Add PyMapping_GetOptionalItem() · Issue #106307 · python/cpython (original) (raw)

PyObject_GetItem() raises a KeyError if the key is not found in a mapping. In some cases it should be treated as any other error, but in other cases it should be caught and suppressed. The repeating pattern of PyObject_GetItem() followed by PyErr_ExceptionMatches(PyExc_KeyError) and PyErr_Clear() occurs 7 times in Python/bytecodes.c and at least 5 times in other files.

I propose to add private helper _PyMapping_LookupItem() which combines these three calls to make the code clearer. It also has a special case for exact dict, so eliminates even more repeating code. For example:

 PyObject *m;

if (PyDict_CheckExact(modules)) {

   m = Py_XNewRef(PyDict_GetItemWithError(modules, name));

}
else {
```
   m = PyObject_GetItem(modules, name);
```

   if (_PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {

```
       _PyErr_Clear(tstate);
```
```
   }
```
}
if (_PyErr_Occurred(tstate)) {

if (_PyMapping_LookupItem(modules, name, &m) < 0) { return NULL; }

The interface of _PyMapping_LookupItem() is very similar to other private helper _PyObject_LookupAttr() (see #76752) and to a proposed new C API for PyDict (see #106004).

Linked PRs

gh-106307: Add PyMapping_GetOptionalItem() #106308