Support for config-aware relative paths

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Added
+- [#1073](https://github.com/plotly/dash/pull/1073) A few function `app.relative_path`. Provides support for providing relative paths that are aware of the config, notably `requests_pathmame_prefix`. This is particularly useful for apps deployed on Dash Enterprise where the apps served under a URL prefix (the app name) which is unlike apps served on localhost:8050. Use `app.relative_path` anywhere you would provide a relative pathname, like `dcc.Link(href=app.relative_path('/page-2'))` or even as an alternative to `app.get_asset_url` with e.g. `html.Img(src=app.relative_path('/assets/logo.png'))`.
+
 ### Changed
 - [#1035](https://github.com/plotly/dash/pull/1035) Simplify our build process.
 

dash/_utils.py

@@ -54,6 +54,21 @@ def get_asset_path(requests_pathname, asset_path, asset_url_path):
     )
 
 
+def get_relative_path(requests_pathname, path):
+    if requests_pathname == '/' and not path.startswith('/'):
+        return path
+    elif not path.startswith('/'):
+        return requests_pathname + path
+    return "/".join(
+        [
+            # Only take the first part of the pathname
+            requests_pathname.rstrip("/"),
+            path.lstrip("/")
+        ]
+    )
+
+
+
 # pylint: disable=no-member
 def patch_collections_abc(member):
     return getattr(collections if utils.PY2 else collections.abc, member)

dash/dash.py

@@ -36,6 +36,7 @@
 from . import _watch
 from ._utils import get_asset_path as _get_asset_path
 from ._utils import create_callback_id as _create_callback_id
+from ._utils import get_relative_path as _get_relative_path
 from ._configs import get_combined_config, pathname_configs
 from .version import __version__
 
@@ -1565,6 +1566,28 @@ def get_asset_url(self, path):
 
         return asset
 
+    def get_relative_path(self, path):
+        """
+        Return a path with `requests_pathname_prefix` prefixed before it.
+        Use this function when specifying local URL paths that will work
+        in environments regardless of what `requests_pathname_prefix` is.
+        In some deployment environments, like Dash Enterprise,
+        `requests_pathname_prefix` is set to the application name,
+        e.g. `my-dash-app`.
+        When working locally, `requests_pathname_prefix` might be unset and
+        so a relative URL like `/page-2` can just be `/page-2`.
+        However, when the app is deployed to a URL like `/my-dash-app`, then
+        `app.get_relative_path('/page-2')` will return `/my-dash-app/page-2`.
+        This can be used as an alternative to `get_asset_url` as well with
+        `app.get_relative_path('/assets/logo.png')`
+        """
+        asset = _get_relative_path(
+            self.config.requests_pathname_prefix,
+            path,
+        )
+
+        return asset
+
     def _setup_dev_tools(self, **kwargs):
         debug = kwargs.get("debug", False)
         dev_tools = self._dev_tools = _AttributeDict()

tests/unit/test_configs.py

@@ -12,7 +12,10 @@
     get_combined_config,
     load_dash_env_vars,
 )
-from dash._utils import get_asset_path
+from dash._utils import (
+    get_asset_path,
+    get_relative_path,
+)
 
 
 @pytest.fixture
@@ -156,3 +159,28 @@ def test_load_dash_env_vars_refects_to_os_environ(empty_environ):
 def test_app_name_server(empty_environ, name, server, expected):
     app = Dash(name=name, server=server)
     assert app.config.name == expected
+
+
+@pytest.mark.parametrize(
+    "prefix, partial_path, expected",
+    [
+        ("/", "/", "/"),
+        ("/my-dash-app", "/", "/my-dash-app/"),
+
+        ("/", "/page-1", "/page-1"),
+        ("/my-dash-app", "/page-1", "/my-dash-app/page-1"),
+
+        ("/", "/page-1/", "/page-1/"),
+        ("/my-dash-app", "/page-1/", "/my-dash-app/page-1/"),
+
+        ("/", "/page-1/sub-page-1", "/page-1/sub-page-1"),
+        ("/my-dash-app", "/page-1/sub-page-1", "/my-dash-app/page-1/sub-page-1"),
+
+        ("/", "relative-page-1", "relative-page-1"),
+        ("/my-dash-app", "relative-page-1", "/my-dash-apprelative-page-1"),
+    ]
+)
+
+def test_pathname_prefix_relative_url(prefix, partial_path, expected):
+    path = get_relative_path(prefix, partial_path)
+    assert path == expected