PEP 553 built-in breakpoint() function (bpo-31353) (#3355)

Implement PEP 553, built-in breakpoint() with support from sys.breakpointhook(), along with documentation and tests.  Closes bpo-31353

Python/sysmodule.c

@@ -96,6 +96,81 @@ PySys_SetObject(const char *name, PyObject *v)
         return PyDict_SetItemString(sd, name, v);
 }
 
+static PyObject *
+sys_breakpointhook(PyObject *self, PyObject **args, Py_ssize_t nargs, PyObject *keywords)
+{
+    assert(!PyErr_Occurred());
+    char *envar = Py_GETENV("PYTHONBREAKPOINT");
+
+    if (envar == NULL || strlen(envar) == 0) {
+        envar = "pdb.set_trace";
+    }
+    else if (!strcmp(envar, "0")) {
+        /* The breakpoint is explicitly no-op'd. */
+        Py_RETURN_NONE;
+    }
+    char *last_dot = strrchr(envar, '.');
+    char *attrname = NULL;
+    PyObject *modulepath = NULL;
+
+    if (last_dot == NULL) {
+        /* The breakpoint is a built-in, e.g. PYTHONBREAKPOINT=int */
+        modulepath = PyUnicode_FromString("builtins");
+        attrname = envar;
+    }
+    else {
+        /* Split on the last dot; */
+        modulepath = PyUnicode_FromStringAndSize(envar, last_dot - envar);
+        attrname = last_dot + 1;
+    }
+    if (modulepath == NULL) {
+        return NULL;
+    }
+
+    PyObject *fromlist = Py_BuildValue("(s)", attrname);
+    if (fromlist == NULL) {
+        Py_DECREF(modulepath);
+        return NULL;
+    }
+    PyObject *module = PyImport_ImportModuleLevelObject(
+        modulepath, NULL, NULL, fromlist, 0);
+    Py_DECREF(modulepath);
+    Py_DECREF(fromlist);
+
+    if (module == NULL) {
+        goto error;
+    }
+
+    PyObject *hook = PyObject_GetAttrString(module, attrname);
+    Py_DECREF(module);
+
+    if (hook == NULL) {
+        goto error;
+    }
+    PyObject *retval = _PyObject_FastCallKeywords(hook, args, nargs, keywords);
+    Py_DECREF(hook);
+    return retval;
+
+  error:
+    /* If any of the imports went wrong, then warn and ignore. */
+    PyErr_Clear();
+    int status = PyErr_WarnFormat(
+        PyExc_RuntimeWarning, 0,
+        "Ignoring unimportable $PYTHONBREAKPOINT: \"%s\"", envar);
+    if (status < 0) {
+        /* Printing the warning raised an exception. */
+        return NULL;
+    }
+    /* The warning was (probably) issued. */
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(breakpointhook_doc,
+"breakpointhook(*args, **kws)\n"
+"\n"
+"This hook function is called by built-in breakpoint().\n"
+);
+
 /* Write repr(o) to sys.stdout using sys.stdout.encoding and 'backslashreplace'
    error handler. If sys.stdout has a buffer attribute, use
    sys.stdout.buffer.write(encoded), otherwise redecode the string and use
@@ -1365,6 +1440,8 @@ sys_getandroidapilevel(PyObject *self)
 
 static PyMethodDef sys_methods[] = {
     /* Might as well keep this in alphabetic order */
+    {"breakpointhook",  (PyCFunction)sys_breakpointhook,
+     METH_FASTCALL | METH_KEYWORDS, breakpointhook_doc},
     {"callstats", (PyCFunction)sys_callstats, METH_NOARGS,
      callstats_doc},
     {"_clear_type_cache",       sys_clear_type_cache,     METH_NOARGS,
@@ -1977,6 +2054,9 @@ _PySys_BeginInit(void)
                                PyDict_GetItemString(sysdict, "displayhook"));
     SET_SYS_FROM_STRING_BORROW("__excepthook__",
                                PyDict_GetItemString(sysdict, "excepthook"));
+    SET_SYS_FROM_STRING_BORROW(
+        "__breakpointhook__",
+        PyDict_GetItemString(sysdict, "breakpointhook"));
     SET_SYS_FROM_STRING("version",
                          PyUnicode_FromString(Py_GetVersion()));
     SET_SYS_FROM_STRING("hexversion",