Fix URL.build not validating that path has a leading slash when passi…

…ng authority

The validation only worked when host was passed

tests/test_url_build.py

@@ -140,12 +140,18 @@ def test_build_with_invalid_host(host: str, is_authority: bool):
 
 
 def test_build_with_authority():
-    url = URL.build(scheme="http", authority="степан:[email protected]:8000", path="path")
+    url = URL.build(scheme="http", authority="степан:[email protected]:8000", path="/path")
     assert (
         str(url) == "http://%D1%81%D1%82%D0%B5%D0%BF%D0%B0%D0%BD:[email protected]:8000/path"
     )
 
 
+def test_build_with_authority_no_leading_flash():
+    msg = r"Path in a URL with authority should start with a slash \('/'\) if set"
+    with pytest.raises(ValueError, match=msg):
+        URL.build(scheme="http", authority="степан:[email protected]:8000", path="path")
+
+
 def test_build_with_authority_without_encoding():
     url = URL.build(
         scheme="http", authority="foo:[email protected]:8000", path="path", encoded=True

yarl/_url.py

@@ -298,7 +298,8 @@ def __new__(
                 if netloc:
                     if "." in path:
                         path = cls._normalize_path(path)
-                    cls._validate_authority_uri_abs_path(host, path)
+                    if path[0] != "/":
+                        cls._raise_for_authority_missing_abs_path()
 
             query = cls._QUERY_REQUOTER(query) if query else query
             fragment = cls._FRAGMENT_REQUOTER(fragment) if fragment else fragment
@@ -390,7 +391,8 @@ def build(
             if path and netloc:
                 if "." in path:
                     path = cls._normalize_path(path)
-                cls._validate_authority_uri_abs_path(host, path)
+                if path[0] != "/":
+                    cls._raise_for_authority_missing_abs_path()
 
             query_string = (
                 cls._QUERY_QUOTER(query_string) if query_string else query_string
@@ -930,15 +932,10 @@ def suffixes(self) -> tuple[str, ...]:
         return tuple(self._UNQUOTER(suffix) for suffix in self.raw_suffixes)
 
     @staticmethod
-    def _validate_authority_uri_abs_path(host: str, path: str) -> None:
-        """Ensure that path in URL with authority starts with a leading slash.
-
-        Raise ValueError if not.
-        """
-        if host and path and path[0] != "/":
-            raise ValueError(
-                "Path in a URL with authority should start with a slash ('/') if set"
-            )
+    def _raise_for_authority_missing_abs_path() -> None:
+        """Raise when he path in URL with authority starts lacks a leading slash."""
+        msg = "Path in a URL with authority should start with a slash ('/') if set"
+        raise ValueError(msg)
 
     def _make_child(self, paths: "Sequence[str]", encoded: bool = False) -> "URL":
         """