Delayed App Config

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## [Unreleased]
 
 ### Changed
+- [#1876](https://github.com/plotly/dash/pull/1876) Delays finalizing `Dash.config` attributes not used in the constructor until `init_app()`.
 - [#1869](https://github.com/plotly/dash/pull/1869), [#1873](https://github.com/plotly/dash/pull/1873) Upgrade Plotly.js to v2.8.3. This includes:
   - [Feature release 2.5.0](https://github.com/plotly/plotly.js/releases/tag/v2.5.0):
     - 3D traces are now compatible with `no-unsafe-eval` CSP rules.

dash/_utils.py

@@ -137,23 +137,37 @@ def __getattr__(self, key):
         raise AttributeError(key)
 
     def set_read_only(self, names, msg="Attribute is read-only"):
-        object.__setattr__(self, "_read_only", names)
-        object.__setattr__(self, "_read_only_msg", msg)
+        """
+        Designate named attributes as read-only with the corresponding msg
+
+        Method is additive. Making additional calls to this method will update
+        existing messages and add to the current set of _read_only names.
+        """
+        new_read_only = {name: msg for name in names}
+        if getattr(self, "_read_only", False):
+            self._read_only.update(new_read_only)
+        else:
+            object.__setattr__(self, "_read_only", new_read_only)
 
     def finalize(self, msg="Object is final: No new keys may be added."):
         """Prevent any new keys being set."""
         object.__setattr__(self, "_final", msg)
 
     def __setitem__(self, key, val):
-        if key in self.__dict__.get("_read_only", []):
-            raise AttributeError(self._read_only_msg, key)
+        if key in self.__dict__.get("_read_only", {}):
+            raise AttributeError(self._read_only[key], key)
 
         final_msg = self.__dict__.get("_final")
         if final_msg and key not in self:
             raise AttributeError(final_msg, key)
 
         return super().__setitem__(key, val)
 
+    def update(self, other):
+        # Overrides dict.update() to use __setitem__ above
+        for k, v in other.items():
+            self[k] = v
+
     # pylint: disable=inconsistent-return-statements
     def first(self, *names):
         for name in names:

dash/dash.py

@@ -356,9 +356,6 @@ def __init__(
                 "assets_folder",
                 "assets_url_path",
                 "eager_loading",
-                "url_base_pathname",
-                "routes_pathname_prefix",
-                "requests_pathname_prefix",
                 "serve_locally",
                 "compress",
             ],
@@ -427,8 +424,19 @@ def __init__(
 
         self.logger.setLevel(logging.INFO)
 
-    def init_app(self, app=None):
+    def init_app(self, app=None, **kwargs):
         """Initialize the parts of Dash that require a flask app."""
+
+        self.config.update(kwargs)
+        self.config.set_read_only(
+            [
+                "url_base_pathname",
+                "routes_pathname_prefix",
+                "requests_pathname_prefix",
+            ],
+            "Read-only: can only be set in the Dash constructor or during init_app()",
+        )
+
         config = self.config
 
         if app is not None:

tests/unit/dash/test_utils.py

@@ -24,12 +24,13 @@ def test_ddut001_attribute_dict():
     assert a.k == 2
     assert a["k"] == 2
 
-    a.set_read_only(["k", "q"], "boo")
+    a.set_read_only(["k"], "boo")
 
     with pytest.raises(AttributeError) as err:
         a.k = 3
     assert err.value.args == ("boo", "k")
     assert a.k == 2
+    assert a._read_only == {"k": "boo"}
 
     with pytest.raises(AttributeError) as err:
         a["k"] = 3
@@ -38,13 +39,11 @@ def test_ddut001_attribute_dict():
 
     a.set_read_only(["q"])
 
-    a.k = 3
-    assert a.k == 3
-
     with pytest.raises(AttributeError) as err:
         a.q = 3
     assert err.value.args == ("Attribute is read-only", "q")
     assert "q" not in a
+    assert a._read_only == {"k": "boo", "q": "Attribute is read-only"}
 
     a.finalize("nope")
 

tests/unit/test_configs.py

@@ -370,3 +370,19 @@ def test_title():
     assert "<title>Hello World</title>" in app.index()
     app = Dash(title="Custom Title")
     assert "<title>Custom Title</title>" in app.index()
+
+
+def test_app_delayed_config():
+    app = Dash(server=False)
+    app.init_app(app=Flask("test"), requests_pathname_prefix="/dash/")
+
+    assert app.config.requests_pathname_prefix == "/dash/"
+
+    with pytest.raises(AttributeError):
+        app.config.name = "cannot update me"
+
+
+def test_app_invalid_delayed_config():
+    app = Dash(server=False)
+    with pytest.raises(AttributeError):
+        app.init_app(app=Flask("test"), name="too late 2 update")