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PyImport_GetModule() returns whatever is in sys.modules, even if the module is still importing and therefore only partially initialized.
One possibility is to reuse the optimization already done in PyImport_ImportModuleLevelObject():
 /* Optimization: only call _bootstrap._lock_unlock_module() if
 __spec__._initializing is true.
 NOTE: because of this, initializing must be set *before*
 stuffing the new module in sys.modules.
 */
 spec = _PyObject_GetAttrId(mod, &PyId___spec__);
 if (_PyModuleSpec_IsInitializing(spec)) {
 PyObject *value = _PyObject_CallMethodIdObjArgs(interp->importlib,
 &PyId__lock_unlock_module, abs_name,
 NULL);
 if (value == NULL) {
 Py_DECREF(spec);
 goto error;
 }
 Py_DECREF(value);
 }
 Py_XDECREF(spec);
Issue originally mentioned in issue34572.