does not change the browser in which the DOM is constructed,
+            so the width will only affect CSS, not JS-driven layout.
+            Defaults to [375, 1280]
         """
+        if widths is None:
+            widths = [375, 1280]
         snapshot_name = "{} - py{}.{}".format(
             name, sys.version_info.major, sys.version_info.minor
         )
@@ -172,7 +190,7 @@ def percy_snapshot(self, name="", wait_for_callbacks=False, convert_canvases=Fal
             """
             )
 
-            self.percy_runner.snapshot(name=snapshot_name)
+            self.percy_runner.snapshot(name=snapshot_name, widths=widths)
 
             self.driver.execute_script(
                 """
@@ -189,7 +207,7 @@ def percy_snapshot(self, name="", wait_for_callbacks=False, convert_canvases=Fal
             )
 
         else:
-            self.percy_runner.snapshot(name=snapshot_name)
+            self.percy_runner.snapshot(name=snapshot_name, widths=widths)
 
     def take_snapshot(self, name):
         """Hook method to take snapshot when a selenium test fails. The