Process-Global State

C-level static variables. Since this is very low-levelmemory storage, it must be managed carefully.

Per-module State

State local to a module object, allocated dynamically as part of amodule object’s initialization. This isolates the state from otherinstances of the module (including those in other subinterpreters).

Accessed byPyModule_GetState().

Static Type

A type object defined as a C-level static variable, i.e. a compiled-in typeobject.

A static type needs to be shared between module instances and has noinformation of what module it belongs to.Static types do not have__dict__ (although their instances might).

Heap Type

A type object created at run time.

Defining Class

The defining class of a method (either bound or unbound) is the class on whichthe method was defined.A class that merely inherits the method from its base is not the defining class.

For example,int is the defining class ofTrue.to_bytes,True.__floor__ andint.__repr__.

In C, the defining class is the one defined with the correspondingtp_methods or “tp slots”[1] entry.For methods defined in Python, the defining class is saved in the__class__ closure cell.

C-API

The “Python/C API” as described in Python documentation.CPython implements the C-API, but other implementations exist.

Slot methods

The above changes don’t cover slot methods, such astp_iter ornb_add.

The problem with slot methods is that their C API is fixed, so we can’tsimply add a new argument to pass in the defining class.Two possible solutions have been proposed to this problem:

• Look up the class through walking the MRO.This is potentially expensive, but will be usable if performance is nota problem (such as when raising a module-level exception).
• Storing a pointer to the defining class of each slot in a separate table,__typeslots__[2]. This is technically feasible and fast,but quite invasive.

Modules affected by this concern also have the option of usingthread-local state orPEP 567 context variables as a caching mechanism, orelse defining their own reload-friendly lookup caching scheme.

Solving the issue generally is deferred to a future PEP.

Adding module references to heap types

A new factory method will be added to the C-API for creating modules:

PyObject*PyType_FromModuleAndSpec(PyObject*module,PyType_Spec*spec,PyObject*bases)

This acts the same asPyType_FromSpecWithBases, and additionally associatesthe provided module object with the new type. (In CPython, this will setht_module described below.)

Additionally, an accessor,PyObject*PyType_GetModule(PyTypeObject*)will be provided.It will return the type’s associated module if one is set,otherwise it will setTypeError and return NULL.When given a static type, it will always setTypeError and return NULL.

To implement this in CPython, thePyHeapTypeObject struct will get anew member,PyObject*ht_module, that will store a pointer to theassociated module.It will beNULL by default and should not be modified after the typeobject is created.

Theht_module member will not be inherited by subclasses; it needs to beset usingPyType_FromSpecWithBases for each individual type that needs it.

Usually, creating a class withht_module set will create a referencecycle involving the class and the module.This is not a problem, as tearing down modules is not a performance-sensitiveoperation, and module-level functions typically also create reference cycles.The existing “set all module globals to None” code that breaks function cyclesthroughf_globals will also break the new cycles throughht_module.

Passing the defining class to extension methods

A new signature flag,METH_METHOD, will be added for use inPyMethodDef.ml_flags. Conceptually, it addsdefining_classto the function signature.To make the initial implementation easier, the flag can only be used as(METH_FASTCALL|METH_KEYWORDS|METH_METHOD).(It can’t be used with other flags likeMETH_O or bareMETH_FASTCALL,though it may be combined withMETH_CLASS orMETH_STATIC).

C functions for methods defined using this flag combination will be calledusing a new C signature calledPyCMethod:

PyObject*PyCMethod(PyObject*self,PyTypeObject*defining_class,PyObject*const*args,size_tnargsf,PyObject*kwnames)

Additional combinations like(METH_VARARGS|METH_METHOD) may be addedin the future (or even in the initial implementation of this PEP).However,METH_METHOD should always be anadditional flag, i.e., thedefining class should only be passed in if needed.

In CPython, a new structure extendingPyCFunctionObject will be addedto hold the extra information:

typedefstruct{PyCFunctionObjectfunc;PyTypeObject*mm_class;/*Passedas'defining_class'argtotheCfunc*/}PyCMethodObject;

ThePyCFunction implementation will passmm_class into aPyCMethod C function when it finds theMETH_METHOD flag being set.A new macroPyCFunction_GET_CLASS(cls) will be added for easier accesstomm_class.

C methods may continue to use the otherMETH_* signatures if they donot require access to their defining class/module.IfMETH_METHOD is not set, casting toPyCMethodObject is invalid.

Argument Clinic

To support passing the defining class to methods using Argument Clinic,a new converter calleddefining_class will be added to CPython’s ArgumentClinic tool.

Each method may only have one argument using this converter, and it mustappear afterself, or, ifself is not used, as the first argument.The argument will be of typePyTypeObject*.

When used, Argument Clinic will selectMETH_FASTCALL|METH_KEYWORDS|METH_METHOD as the calling convention.The argument will not appear in__text_signature__.

The new converter will initially not be compatible with__init__ and__new__ methods, which cannot use theMETH_METHOD convention.

Helpers

Getting toper-module state from a heap type is a very common task. To makethis easier, a helper will be added:

void*PyType_GetModuleState(PyObject*type)

This function takes a heap type and on success, it returns pointer to the stateof the module that the heap type belongs to.

On failure, two scenarios may occur. When a non-type object, or a type without amodule is passed in,TypeError is set andNULL returned. If the moduleis found, the pointer to the state, which may beNULL, is returned withoutsetting any exception.

Modules Converted in the Initial Implementation

To validate the approach, the_elementtree module will be modified duringthe initial implementation.

Slot methods

A way of passing defining class (or module state) to slot methods may beadded in the future.

A previous version of this PEP proposed a helper function that would determinea defining class by searching the MRO for a class that defines a slot to aparticular function. However, this approach would fail if a class is mutated(which is, for heap types, possible from Python code).Solving this problem is left to future discussions.

Easy creation of types with module references

It would be possible to add aPEP 489 execution slot type to makecreating heap types significantly easier than callingPyType_FromModuleAndSpec.This is left to a future PEP.

It may be good to add a good way to create static exception types from thelimited API. Such exception types could be shared between subinterpreters,but instantiated without needing specific module state.This is also left to possible future discussions.

Optimization

As proposed here, methods defined with theMETH_METHOD flag only supportone specific signature.

If it turns out that other signatures are needed for performance reasons,they may be added.