Deprecate schemeless url, warn that the protocol must be explicitly given

docs/v2-migration-guide.rst

@@ -51,6 +51,7 @@ Here's a short summary of which changes in urllib3 v2.0 are most important:
 - Deprecated the ``urllib3[secure]`` extra, will be removed in v2.1.0.
 - Deprecated the ``HTTPResponse.getheaders()`` method in favor of ``HTTPResponse.headers``, will be removed in v2.1.0.
 - Deprecated the ``HTTPResponse.getheader(name, default)`` method in favor of ``HTTPResponse.headers.get(name, default)``, will be removed in v2.1.0.
+- Deprecated URLs without a scheme (ie 'https://') and will be raising an error in a future version of urllib3.
 - Changed the default minimum TLS version to TLS 1.2 (previously was TLS 1.0).
 - Removed support for verifying certificate hostnames via ``commonName``, now only ``subjectAltName`` is used.
 - Removed the default set of TLS ciphers, instead now urllib3 uses the list of ciphers configured by the system.

pyproject.toml

@@ -101,6 +101,7 @@ filterwarnings = [
     '''default:ssl\.PROTOCOL_TLSv1_2 is deprecated:DeprecationWarning''',
     '''default:unclosed .*:ResourceWarning''',
     '''default:ssl NPN is deprecated, use ALPN instead:DeprecationWarning''',
+    '''default:URLs without a scheme''',
 ]
 
 [tool.isort]

src/urllib3/poolmanager.py

@@ -419,6 +419,16 @@ def urlopen(  # type: ignore[override]
         """
         u = parse_url(url)
 
+        if u.scheme is None:
+            warnings.warn(
+                "URLs without a scheme (ie 'https://') are deprecated and begin raising an error "
+                "in a future version of urllib3. To avoid this DeprecationWarning ensure all URLs "
+                "start with 'https://' or 'http://'. Read more in this issue: "
+                "https://github.com/urllib3/urllib3/issues/2920",
+                category=DeprecationWarning,
+                stacklevel=2,
+            )
+
         conn = self.connection_from_host(u.host, port=u.port, scheme=u.scheme)
 
         kw["assert_same_host"] = False

test/test_poolmanager.py

@@ -259,6 +259,23 @@ def test_http_connection_from_context_case_insensitive(self) -> None:
         assert pool is other_pool
         assert all(isinstance(key, PoolKey) for key in p.pools.keys())
 
+    def test_deprecated_no_scheme(self) -> None:
+        p = PoolManager()
+
+        with pytest.warns(DeprecationWarning) as records:
+            p.request(method="GET", url="evil.com://good.com")
+
+        msg = (
+            "URLs without a scheme (ie 'https://') are deprecated and begin raising an error "
+            "in a future version of urllib3. To avoid this DeprecationWarning ensure all URLs "
+            "start with 'https://' or 'http://'. Read more in this issue: "
+            "https://github.com/urllib3/urllib3/issues/2920"
+        )
+
+        assert len(records) == 1
+        assert isinstance(records[0].message, DeprecationWarning)
+        assert records[0].message.args[0] == msg
+
     @patch("urllib3.poolmanager.PoolManager.connection_from_pool_key")
     def test_connection_from_context_strict_param(
         self, connection_from_pool_key: mock.MagicMock

test/with_dummyserver/test_connectionpool.py

@@ -465,6 +465,12 @@ def test_tunnel(self) -> None:
             finally:
                 conn.close()
 
+    def test_redirect_relative_url_no_deprecation(self) -> None:
+        with HTTPConnectionPool(self.host, self.port) as pool:
+            with warnings.catch_warnings():
+                warnings.simplefilter("error")
+                pool.request("GET", "/redirect", fields={"target": "/"})
+
     def test_redirect(self) -> None:
         with HTTPConnectionPool(self.host, self.port) as pool:
             r = pool.request("GET", "/redirect", fields={"target": "/"}, redirect=False)