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feat: fall back to system Python when pre-built binaries unavailable #1289

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156786c
feat: fall back to system Python when pre-built binaries unavailable
gounthar Mar 18, 2026
02cd31a
fix: address review feedback on system Python fallback
gounthar Mar 19, 2026
cb5cf22
test: add 5 tests for system Python fallback
gounthar Mar 19, 2026
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24 changes: 24 additions & 0 deletions dist/setup/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -82997,6 +82997,30 @@ async function useCpythonVersion(version, architecture, updateEnvironment, check
installDir = tc.find('Python', semanticVersionSpec, architecture);
}
}
if (!installDir) {
// Try system Python as fallback (e.g., on architectures without pre-built binaries)
try {
const { exitCode, stdout } = await exec.getExecOutput('python3', [
'-c',
'import sys; print(sys.prefix)'
]);
if (exitCode === 0) {
const systemPrefix = stdout.trim();
const systemVersion = await exec.getExecOutput('python3', [
'-c',
'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}")'
]);
if (systemVersion.exitCode === 0 &&
semver.satisfies(systemVersion.stdout.trim(), semanticVersionSpec)) {
installDir = systemPrefix;
core.warning(`Pre-built Python not available for architecture '${architecture}'. Using system Python ${systemVersion.stdout.trim()} at ${systemPrefix}.`);
}
}
}
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@weiji14 weiji14 Mar 30, 2026 •
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Is this somewhat hardcoded to whatever the default Python version on the runner image is at the moment? I.e. Python 3.12 on the ubuntu-24.04-riscv images? I was testing this branch out using python-version: "3.13" at weiji14/cog3pio@c40e8af, but got the following error output:

Run gounthar/setup-python@cb5cf223af5f1712fe2914b9c669d2008e051022
Installed versions
  Version 3.13 was not found in the local cache
  /usr/bin/python3 -c import sys, os; print(sys.executable + "\n" + f"{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}" + "\n" + sys.prefix + "\n" + os.path.dirname(sys.executable))
  /usr/bin/python3
  3.12.13
  /opt/python-3.12
  /usr/bin
  Error: The version '3.13' with architecture 'riscv64' was not found for Ubuntu 24.04.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
0s

If I change to using python-version: "3.x" (see this commit), it picks up Python 3.12:

  Version 3.x was not found in the local cache
  /usr/bin/python3 -c import sys, os; print(sys.executable + "\n" + f"{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}" + "\n" + sys.prefix + "\n" + os.path.dirname(sys.executable))
  /usr/bin/python3
  3.12.13
  /opt/python-3.12
  /usr/bin
  Warning: Pre-built Python not available for architecture 'riscv64'. Using system Python 3.12.13 at /usr/bin/python3.
  Successfully set up CPython (3.12.13)

but I hit into another error with PyO3/maturin-action (which is a separate issue 🙂). I'm wondering how much effort it would be to allow a different Python version?

catch {
// System Python not available, fall through to error
}
}
if (!installDir) {
const osInfo = await (0, utils_1.getOSInfo)();
const msg = [
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28 changes: 28 additions & 0 deletions src/find-python.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -122,6 +122,34 @@ export async function useCpythonVersion(
}
}

if (!installDir) {
// Try system Python as fallback (e.g., on architectures without pre-built binaries)
try {
const {exitCode, stdout} = await exec.getExecOutput('python3', [
'-c',
'import sys; print(sys.prefix)'
]);
if (exitCode === 0) {
const systemPrefix = stdout.trim();
const systemVersion = await exec.getExecOutput('python3', [
'-c',
'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}")'
]);
if (
systemVersion.exitCode === 0 &&
semver.satisfies(systemVersion.stdout.trim(), semanticVersionSpec)
) {
installDir = systemPrefix;
core.warning(
`Pre-built Python not available for architecture '${architecture}'. Using system Python ${systemVersion.stdout.trim()} at ${systemPrefix}.`
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@gounthar gounthar Mar 19, 2026

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Fixed — the fallback now returns early with outputs captured directly from sys.executable, sys.prefix, and os.path.dirname(sys.executable). No longer sets installDir or relies on versionFromPath()/binDir().

);
}
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@gounthar gounthar Mar 19, 2026

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Fixed — added !freethreaded guard. The fallback is now skipped entirely for free-threaded builds.

}
} catch {
// System Python not available, fall through to error
}
}
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@gounthar gounthar Mar 19, 2026

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Addressed — tests will be added in a follow-up. The structural fixes in this push are the priority.

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@gounthar gounthar Mar 19, 2026

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Fixed — the fallback now only triggers when the manifest has zero entries for the current architecture (!archHasManifestEntries). On x86_64/arm64 where the manifest has entries, the original error behavior is preserved.


if (!installDir) {
const osInfo = await getOSInfo();
const msg = [
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