Use URL.join()

aiohttp/client.py

@@ -311,13 +311,11 @@ def request(
         """Perform HTTP request."""
         return _RequestContextManager(self._request(method, url, **kwargs))
 
-    def _build_url(self, url: str) -> URL:
+    def _build_url(self, str_or_url: StrOrURL) -> URL:
         if self._base_url is None:
-            return URL(url)
-        elif url.startswith("/"):
-            return self._base_url / url[1:]
+            return URL(str_or_url)
         else:
-            raise ValueError("url must start with /")
+            return self._base_url.join(URL(str_or_url))
 
     async def _request(
         self,
@@ -378,10 +376,7 @@ async def _request(
         proxy_headers = self._prepare_headers(proxy_headers)
 
         try:
-            if isinstance(str_or_url, URL):
-                url = str_or_url
-            else:
-                url = self._build_url(str_or_url)
+            url = self._build_url(str_or_url)
         except ValueError as e:
             raise InvalidURL(str_or_url) from e
 

tests/test_client_session.py

@@ -725,12 +725,6 @@ async def test_build_url_returns_expected_url(
     assert session._build_url(url) == expected_url
 
 
-async def test_build_url_raises_when_url_not_starts_with_slash(create_session) -> None:
-    session = await create_session("http://example.com")
-    with pytest.raises(ValueError, match="url must start with /"):
-        session._build_url("test")
-
-
 async def test_request_uses_base_url_when_url_is_str(create_session) -> None:
     request_class = mock.MagicMock()
     session = await create_session("http://example.com", request_class=request_class)