From fd36f32a30ca4d0fea3c0855c9b0ed42bc1dfb8b Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Fri, 24 Apr 2026 20:52:37 +0100
Subject: [PATCH 1/4] Add a switcher for 3.15/3.14

---
 .github/workflows/cpython-wasm.yml | 135 +++++++++++++++--------------
 Makefile                           |   7 ++
 web/index.html                     |  46 ++++++++++
 web/python.worker.mjs              |  15 ++--
 4 files changed, 133 insertions(+), 70 deletions(-)

diff --git a/.github/workflows/cpython-wasm.yml b/.github/workflows/cpython-wasm.yml
index 06855b1..445e9f7 100644
--- a/.github/workflows/cpython-wasm.yml
+++ b/.github/workflows/cpython-wasm.yml
@@ -13,10 +13,6 @@ on:
       - reopened
   workflow_dispatch:
     inputs:
-      cpython_ref:
-        required: true
-        default: "main"
-        description: "Ref of CPython to build"
       deploy_channel:
         required: true
         default: "stable"
@@ -35,23 +31,13 @@ concurrency:
   cancel-in-progress: false
 
 jobs:
-  get-emscripten-version:
-    name: "Read pinned Emscripten/Node versions from CPython"
+  setup:
+    name: "Resolve deploy channel"
     runs-on: ubuntu-latest
     outputs:
-      emscripten_version: ${{ steps.versions.outputs.emscripten_version }}
-      node_version: ${{ steps.versions.outputs.node_version }}
-      cpython_ref: ${{ steps.ref.outputs.cpython_ref }}
       deploy_channel: ${{ steps.channel.outputs.deploy_channel }}
       deploy_destination: ${{ steps.channel.outputs.deploy_destination }}
     steps:
-      - name: "Resolve cpython ref"
-        id: ref
-        env:
-          REF_INPUT: ${{ inputs.cpython_ref }}
-        run: |
-          ref="${REF_INPUT:-main}"
-          echo "cpython_ref=$ref" >> "$GITHUB_OUTPUT"
       - name: "Resolve deploy channel"
         id: channel
         env:
@@ -71,37 +57,23 @@ jobs:
           fi
           echo "deploy_channel=$channel" >> "$GITHUB_OUTPUT"
           echo "deploy_destination=$destination" >> "$GITHUB_OUTPUT"
-      - name: "Checkout cpython @ ${{ steps.ref.outputs.cpython_ref }}"
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-        with:
-          repository: "python/cpython"
-          ref: ${{ steps.ref.outputs.cpython_ref }}
-          persist-credentials: false
-      - name: "Install Python"
-        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
-        with:
-          python-version: "3.x"
-      - name: "Read config.toml"
-        id: versions
-        shell: python
-        run: |
-          import os, tomllib
-          from pathlib import Path
-          cfg = tomllib.loads(Path("Platforms/emscripten/config.toml").read_text())
-          with open(os.environ["GITHUB_OUTPUT"], "a") as f:
-              f.write(f"emscripten_version={cfg['emscripten-version']}\n")
-              f.write(f"node_version={cfg['node-version']}\n")
 
   build:
-    name: "Cross-compile CPython to wasm32-emscripten"
-    needs: get-emscripten-version
+    name: "Build CPython ${{ matrix.version }}"
+    needs: setup
     runs-on: ubuntu-latest
     timeout-minutes: 120
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - version: "3.14"
+            ref: "3.14"
+          - version: "3.15"
+            ref: "main"
     env:
-      CPYTHON_REF: ${{ needs.get-emscripten-version.outputs.cpython_ref }}
-    defaults:
-      run:
-        working-directory: cpython
+      CPYTHON_REF: ${{ matrix.ref }}
+      CPYTHON_VERSION: ${{ matrix.version }}
     steps:
       - name: "Checkout codoscope"
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
@@ -122,42 +94,77 @@ jobs:
         with:
           python-version: "3"
 
