rules_python not able to build offline from vendor only directory

[amfern]

🐞 bug report

Description

rules_python isn't able to build things completely offline, and still downloads python dependencies even through everything supposed to be available in the --vendor_dir to support complete offline build.

🔬 Minimal Reproduction

git clone git@github.com:bazel-contrib/rules_python.git

# prepare vendor dir
docker run -e USER="$(id -u)" -u="$(id -u)" -v $HOME/.netrc:/.netrc -v $(pwd):/src/workspace -w /src/workspace/examples/bzlmod gcr.io/bazel-public/bazel:8.4.2 --output_user_root=/tmp/build_output vendor --vendor_dir=my_vendor_dir //...

# run build offline using only the data from my_vendor_dir
docker run --network none -e USER="$(id -u)" -u="$(id -u)" -v $HOME/.netrc:/.netrc -v $(pwd):/src/workspace -w /src/workspace/examples/bzlmod gcr.io/bazel-public/bazel:8.4.2 --output_user_root=/tmp/build_output build --lockfile_mode=error --vendor_dir=my_vendor_dir //...

🔥 Exception or Error

Extracting Bazel installation...
Starting local Bazel server and connecting to it...
Computing main repo mapping:
WARNING: For repository 'bazel_skylib', the root module requires module version bazel_skylib@1.7.1, but got bazel_skylib@1.8.2 in the resolved dependency graph.
WARNING: For repository 'platforms', the root module requires module version platforms@0.0.4, but got platforms@1.0.0 in the resolved dependency graph.
WARNING: For repository 'rules_shell', the root module requires module version rules_shell@0.3.0, but got rules_shell@0.6.1 in the resolved dependency graph.
Loading:
Loading: 0 packages loaded
WARNING: Download from https://pypi.org/simple/ failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException Unknown host: pypi.org
ERROR: /tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/simpleapi_download.bzl:258:39: Traceback (most recent call last):
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/extension.bzl", line 378, column 25, in _pip_impl
                mods = parse_modules(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/extension.bzl", line 267, column 30, in parse_modules
                builder.pip_parse(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/hub_builder.bzl", line 60, column 47, in lambda
                pip_parse = lambda *a, **k: _pip_parse(self, *a, **k),
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/hub_builder.bzl", line 193, column 22, in _pip_parse
                _create_whl_repos(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/hub_builder.bzl", line 542, column 50, in _create_whl_repos
                requirements_by_platform = parse_requirements(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/parse_requirements.bzl", line 196, column 36, in parse_requirements
                index_urls = get_index_urls(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/hub_builder.bzl", line 381, column 144, in lambda
                self._get_index_urls[python_version] = lambda ctx, distributions, *, index_url = None, extra_index_urls = None: self._simpleapi_download_fn(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/simpleapi_download.bzl", line 92, column 31, in simpleapi_download
                dist_urls = _get_dist_urls(
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/simpleapi_download.bzl", line 164, column 43, in _get_dist_urls
                results[index_url] = download.wait()
        File "/tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/external/rules_python~/python/private/pypi/simpleapi_download.bzl", line 258, column 39, in lambda
                result = download.wait(),
Error in wait: java.io.IOException: Error downloading [https://pypi.org/simple/] to /tmp/build_output_offline/6a4437257408fba6059e0f41368f396b/modextwd/rules_python~~pip/https___pypi_org_simple.html: Unknown host: pypi.org
ERROR: error evaluating module extension pip in @@rules_python~//python/extensions:pip.bzl
INFO: Elapsed time: 7.499s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
FAILED:

🌍 Your Environment

**Operating System: **

  
ubuntu 24.04
  

Output of bazel version:

  
bazel 8.4.2
  

Rules_python version:

  
https://github.com/bazel-contrib/rules_python/commit/c33ba2383b7ce23f92fd97fbaa58132341dec2ca
  

Anything else relevant?

There is a test for building from vendor, however it allows network access which makes it ineffective.

rules_python/.bazelci/presubmit.yml

Lines 360 to 362 in c33ba23

	- "bazel vendor --vendor_dir=./vendor //..."
	- "bazel build --vendor_dir=./vendor //..."
	- "rm -rf ./vendor"