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bpo-38787: Add PyCFunction_CheckExact() macro for exact type checks #20024

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May 12, 2020
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bpo-38787: Add PyCFunction_CheckExact() macro for exact type checks #20024

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bpo-38787: Add PyCFunction_CheckExact() macro for exact type checks n…
scoder May 10, 2020
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Add news entry.
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Apply suggestions from code review: move News entry, move PyCMethod_C…
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3 changes: 3 additions & 0 deletions Include/cpython/methodobject.h
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,9 @@

PyAPI_DATA(PyTypeObject) PyCMethod_Type;

#define PyCMethod_CheckExact(op) Py_IS_TYPE(op, &PyCMethod_Type)
#define PyCMethod_Check(op) PyObject_TypeCheck(op, &PyCMethod_Type)

/* Macros for direct access to these values. Type checks are *not*
done, so use with care. */
#define PyCFunction_GET_FUNCTION(func) \
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3 changes: 2 additions & 1 deletion Include/methodobject.h
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Expand Up @@ -13,7 +13,8 @@ extern "C" {

PyAPI_DATA(PyTypeObject) PyCFunction_Type;

#define PyCFunction_Check(op) (Py_IS_TYPE(op, &PyCFunction_Type) || (PyType_IsSubtype(Py_TYPE(op), &PyCFunction_Type)))
#define PyCFunction_CheckExact(op) Py_IS_TYPE(op, &PyCFunction_Type)
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I would prefer to exclude PyCFunction_CheckExact() from the limited C API.

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I'm not so sure. We are changing the semantics of PyCFunction_Check() by adding a subtype that it covers. The macro is a part of the limited C-API and thus, the semantic change is a problem all by itself. Either we accept that, then we should add the "obvious" related CheckExact macro, also to the limited API. Or, we reject that change all together. I don't think the latter is an option, though.

Personally, I'd say, since it's a macro, it doesn't hurt, since it has no impact at all on the ABI. Everyone can safely copy and use that macro also on their side, so why not just add it?

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I will not hold this PR just for that. We can should PyCFunction_CheckExact() as it is in Python 3.9, and revisit the issue later. I created https://bugs.python.org/issue40601 to explain my concern: I plan to work on that in Python 3.10, it's too late for Python 3.9.

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Do note that PyCMethod is a bit of a hack in that PyCFunction is still not an "acceptable base type". You can't subclasstype(print), but static types happen to bypass the Py_TPFLAGS_BASETYPE check. This is one more thing to be resolved if we want all types can become heap types.

#define PyCFunction_Check(op) PyObject_TypeCheck(op, &PyCFunction_Type)

typedef PyObject *(*PyCFunction)(PyObject *, PyObject *);
typedef PyObject *(*_PyCFunctionFast) (PyObject *, PyObject *const *, Py_ssize_t);
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/C API/2020-05-10-16-39-08.bpo-38787.XzQ59O.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Add PyCFunction_CheckExact() macro for exact type checks now that we allow subtypes of PyCFunction,
as well as PyCMethod_CheckExact() and PyCMethod_Check() for the new PyCMethod subtype.
2 changes: 1 addition & 1 deletion Objects/abstract.c
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Expand Up @@ -900,7 +900,7 @@ binary_op(PyObject *v, PyObject *w, const int op_slot, const char *op_name)
Py_DECREF(result);

if (op_slot == NB_SLOT(nb_rshift) &&
PyCFunction_Check(v) &&
PyCFunction_CheckExact(v) &&
strcmp(((PyCFunctionObject *)v)->m_ml->ml_name, "print") == 0)
{
PyErr_Format(PyExc_TypeError,
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4 changes: 2 additions & 2 deletions Python/ceval.c
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Expand Up @@ -5054,7 +5054,7 @@ trace_call_function(PyThreadState *tstate,
PyObject *kwnames)
{
PyObject *x;
if (PyCFunction_Check(func)) {
if (PyCFunction_CheckExact(func) || PyCMethod_CheckExact(func)) {
C_TRACE(x, PyObject_Vectorcall(func, args, nargs, kwnames));
return x;
}
Expand Down Expand Up @@ -5115,7 +5115,7 @@ do_call_core(PyThreadState *tstate, PyObject *func, PyObject *callargs, PyObject
{
PyObject *result;

if (PyCFunction_Check(func)) {
if (PyCFunction_CheckExact(func) || PyCMethod_CheckExact(func)) {
C_TRACE(result, PyObject_Call(func, callargs, kwdict));
return result;
}
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