+      - name: "Read pinned Emscripten/Node versions from CPython"
+        id: versions
+        shell: python
+        run: |
+          import os, tomllib
+          from pathlib import Path
+          cfg = tomllib.loads(Path("cpython/Platforms/emscripten/config.toml").read_text())
+          with open(os.environ["GITHUB_OUTPUT"], "a") as f:
+              f.write(f"emscripten_version={cfg['emscripten-version']}\n")
+              f.write(f"node_version={cfg['node-version']}\n")
+
       - name: "Install Node"
         uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
         with:
-          node-version: ${{ needs.get-emscripten-version.outputs.node_version }}
+          node-version: ${{ steps.versions.outputs.node_version }}
 
-      - name: "Install Emscripten SDK ${{ needs.get-emscripten-version.outputs.emscripten_version }}"
+      - name: "Install Emscripten SDK ${{ steps.versions.outputs.emscripten_version }}"
         uses: mymindstorm/setup-emsdk@6ab9eb1bda2574c4ddb79809fc9247783eaf9021 # v14
         with:
-          version: ${{ needs.get-emscripten-version.outputs.emscripten_version }}
-          actions-cache-folder: emsdk-cache
+          version: ${{ steps.versions.outputs.emscripten_version }}
+          actions-cache-folder: emsdk-${{ matrix.version }}
 
       - name: "Build"
+        working-directory: cpython
         run: python3 Tools/wasm/emscripten build --quiet
 
-      - name: "Stage site"
-        working-directory: ${{ github.workspace }}
+      - name: "Stage runtime for ${{ matrix.version }}"
         run: |
-          mkdir _site
-          cp -a cpython/cross-build/wasm32-emscripten/build/python/web_example/. _site/
-          rm _site/server.py
-          cp codoscope/web/index.html _site/index.html
-          cp codoscope/web/driver.py  _site/driver.py
-          if [ -f codoscope/web/python.worker.mjs ]; then
-            cp codoscope/web/python.worker.mjs _site/python.worker.mjs
-          fi
+          set -euo pipefail
+          dest="runtime/cpython/${CPYTHON_VERSION}"
+          src="cpython/cross-build/wasm32-emscripten/build/python/web_example"
+          mkdir -p "$dest"
+          cp "$src/python.mjs"  "$dest/"
+          cp "$src/python.wasm" "$dest/"
+          cp "$src/python${CPYTHON_VERSION}.zip" "$dest/"
+
+      - name: "Upload runtime artifact"
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: cpython-runtime-${{ matrix.version }}
+          path: runtime
+          if-no-files-found: error
+          retention-days: 7
+
+  assemble:
+    name: "Assemble site"
+    needs: [setup, build]
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout codoscope"
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: "Download runtime artifacts"
+        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4
+        with:
+          pattern: cpython-runtime-*
+          path: runtime-artifacts
+          merge-multiple: true
 
-      - name: "Add Pages bits"
-        shell: bash
-        working-directory: ${{ github.workspace }}/_site
+      - name: "Assemble site"
         run: |
           set -euo pipefail
+          cp -a web _site
+          cp -a runtime-artifacts/cpython _site/cpython
           # GitHub Pages cannot set COOP/COEP headers, but the Emscripten
           # thingy needs SharedArrayBuffer. coi-serviceworker installs
           # those headers client-side (which costs us one automatic reload).
           # The <script src="coi-serviceworker.js"> tag lives in web/index.html.
-          curl -fsSL -o coi-serviceworker.js \
+          curl -fsSL -o _site/coi-serviceworker.js \
             https://cdn.jsdelivr.net/gh/gzuidhof/coi-serviceworker@7b1d2a092d0d2dd2b7270b6f12f13605de26f214/coi-serviceworker.min.js
 
