Porting guide

Porting PyObject * to HPy API constructs

While in CPython one always uses PyObject * to reference to Python objects, in HPy there are several types of handles that should be used depending on the life-time of the handle: HPy, HPyField, and HPyGlobal.

  • HPy represents short lived handles that live no longer than the duration of one call from Python to HPy extension function. Rule of thumb: use for local variables, arguments, and return values.

  • HPyField represents handles that are Python object struct fields, i.e., live in native memory attached to some Python object.

  • HPyGlobal represents handles stored in C global variables. HPyGlobal can provide isolation between subinterpreters.

WARNING: never use a local variable of type HPyField, for any reason! If the GC kicks in, it might become invalid and become a dangling pointer.

WARNING: never store HPy handles to a long-lived memory, for example: C global variables or Python object structs.

The HPy/HPyField dichotomy might seem arbitrary at first, but it is needed to allow Python implementations to use a moving GC, such as PyPy. It is easier to explain and understand the rules by thinking about how a moving GC interacts with the C code inside an HPy extension.

It is worth remembering that during the collection phase, a moving GC might move an existing object to another memory location, and in that case it needs to update all the places which store a pointer to it. In order to do so, it needs to know where the pointers are. If there is a local C variable which is unknown to the GC but contains a pointer to a GC-managed object, the variable will point to invalid memory as soon as the object is moved.

Back to HPy vs HPyField vs HPyGlobal:

  • HPy handles must be used for all C local variables, function arguments and function return values. They are supposed to be short-lived and closed as soon as they are no longer needed. The debug mode will report a long-lived HPy as a potential memory leak.

  • In PyPy and GraalPython, HPy handles are implemented using an indirection: they are indexes inside a big list of GC-managed objects: this big list is tracked by the GC, so when an object moves its pointer is correctly updated.

  • HPyField is for long-lived references, and the GC must be aware of their location in memory. In PyPy, an HPyField is implemented as a direct pointer to the object, and thus we need a way to inform the GC where it is in memory, so that it can update its value upon moving: this job is done by tp_traverse, as explained in the next section.

  • HPyGlobal is for long-lived references that are supposed to be closed implicitly when the module is unloaded (once module unloading is actually implemented). HPyGlobal provides indirection to isolate subinterpreters. Implementation wise, HPyGlobal will usually contain an index to a table with Python objects stored in the interpreter state.

  • On CPython without subinterpreters support, HPy, HPyGlobal, and HPyField are implemented as PyObject *.

  • On CPython with subinterpreters support, HPyGlobal will be implemented by an indirection through the interpreter state. Note that thanks to the HPy design, switching between this and the more efficient implementation without subinterpreter support will not require rebuilding of the extension (in HPy universal mode), nor rebuilding of CPython.

IMPORTANT: if you write a custom type having HPyField s, you MUST also write a tp_traverse slot. Note that this is different than the old Python.h API, where you need tp_traverse only under certain conditions. See the next section for more details.

IMPORTANT: the contract of tp_traverse is that it must visit all the HPyFields contained within given struct, or more precisely “owned” by given Python object (in the sense of the “owner” argument to HPyField_Store), and nothing more, nothing less. Some Python implementations may choose to not call the provided tp_traverse if they know how to visit all the HPyFields by other means (for example, when they track them internally already). The debug mode will check this contract.

tp_traverse, tp_clear, Py_TPFLAGS_HAVE_GC

Let’s quote the Python.h documentation about GC support

Python’s support for detecting and collecting garbage which involves circular references requires support from object types which are “containers” for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection.

A good rule of thumb is that if your type contains PyObject * fields, you need to:

  1. provide a tp_traverse slot;

  2. provide a tp_clear slot;

  3. add the Py_TPFLAGS_GC to the tp_flags.

However, if you know that your PyObject * fields will contain only “atomic” types, you can avoid these steps.

In HPy the rules are slightly different:

  1. if you have a field of type HPyField, you always MUST provide a tp_traverse. This is needed so that a moving GC can track the relevant areas of memory. However, you MUST NOT rely on tp_traverse to be called;

  2. tp_clear does not exist. On CPython, HPy automatically generates one for you, by using tp_traverse to know which are the fields to clear. Other implementations are free to ignore it, if it’s not needed;

  3. HPy_TPFLAGS_GC is still needed, especially on CPython. If you don’t specify it, your type will not be tracked by CPython’s GC and thus it might cause memory leaks if it’s part of a reference cycle. However, other implementations are free to ignore the flag and track the objects anyway, if their GC implementation allows it.

tp_dealloc and Py_DECREF

Generally speaking, if you have one or more PyObject * fields in the old Python.h, you must provide a tp_dealloc slot where you Py_DECREF all of them. In HPy this is not needed and will be handled automatically by the system.

In particular, when running on top of CPython, HPy will automatically provide a tp_dealloc which decrefs all the fields listed by tp_traverse.


PyModule_AddObject() is replaced with a regular HPy_SetAttr_s(). There is no HPyModule_AddObject() because it has an unusual refcount behaviour (stealing a reference but only when it returns 0).


Py_tp_dealloc becomes HPy_tp_destroy. We changed the name a little bit because only “lightweight” destructors are supported. Use tp_finalize if you really need to do things with the context or with the handle of the object.

Py_tp_methods, Py_tp_members and Py_tp_getset

Py_tp_methods, Py_tp_members and Py_tp_getset are no longer needed. Methods, members and getsets are specified “flatly” together with the other slots, using the standard mechanism of HPyDef_{METH,MEMBER,GETSET} and HPyType_Spec.defines.


PyList_New(5)/PyList_SET_ITEM() becomes:

HPyListBuilder builder = HPyListBuilder_New(ctx, 5);
HPyListBuilder_Set(ctx, builder, 0, h_item0);
HPyListBuilder_Append(ctx, builder, h_item5);
HPy h_list = HPyListBuilder_Build(ctx, builder);

For lists of (say) integers:

HPyListBuilder_i builder = HPyListBuilder_i_New(ctx, 5);
HPyListBuilder_i_Set(ctx, builder, 0, 42);
HPy h_list = HPyListBuilder_i_Build(ctx, builder);

And similar for building tuples or bytes

PyObject_Call and PyObject_CallObject

Both PyObject_Call and PyObject_CallObject are replaced by HPy_CallTupleDict(callable, args, kwargs) in which either or both of args and kwargs may be null handles.

PyObject_Call(callable, args, kwargs) becomes:

HPy result = HPy_CallTupleDict(ctx, callable, args, kwargs);

PyObject_CallObject(callable, args) becomes:

HPy result = HPy_CallTupleDict(ctx, callable, args, HPy_NULL);

If args is not a handle to a tuple or kwargs is not a handle to a dictionary, HPy_CallTupleDict will return HPy_NULL and raise a TypeError. This is different to PyObject_Call and PyObject_CallObject which may segfault instead.


The buffer API in HPy is implemented using the HPy_buffer struct, which looks very similar to Py_buffer (refer to the CPython documentation for the meaning of the fields):

typedef struct {
    void *buf;
    HPy obj;
    HPy_ssize_t len;
    HPy_ssize_t itemsize;
    int readonly;
    int ndim;
    char *format;
    HPy_ssize_t *shape;
    HPy_ssize_t *strides;
    HPy_ssize_t *suboffsets;
    void *internal;
} HPy_buffer;

Buffer slots for HPy types are specified using slots HPy_bf_getbuffer and HPy_bf_releasebuffer on all supported Python versions, even though the matching PyType_Spec slots, Py_bf_getbuffer and Py_bf_releasebuffer, are only available starting from CPython 3.9.