update get_asset_url function

CHANGELOG.md

@@ -694,3 +694,9 @@ app = dash.Dash(...)
 
 ## 0.17.3 - 2017-06-22
 ✨ This is the initial open-source release of Dash.
+
+### Fixed
+- [#1527](https://github.com/plotly/dash/issues/1527)🐛 `get_asset_url` now pulls from an external source if `assets_external_path` is set.
+- updated `_add_assets_resource` to build asset urls the same way as `get_asset_url`.
+- updated doc string for `assets_external_path` Dash argument to be more clear that it will allways be joined with 
+the `assets_url_path` argument when determining the url to an external asset.

dash/dash.py

@@ -150,10 +150,12 @@ class Dash(object):
     :type assets_ignore: string
 
     :param assets_external_path: an absolute URL from which to load assets.
-        Use with ``serve_locally=False``. Dash can still find js and css to
-        automatically load if you also keep local copies in your assets
-        folder that Dash can index, but external serving can improve
-        performance and reduce load on the Dash server.
+        Use with ``serve_locally=False``. assets_external_path is joined
+        with assets_url_path to determine the absolute url to the
+        asset folder. Dash can still find js and css to automatically load
+        if you also keep local copies in your assets folder that Dash can index,
+        but external serving can improve performance and reduce load on
+        the Dash server.
         env: ``DASH_ASSETS_EXTERNAL_PATH``
     :type assets_external_path: string
 
@@ -1098,8 +1100,9 @@ def _setup_server(self):
     def _add_assets_resource(self, url_path, file_path):
         res = {"asset_path": url_path, "filepath": file_path}
         if self.config.assets_external_path:
-            res["external_url"] = "{}{}".format(
-                self.config.assets_external_path, url_path
+            res["external_url"] = "/".join([
+                self.config.assets_external_path.rstrip('/'), self.config.assets_url_path.strip('/'), url_path.lstrip('/')
+            ]
             )
         self._assets_files.append(file_path)
         return res
@@ -1185,10 +1188,15 @@ def csp_hashes(self, hash_algorithm="sha256"):
         ]
 
     def get_asset_url(self, path):
+        if self.config.assets_external_path:
+            prefix = self.config.assets_external_path
+        else:
+            prefix = self.config.requests_pathname_prefix
+
         asset = get_asset_path(
-            self.config.requests_pathname_prefix,
+            prefix,
             path,
-            self.config.assets_url_path.lstrip("/"),
+            self.config.assets_url_path.lstrip("/")
         )
 
         return asset

tests/unit/test_configs.py

@@ -105,6 +105,28 @@ def test_pathname_prefix_assets(empty_environ, req, expected):
     assert path == expected
 
 
+@pytest.mark.parametrize(
+    "requests_pathname_prefix, assets_external_path, assets_url_path, expected",
+    [
+        (None    , None                        , "assets" , "/assets/reset.css"),
+        ("/app/" , None                        , "assets" , '/app/assets/reset.css'),
+        (None    , None                        , "css"    , "/css/reset.css"),
+        ("/app/" , None                        , "css"    , "/app/css/reset.css"),
+        (None    , "http:\\external.com/", "assets" , "http:\\external.com/assets/reset.css"),
+        ("/app/" , "http:\\external.com/", "css"    , "http:\\external.com/css/reset.css"),
+
+    ],)
+def test_asset_url(empty_environ, requests_pathname_prefix, assets_external_path, assets_url_path, expected):
+    app = Dash('Dash',
+               requests_pathname_prefix=requests_pathname_prefix,
+               assets_external_path=assets_external_path,
+               assets_url_path=assets_url_path
+               )
+
+    path = app.get_asset_url('reset.css')
+    assert path == expected
+
+
 def test_get_combined_config_dev_tools_ui(empty_environ):
     val1 = get_combined_config("ui", None, default=False)
     assert (