       - name: "Upload site artifact"
@@ -171,15 +178,15 @@ jobs:
 
   deploy:
     name: "Publish site"
-    needs: [get-emscripten-version, build]
+    needs: [setup, assemble]
     runs-on: ubuntu-latest
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
     permissions:
       contents: write
       pull-requests: write
     env:
-      DEPLOY_CHANNEL: ${{ needs.get-emscripten-version.outputs.deploy_channel }}
-      DEPLOY_DESTINATION: ${{ needs.get-emscripten-version.outputs.deploy_destination }}
+      DEPLOY_CHANNEL: ${{ needs.setup.outputs.deploy_channel }}
+      DEPLOY_DESTINATION: ${{ needs.setup.outputs.deploy_destination }}
     steps:
       - name: "Checkout"
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
@@ -213,7 +220,7 @@ jobs:
           publish_branch: gh-pages
           keep_files: true
           force_orphan: false
-          commit_message: "Publish site from main (cpython@${{ needs.get-emscripten-version.outputs.cpython_ref || 'main' }})"
+          commit_message: "Publish site from main (cpython 3.14/3.15)"
 
       - name: "Comment PR preview URL"
         if: env.DEPLOY_CHANNEL == 'preview'
diff --git a/Makefile b/Makefile
index 15a9fdc..443a435 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,6 @@
 PYTHON ?= python3.13
 WEB_PORT ?= 8000
+CPY_VERSION ?= 3.15
 
 .PHONY: test web web-sync web-kill web-restart
 test:
@@ -10,6 +11,12 @@ web-sync:
 	cp web/index.html _site/index.html
 	cp web/driver.py _site/driver.py
 	cp web/python.worker.mjs _site/python.worker.mjs
+	@mkdir -p _site/cpython/$(CPY_VERSION)
+	@for f in python.mjs python.wasm python$(CPY_VERSION).zip; do \
+		if [ -f _site/$$f ] && [ ! -e _site/cpython/$(CPY_VERSION)/$$f ]; then \
+			mv _site/$$f _site/cpython/$(CPY_VERSION)/$$f; \
+		fi; \
+	done
 
 web: web-sync
 	@echo "Serving codoscope web UI at http://localhost:$(WEB_PORT)"
diff --git a/web/index.html b/web/index.html
index a23b275..e79cc69 100644
--- a/web/index.html
+++ b/web/index.html
@@ -79,6 +79,22 @@
             }
             .toggles input { accent-color: var(--accent); }
             .toggles input:checked + span { color: var(--text); }
+            .pyversion {
+                display: inline-flex;
+                align-items: center;
+                gap: 6px;
+                color: var(--muted);
+            }
+            .pyversion select {
+                background: #232a35;
+                color: var(--text);
+                border: 1px solid var(--panel-border);
+                padding: 4px 6px;
+                border-radius: 4px;
+                font: inherit;
+                cursor: pointer;
+            }
+            .pyversion select:disabled { opacity: 0.5; cursor: wait; }
             main {
                 padding: 14px;
                 display: grid;
@@ -167,6 +183,10 @@
             <h1><strong>codoscope</strong> — <span id="pyver">loading CPython…</span></h1>
             <span class="status" id="status"></span>
             <span class="spacer"></span>
+            <label class="pyversion">
+                <span>Python</span>
+                <select id="pyversion" disabled></select>
+            </label>
             <button id="run" disabled>Run</button>
             <div class="toggles" id="toggles">
                 <label><input type="checkbox" data-view="source"     checked><span>Source</span></label>
@@ -213,13 +233,31 @@ <h2>Code Object</h2><pre class="content"></pre>
             const runBtn   = document.getElementById("run");
             const status   = document.getElementById("status");
             const pyver    = document.getElementById("pyver");
+            const pyversionSelect = document.getElementById("pyversion");
             const panels   = Array.from(document.querySelectorAll(".panel"));
             const toggles  = Array.from(document.querySelectorAll("#toggles input"));
 
             const VIEWS = ["tokens", "ast", "ast-opt", "pseudo", "pseudo-opt", "compiled"];
+            const PYTHON_VERSIONS = ["3.14", "3.15"];
+            const DEFAULT_PYTHON_VERSION = "3.15";
+            const VERSION_STORAGE_KEY = "codoscope:python-version";
 
             let driverSource = null;
 
