check that url prefix starts with /

Author: rmorshea

src/idom/backend/_common.py

@@ -128,3 +128,7 @@ class CommonOptions:
 
     url_prefix: str = ""
     """The URL prefix where IDOM resources will be served from"""
+
+    def __post_init__(self):
+        if self.url_prefix and not self.url_prefix.startswith("/"):
+            raise ValueError("Expected 'url_prefix' to start with '/'")

tests/test_backend/test__common.py

@@ -1,7 +1,19 @@
 import pytest
 
 from idom import html
-from idom.backend._common import traversal_safe_path, vdom_head_elements_to_html
+from idom.backend._common import (
+    CommonOptions,
+    traversal_safe_path,
+    vdom_head_elements_to_html,
+)
+
+
+def test_common_options_url_prefix_starts_with_slash():
+    # no prefix specified
+    CommonOptions(url_prefix="")
+
+    with pytest.raises(ValueError, match="start with '/'"):
+        CommonOptions(url_prefix="not-start-withslash")
 
 
 @pytest.mark.parametrize(