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Add helpers to populate modules in C #86542
Description
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It's currently inconvenient and sometimes error-prone to initialize module members in C. Even stdlib modules are sometimes missing error checks or have reference counting issues in error path. I propose add three helpers to deal with common cases:
- declaration of simple attributes (int, str, bool, float) in an array of structs
- creation and addition of a new type from a type spec
- creation and addition a new exception object
(1) Simple attribute declaration uses a NULL terminated array of PyModuleConst_Def. The internal definition of the type can be hidden by macros. Example:
static PyModuleConst_Def example_constants[] = {
PyModuleConst_None("none_value"),
PyModuleConst_Long("integer", 42),
PyModuleConst_Bool("false_value", 0),
PyModuleConst_Bool("true_value", 1),
PyModuleConst_String("somestring", "Hello"),
PyModuleConst_LongMacro(EXAMPLE_INT),
PyModuleConst_StringMacro(EXAMPLE_STRING),
{NULL},
}A new function "int PyModule_AddConstants(PyObject *module, PyModuleConst_Def *def)" populates the module object with attributes.
(2) Type initialization
The function "PyTypeObject * PyModule_AddNewTypeFromSpec(PyObject *module, PyType_Spec *spec, PyObject *base)" is PyType_FromModuleAndSpec() + PyModule_AddType(). Additionally to PyType_FromModuleAndSpec() it also handles "base" argument with single type instance. The extra feature avoids "PyTuple_Pack(1, baseclass)" in the caller. The function adds a strong reference to the module object and returns a borrowed reference. The borrowed reference is designed module state assignment.
(3) exception creation
The function "PyObject* PyModule_AddNewException(PyObject *module, const char *name, const char *doc, PyObject *base, PyObject *dict)" creates a new exception, calculates the attribute name for the qualified exception name, and adds the exception to the module. It's PyErr_NewExceptionWithDoc() + PyModule_AddObjectRef().
Note about (1): I first proposed to add PyModuleDef.m_constants, however we cannot extend the struct easily becaues it conflicts with stable ABI. Petr mentioned that he has plans to deal with the issue. Also see discussion https://discuss.python.org/t/define-module-constants-in-pymoduledef/5749
With 2 years of extra experience, I see issues in this proposal -- see capi-workgroup/decisions#6 (comment)
If anyone would like to move any part of this proposal forward, feel free to bounce ideas off me. I recommend tackling adding types separately from the module constants.
With 2 years of extra experience, I see issues in this proposal -- see capi-workgroup/decisions#6 (comment)
If anyone would like to move any part of this proposal forward, feel free to bounce ideas off me. I recommend tackling adding types separately from the module constants.
Can you explain your viewpoint? That is,
separate types from module constants. When I was working on #127435, I had to review the content of the proposal. In other words, I want to take over part of the proposal.I suggest to come up with a different mechanism for constants (where creating the Python object is rather trivial and unchanging) and for types (every few releases we get a new
PyType_From*type-creation function). I don't think they should be bundled together in one issue like here.
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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