+            const getPythonDir = () => `./cpython/${pyversionSelect.value}/`;
+
+            const initVersionSelect = () => {
+                for (const v of PYTHON_VERSIONS) {
+                    const opt = document.createElement("option");
+                    opt.value = v;
+                    opt.textContent = v;
+                    pyversionSelect.appendChild(opt);
+                }
+                const stored = localStorage.getItem(VERSION_STORAGE_KEY);
+                pyversionSelect.value = PYTHON_VERSIONS.includes(stored) ? stored : DEFAULT_PYTHON_VERSION;
+                pyversionSelect.disabled = false;
+            };
+
             const applyToggles = () => {
                 for (const box of toggles) {
                     const view = box.dataset.view;
@@ -377,6 +415,7 @@ <h2>Code Object</h2><pre class="content"></pre>
                 worker.postMessage({
                     type: "run",
                     args: ["driver.py"],
+                    pythonDir: getPythonDir(),
                     files: {
                         "driver.py": driverSource,
                         "user.py":   window.editor.getValue(),
@@ -391,6 +430,13 @@ <h2>Code Object</h2><pre class="content"></pre>
 
             runBtn.addEventListener("click", run);
 
+            pyversionSelect.addEventListener("change", () => {
+                localStorage.setItem(VERSION_STORAGE_KEY, pyversionSelect.value);
+                run();
+            });
+
+            initVersionSelect();
+
             window.addEventListener("load", async () => {
                 try {
                     driverSource = await fetch("driver.py", { cache: "no-store" }).then((r) => r.text());
diff --git a/web/python.worker.mjs b/web/python.worker.mjs
index b354ca6..580f3be 100644
--- a/web/python.worker.mjs
+++ b/web/python.worker.mjs
@@ -1,5 +1,3 @@
-import createEmscriptenModule from "./python.mjs";
-
 class StdinBuffer {
   constructor() {
     this.sab = new SharedArrayBuffer(128 * Int32Array.BYTES_PER_ELEMENT);
@@ -61,7 +59,7 @@ const stderr = (charCode) => {
 
 const stdinBuffer = new StdinBuffer();
 
-const emscriptenSettings = {
+const buildEmscriptenSettings = (pythonDir) => ({
   noInitialRun: true,
   stdin: stdinBuffer.stdin,
   stdout: stdout,
@@ -81,7 +79,7 @@ const emscriptenSettings = {
     try {
       const controller = new AbortController();
       const timeoutId = setTimeout(() => controller.abort(), 30000);
-      const resp = await fetch(stdlibName, { signal: controller.signal });
+      const resp = await fetch(`${pythonDir}${stdlibName}`, { signal: controller.signal });
       clearTimeout(timeoutId);
       if (!resp.ok) {
         throw new Error(`failed to fetch ${stdlibName}: HTTP ${resp.status}`);
@@ -96,13 +94,18 @@ const emscriptenSettings = {
       Module.removeRunDependency(depName);
     }
   },
-};
+});
 
-const modulePromise = createEmscriptenModule(emscriptenSettings);
+let modulePromise = null;
 
 onmessage = async (event) => {
   if (event.data.type === "run") {
     try {
+      const pythonDir = event.data.pythonDir || "./";
+      if (!modulePromise) {
+        const mod = await import(`${pythonDir}python.mjs`);
+        modulePromise = mod.default(buildEmscriptenSettings(pythonDir));
+      }
       const Module = await modulePromise;
       if (event.data.files) {
         for (const [filename, contents] of Object.entries(event.data.files)) {

From af475b2f42bdce38fb3d50e3f98dcb3047cac14e Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Fri, 24 Apr 2026 21:22:03 +0100
Subject: [PATCH 2/4] Fix 3.14

