Vendor JS files in python workers SDK as esm modules

[ryanking13]

This updates the vendoring logic of Python workers so that we can include some JavaScript files in our Python SDK package.

Currently, all the files in the python_modules directory (packages installed for Python workers) are uploaded as buffer types. But we would like to make some JavaScript files in our SKD package to be uploaded as ESM types so that they can be imported in the runtime.

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: This is an internal change for python workers

[changeset-bot]

🦋 Changeset detected

Latest commit: 2c0f85f

The changes in this PR will be included in the next version bump.

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[ask-bonk]

There's a minor edge case: for a file like workers/noext (no extension), lastIndexOf(".") returns -1, and name.slice(-1) returns the last character. However, it won't match any entry in VENDORED_JS_EXTENSIONS, so it just falls through to fallback. Not a bug in practice.

The logic is sound. The test coverage is good. The code is well-scoped and doesn't introduce any backward compatibility risks (it only changes behavior for Python workers with files specifically under python_modules/workers/ with .js or .mjs extensions).

LGTM

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commit: 2c0f85f

[workers-devprod]

Codeowners approval required for this PR:

• ✅ @cloudflare/wrangler

Show detailed file reviewers

[github-actions]

⚠️ Issues found

.changeset/poor-feet-like.md

1. Grammar issue: "JS files ... is now handled" should be "JS files ... are now handled" (subject-verb agreement).

2. Experimental feature not called out: This change appears to relate to the "Python workers runtime SDK" which is likely an experimental/beta feature. Per the guidelines, changesets for experimental features should explicitly call this out and note how users can opt in.

3. Limited context: The description could benefit from:

   • What was the previous behavior? (JS files were presumably handled as CommonJS?)
   • Why is this change being made?
   • Any potential impact on users?

Suggested revision:

---
"wrangler": minor
---

JS files under the Python Workers runtime SDK are now handled as ESM modules

This is a change to the experimental Python Workers runtime. Previously, JS files were handled as CommonJS modules; they are now treated as ESM modules for better interoperability.

To opt in to the Python Workers runtime, add `python_workers = true` to your `wrangler.toml` or enable it in your Cloudflare dashboard.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[workers-devprod]

Codeowners reviews satisfied