Fix joining a path when the existing path is empty (#1118)

CHANGES/1118.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed joining a path when the existing path was empty -- by :user:`bdraco`.
+
+A regression in :meth:`URL.join() <yarl.URL.join>` was introduced in :issue:`1082`.

tests/test_url.py

@@ -1767,6 +1767,14 @@ def test_join_cpython_urljoin(base, url, expected):
     assert joined == expected
 
 
+def test_join_preserves_leading_slash():
+    """Test that join preserves leading slash in path."""
+    base = URL.build(scheme="https", host="localhost", port=443)
+    new = base.join(URL("") / "_msearch")
+    assert str(new) == "https://localhost/_msearch"
+    assert new.path == "/_msearch"
+
+
 def test_empty_authority():
     assert URL("http:///").authority == ""
 

yarl/_url.py

@@ -1379,8 +1379,10 @@ def join(self, url: "URL") -> "URL":
         if not other_val.path:
             return URL(val._replace(**parts), encoded=True)
 
-        if other_val.path[0] == "/" or not val.path:
+        if other_val.path[0] == "/":
             path = other_val.path
+        elif not val.path:
+            path = f"/{other_val.path}"
         elif val.path[-1] == "/":
             path = f"{val.path}{other_val.path}"
         else: