From 156786c425f4b4169717def2a3b322fd7e1ab675 Mon Sep 17 00:00:00 2001
From: Bruno Verachten <gounthar@gmail.com>
Date: Wed, 18 Mar 2026 15:48:13 +0100
Subject: [PATCH] feat: fall back to system Python when pre-built binaries
 unavailable

When pre-built Python binaries are not available for the current
architecture (e.g., riscv64), try using system Python as a fallback
instead of failing with an error.

This enables setup-python to work on architectures that don't yet
have pre-built binaries in actions/python-versions, such as riscv64
self-hosted runners. The fallback only activates when:
1. No cached version is found
2. No manifest entry exists for the architecture
3. System python3 exists and satisfies the requested version spec

A warning is emitted so users know the fallback was used.

Fixes #1288

Signed-off-by: Bruno Verachten <gounthar@gmail.com>
---
 dist/setup/index.js | 24 ++++++++++++++++++++++++
 src/find-python.ts  | 28 ++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/dist/setup/index.js b/dist/setup/index.js
index 41363d8b..b53c01a8 100644
--- a/dist/setup/index.js
+++ b/dist/setup/index.js
@@ -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}.`);
+                }
+            }
+        }
+        catch {
+            // System Python not available, fall through to error
+        }
+    }
     if (!installDir) {
         const osInfo = await (0, utils_1.getOSInfo)();
         const msg = [
diff --git a/src/find-python.ts b/src/find-python.ts
index 99c6a7f2..9e8de0da 100644
--- a/src/find-python.ts
+++ b/src/find-python.ts
@@ -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}.`
+          );
+        }
+      }
+    } catch {
+      // System Python not available, fall through to error
+    }
+  }
+
   if (!installDir) {
     const osInfo = await getOSInfo();
     const msg = [