ImportError: cannot import name 'encodestring' from 'base64'

TL;DR: Python 3.9 removed base64.encodestring and your nbformat is too old. Upgrade nbformat and jupyter starts.

The error

ImportError: cannot import name 'encodestring' from 'base64'

Fix it

1. Upgrade nbformat (and the jupyter stack with it):

   pip install -U nbformat notebook

   Expected: installs nbformat new enough to use encodebytes.

2. Verify:

   python -c "import nbformat; print(nbformat.__version__)"

   Expected: imports cleanly.

3. Start jupyter again. Expected: no import error.

When this applies

Use this when jupyter or nbformat raises the encodestring error on Python 3.9+.

When it does not apply

If your own code uses base64.encodestring, switch to base64.encodebytes. If you are on Python 3.8 or older, something else is wrong.

Compatibility

Python 3.9+. base64.encodebytes exists since 3.1, so the new code works everywhere.

Root cause

base64.encodestring was a deprecated alias of encodebytes since Python 3.1 and was deleted in 3.9. Old nbformat imported the alias, so jupyter startup broke on new Python.

Edge cases

The same 3.9 cleanup removed base64.decodestring (use decodebytes); grep old code for both. Upgrading the whole jupyter stack at once avoids the next version-mismatch error.

GitHub issue jupyter/nbformat#215 reports ImportError: cannot import name 'encodestring' from 'base64' when starting jupyter on Python 3.9+, because old nbformat imported the removed alias. The verified fix: base64.encodestring was removed in Python 3.9 (deprecated since 3.1 in favor of encodebytes), so upgrading nbformat to a release that uses the new name resolved the startup failure. Full thread: https://github.com/jupyter/nbformat/issues/215