Hi,
I would like to replace runtime checks on C function arguments with
assertions. For example, replace:
PyObject *
PyDict_GetItemWithError(PyObject *op, PyObject *key)
{
if (!PyDict_Check(op)) {
PyErr_BadInternalCall();
return NULL;
}
...
}
with:
PyObject *
PyDict_GetItemWithError(PyObject *op, PyObject *key)
{
assert(PyDict_Check(op));
...
}
I'm not asking to remove all of them. For example,
PyList_GetItem(list, index) will continuing raising an IndexError if
the index is out of range.
I'm asking to replace runtime checks with assertions when the C API is
"obviously" misused: replace PyErr_BadInternalCall(),
_Py_CheckFunctionResult() and _Py_CheckSlotResult() with assertions.
The exact scope should be defined.
--
Python tries to be nice to C extensions authors and calls
PyErr_BadInternalCall() to emit a nice SystemError exception when the
C API is misused. There are many sanity checks run at runtime. We pay
the cost of these runtime checks even if a C extension is well written
(has no bug).
In Python 3.6, I added the _Py_CheckFunctionResult() function to check
the result of any function call. A function must return a non-NULL
object and not raises an exception, or return NULL and raises an
exception. Otherwise, SystemError is raised. For example,
PyObject_Call() uses it to check the PyTypeObject.tp_call result.
In Python 3.10, I added a similar _Py_CheckSlotResult() function to
check the result of slot functions. For example, PyObject_Size()
checks the result of the PyTypeObject.tp_as_sequence.sq_length slot
function.
--
I modified Python 3.8 to be able to use a debug Python build as a
drop-in replacement of a Python release build: debug and release
builds are now ABI compatible. It is no longer needed to rebuild C
extensions in debug mode. My first goal was to be able to use gdb on a
Python binary built with -O0:
https://developers.redhat.com/articles/2021/09/08/debugging-python-c-extensions-gdb
A debug Python build adds many other debug features useful to develop
C extensions:
https://docs.python.org/dev/using/configure.html#python-debug-build
IMO with Python 3.11, it's now easy enough to get a Python debug build
to develop or debug your C extensions. End users should not have to
pay the price of these runtime checks.
On most Linux distribution, a Python debug build is available as
"python3-debug" executable. To check if you are using a debug build,
check if the sys.gettotalrefcount() function is available.
By the way, it is also possible to build Python in release mode with
assertions using "./configure --with-assertions":
https://docs.python.org/dev/using/configure.html#cmdoption-with-assertions
So, what do you think?
--
In 2019, I already proposed this idea on the bug tracker, but I
abandoned my idea:
https://bugs.python.org/issue37406
Victor
--
Night gathers, and now my watch begins. It shall not end until my death.
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