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HPy Status Update

HPy's first git commit was on Friday, 12 July 2019. Despite the crazy state of the world, HPy has come a long way since then. This is a quick update to say where we are now and what's planned for the immediate future.

What can one do with HPy right now?

One can already port non-trivial Python extensions to the HPy API, but there is also still a lot of work to do. Things one can do right now with HPy include:

  • Initialize an extension module
  • Create custom extension types
  • Create custom methods and functions
  • Interact with many built-in types (including dictionaries, tuples, lists, strings, bytes and longs)
  • Raise and check exceptions
  • Parse arguments to C functions
  • Call Python functions
  • Compile an extension against either the "universal" or "cpython" ABIs (see below)
  • Load a single compiled "universal" extension into different Python implementations.
  • Put a "universal" extension into debug mode at runtime (see below for how this is possible without a performance penalty).
  • Compile extensions for POSIX and Windows (MSVC).

An extension written using HPy may be compiled against either the "cpython" or "universal" ABI:

  • compiling against the "cpython" ABI produces an ordinary Python C extension. The extension uses the HPy API only internally. From the point of view of the Python implementation the compiled module is an ordinary C extension that just happens to have been compiled using HPy. Note that as is the case for ordinary C extensions, the modules produced target a specific ABI tag.

  • compiling against the "universal" ABI produces an HPy C extension. This extension does not use the Python C API directly at all and the ABI is unique (up to platform and HPy version number). The same compiled module may thus be loaded by multiple Python versions and implementations. A Python implementation must know how to load such an extension and have implemented the HPy universal ABI. Universal ABI implementations exist for CPython, PyPy and GraalPython.

Debug mode is implemented by replacing the HPy context with a debug mode context at runtime. The debug context wraps the original context and tracks all HPy API calls. Debug mode already supports finding handles that are leaked (i.e. not closed) and handles that are used after being closed. Additional debug features will be fairly straightforward to add, like for example checking for NULL pointers or detecting whether the user is trying to call a function without holding the GIL.

The debug mode is written using HPy itself, so its implementation can be shared by different Python implementations that support HPy.

What are the next steps?

The big picture is that we're working towards writing a Cython backend and porting core features of numpy to HPy. There is still some way to go but the immediate next steps planned are:

  • Add support for storing long-lived references to other Python objects in custom types, i.e. the equivalent of storing a PyObject * in one of the fields with the struct of a custom type. HPy handles are only for short-lived references, so we need a GC-friendly way to store long-lived references. See issue #9.

  • Implement isolated module level state, to avoid using C static global variables. This will be the HPy equivalent of PEP 573 and PEP 630.

  • Implement multi-phase Python module initialization (HPy currently uses the legacy module initialization). See issue #183.

  • Update the PyPy and GraalPython HPy implementations to support all the latest HPy features.

  • Start work on an HPy backend for Cython.

  • Continuing work on a minimal port of numpy's array type to HPy.

  • Ask the CPython core developers whether we can find a way to support HPy universal extensions directly within in CPython (currently we write a small .py file that knows how to load the universal extension, but the presence of this stub file causes many small edge cases that are annoying for HPy developers and users)

If you'd like to work on any of these with us, drop us a note in the comments or mailing list or join us on IRC.

Happy hacking, The HPy Team.