fix(pypi): allow disabling Simple API downloader with index_url = ""

news/pip-default-empty-index-url.fixed.md

@@ -0,0 +1,2 @@
+(pypi) Fixed {obj}`pip.default` so setting `index_url = ""` disables the Simple
+API downloader and falls back to `pip` instead of being ignored.

python/private/pypi/extension.bzl

@@ -33,6 +33,8 @@ load(":simpleapi_download.bzl", "simpleapi_download")
 load(":unified_hub_repo.bzl", "unified_hub_repo")
 load(":whl_library.bzl", "whl_library")
 
+_DEFAULT_INDEX_URL = "https://pypi.org/simple"
+
 def _whl_mods_impl(whl_mods_dict):
     """Implementation of the pip.whl_mods tag class.
 
@@ -249,10 +251,14 @@ def build_config(
                 if platform and not (tag.arch_name or tag.config_settings or tag.env or tag.os_name or tag.whl_abi_tags or tag.whl_platform_tags or tag.marker):
                     defaults["platforms"].pop(platform)
 
+            if tag.index_url != _DEFAULT_INDEX_URL and (
+                mod.is_root or "index_url" not in defaults
+            ):
+                defaults["index_url"] = tag.index_url
+
             _configure(
                 defaults,
                 override = mod.is_root,
-                index_url = tag.index_url,
                 # extra values that we just add
                 auth_patterns = tag.auth_patterns,
                 netrc = tag.netrc,
@@ -264,7 +270,7 @@ def build_config(
     return struct(
         auth_patterns = defaults.get("auth_patterns", {}),
         default_hub = default_hub,
-        index_url = defaults.get("index_url", "https://pypi.org/simple").rstrip("/"),
+        index_url = defaults.get("index_url", _DEFAULT_INDEX_URL).rstrip("/"),
         netrc = defaults.get("netrc", None),
         python_version = defaults.get("python_version", None),
         platforms = {
@@ -687,6 +693,7 @@ Supported keys:
 """,
     ),
     "index_url": attr.string(
+        default = _DEFAULT_INDEX_URL,
         doc = """\
 The index URL to use as a default when downloading packages from PyPI. This is used if nothing is
 specified via `--index-url` or `--extra-index-url` parameters in the `requirements.txt` file or via
@@ -701,13 +708,21 @@ The indexes must support Simple API as described here:
 Index metadata will be used to get the hash digest values for packages even
 if the `--hash` values are not present in the requirements.txt lock file.
 
+Set to `""` to disable the Simple API downloader and fall back to using `pip`
+to download wheels.
+
 Defaults to `https://pypi.org/simple`.
 
 :::{versionadded} 2.0.0
 This has been added as a replacement for 
 {obj}`pip.parse.experimental_index_url` and 
 {obj}`pip.parse.experimental_extra_index_urls`.
 :::
+
+:::{versionchanged} VERSION_NEXT_PATCH
+Setting `index_url = ""` now disables the Simple API downloader instead of
+being ignored.
+:::
 """,
     ),
     "marker": attr.string(

python/private/pypi/hub_builder.bzl

@@ -377,7 +377,11 @@ def _set_get_index_urls(self, mctx, pip_attr):
         pip_attr.experimental_index_url,
         pip_attr.envsubst,
         mctx.getenv,
-    ) or self._config.index_url
+    ) or envsubst(
+        self._config.index_url,
+        pip_attr.envsubst,
+        mctx.getenv,
+    )
     default_extra_index_urls = pip_attr.experimental_extra_index_urls or []
 
     if not default_index_url:

tests/pypi/extension/extension_tests.bzl

@@ -46,7 +46,7 @@ def _default(
         config_settings = None,
         default_hub = "",
         env = None,
-        index_url = None,
+        index_url = "https://pypi.org/simple",
         marker = None,
         netrc = None,
         os_name = None,
@@ -61,7 +61,7 @@ def _default(
         config_settings = config_settings,
         default_hub = default_hub,
         env = env or {},
-        index_url = index_url or "",
+        index_url = index_url,
         marker = marker or "",
         netrc = netrc,
         os_name = os_name,
@@ -761,6 +761,57 @@ simple==0.0.1 \
 
 _tests.append(_test_default_index_setting)
 
+def _test_default_index_url_empty_disables_simpleapi(env):
+    def fail_simpleapi_download(*_args, **_kwargs):
+        fail("simpleapi_download should not be called when index_url is empty")
+
+    for index_url, envsubst, want_whl_library in [
+        ("", [], {}),
+        (
+            "$RULES_PYTHON_PIP_INDEX_URL",
+            ["RULES_PYTHON_PIP_INDEX_URL"],
+            {"envsubst": ["RULES_PYTHON_PIP_INDEX_URL"]},
+        ),
+    ]:
+        pypi = _parse_modules(
+            env,
+            module_ctx = _pypi_mock_mctx(
+                _mod(
+                    name = "my_module",
+                    default = _default_tags_default + [
+                        _default(index_url = index_url),
+                        _default(default_hub = "pypi_a"),
+                    ],
+                    parse = [
+                        _parse(
+                            hub_name = "pypi_a",
+                            python_version = "3.15",
+                            envsubst = envsubst,
+                            requirements_lock = "requirements.txt",
+                        ),
+                    ],
+                ),
+                os_name = "linux",
+                arch_name = "x86_64",
+            ),
+            available_interpreters = {
+                "python_3_15_host": "unit_test_interpreter_target",
+            },
+            minor_mapping = {"3.15": "3.15.19"},
+            simpleapi_download = fail_simpleapi_download,
+        )
+
+        pypi.whl_libraries().contains_exactly({
+            "pypi_a_315_simple": {
+                "config_load": "@pypi_a//:config.bzl",
+                "dep_template": "@pypi_a//{name}:{target}",
+                "python_interpreter_target": "unit_test_interpreter_target",
+                "requirement": "simple==0.0.1 --hash=sha256:deadbeef --hash=sha256:deadbaaf",
+            } | want_whl_library,
+        })
+
+_tests.append(_test_default_index_url_empty_disables_simpleapi)
+
 def extension_test_suite(name):
     """Create the test suite.