Python 3.15: sentinel, frozendict, UTF-8 by default and experimental JIT

What happened and what to do

Python 3.15 has been released, according to the LWN article published on October 9, 2026. Notable changes include the addition of the built-in sentinel and frozendict types, UTF-8 as the default encoding, package initialization configuration files and improvements to the experimental JIT compiler. The article points to the 'What's new in Python 3.15' document for details on each feature and to the changelog for the full list of changes.

For teams maintaining Python systems, the move to UTF-8 by default requires reviewing file reads and writes, environment variables and integrations with legacy systems that assume another encoding. A useful practice is to set up a CI test matrix with Python 3.15 running alongside the current version, classify failures by category and only then plan the migration. The experimental JIT should be measured on real workloads before being enabled in production. At Wendelmaques, this translates into a review of the codebase, a compatibility test pipeline and a benchmark dashboard so the adoption decision is based on data.

How the consultancy can help

Compatibility diagnosis of your code against Python 3.15: dependency inventory, points sensitive to UTF-8 encoding and a migration plan. Implementation of a CI test matrix and controlled benchmarks of the experimental JIT, with operation and follow-up.

Next step

Send a short description of your Python system, the version in use and your deadline for upgrading. With that, we prepare a scoped proposal for the diagnosis and implementation.