The first year of free-threaded Python

Summary

Last week, the CPython developers rolled out CPython 3.14.0b1. This week, PyCon 2025 kicks off in Pittsburgh, PA. Both events mark a significant milestone for the effort to ship and stabilize free-threaded Python. This is the story of the first year of that effort and how our team at Quansight played a key role in enabling experimental use of the free-threaded build with real production workflows that depend on a complex set of dependencies. Support for free-threaded Python unlocks the full compute power of modern hardware with multicore CPUs and GPUs now commonplace. In the GIL-enabled build, making full use of parallel algorithms that exploit all available compute resources in Python requires workarounds and careful tuning. The Python threading module is often not used because the GIL prevents useful parallel scaling. Instead, many reach for multiprocessing, but spawning processes is expensive and communicating across processes often requires making expensive copies of data that would not be necessary in a multithreaded program where data is implicitly shared between threads. Having said that, it is not possible for packages that ship compiled code in their release distributions to support the free-threaded build out of the box. Any package shipping native code (many Python packages do that) need to be audited to ensure the package builds and does not suffer thread-safety issues that are impossible on the GIL-enabled build. Disabling the GIL required deep structural changes to the CPython interpreter. Fully supporting the free-threaded build in existing packages similarly requires fixing structural issues that until now were not big problems. Things like use of global state in the implementation of a C extension for convenience or for performance are no longer safe, since the GIL does not protect simultaneous access from Python to the global state, allowing undefined behavior via data races. While it is possible to trigger thread safety issues like this using the ...