---
 web/index.html        |  4 ++--
 web/python.worker.mjs | 10 ++++------
 2 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/web/index.html b/web/index.html
index e79cc69..d25acf3 100644
--- a/web/index.html
+++ b/web/index.html
@@ -346,7 +346,8 @@ <h2>Code Object</h2><pre class="content"></pre>
                 const stdout = [];
                 const stderr = [];
                 let finished = false;
-                const worker = new Worker("./python.worker.mjs", { type: "module" });
+                const workerUrl = `./python.worker.mjs?dir=${encodeURIComponent(getPythonDir())}`;
+                const worker = new Worker(workerUrl, { type: "module" });
                 const startedAt = Date.now();
                 const watchdog = setInterval(() => {
                     if (finished) {
@@ -415,7 +416,6 @@ <h2>Code Object</h2><pre class="content"></pre>
                 worker.postMessage({
                     type: "run",
                     args: ["driver.py"],
-                    pythonDir: getPythonDir(),
                     files: {
                         "driver.py": driverSource,
                         "user.py":   window.editor.getValue(),
diff --git a/web/python.worker.mjs b/web/python.worker.mjs
index 580f3be..8ad6223 100644
--- a/web/python.worker.mjs
+++ b/web/python.worker.mjs
@@ -96,16 +96,14 @@ const buildEmscriptenSettings = (pythonDir) => ({
   },
 });
 
-let modulePromise = null;
+const pythonDir = new URL(self.location.href).searchParams.get("dir");
+const modulePromise = import(`${pythonDir}python.mjs`).then((mod) =>
+  mod.default(buildEmscriptenSettings(pythonDir)),
+);
 
 onmessage = async (event) => {
   if (event.data.type === "run") {
     try {
-      const pythonDir = event.data.pythonDir || "./";
-      if (!modulePromise) {
-        const mod = await import(`${pythonDir}python.mjs`);
-        modulePromise = mod.default(buildEmscriptenSettings(pythonDir));
-      }
       const Module = await modulePromise;
       if (event.data.files) {
         for (const [filename, contents] of Object.entries(event.data.files)) {

From 0321419e5429ec5b409b82e36784067798df14ec Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Fri, 24 Apr 2026 21:50:10 +0100
Subject: [PATCH 3/4] Get full traceback

---
 web/index.html | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/web/index.html b/web/index.html
index d25acf3..28f956b 100644
--- a/web/index.html
+++ b/web/index.html
@@ -367,7 +367,10 @@ <h2>Code Object</h2><pre class="content"></pre>
                         finished = true;
                         clearInterval(watchdog);
                         worker.terminate();
-                        showError(`Worker initialization failed: ${d.error || "unknown error"}`);
+                        const stderrText = String.fromCharCode(...stderr).trimEnd();
+                        const parts = [`Worker error: ${d.error || "unknown error"}`];
+                        if (stderrText) parts.push("", "stderr:", stderrText);
+                        showError(parts.join("\n"));
                         runBtn.disabled = false;
                     } else if (d.type === "ready") {
                         status.textContent = "runtime ready, running…";

From 56070f40f5697b036f302194be4117c4e597457e Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Fri, 24 Apr 2026 22:22:10 +0100
Subject: [PATCH 4/4] Just disable it for now

---
 web/driver.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/web/driver.py b/web/driver.py
index 183489c..4d8a8ed 100644
--- a/web/driver.py
+++ b/web/driver.py
@@ -334,7 +334,7 @@ def view_pseudo(code: str, *, optimize: bool = False) -> dict[str, Any]:
     # On some newer WASM builds (e.g. CPython 3.15 snapshots), optimize_cfg can
     # trap at runtime with low-level wasm errors. Prefer a stable pseudo view
     # there instead of crashing the whole worker process.
-    if optimize and sys.version_info < (3, 15):
+    if optimize and sys.version_info < (3, 14):
         insts = optimize_cfg(insts, co_consts, 0)
     items = _instruction_items(insts)
     adjusted_consts = _apply_annotations_const_workaround(items, co_consts)