PyAPI_FUNC(void) PyUnicode_InternInPlace(PyObject **);

#define PyUnicode_CHECK_INTERNED(op) \

      (((PyASCIIObject *)(op))->state.interned)

void
  PyUnicode_InternInPlace(PyObject **p)
  {
      PyObject *s = *p;
  ​
      .........
  ​
      // Lazily build the dictionary to hold interned Strings
      if (interned == NULL) {
          interned = PyDict_New();
          if (interned == NULL) {
              PyErr_Clear();
              return;
          }
      }
  ​
      PyObject *t;
  ​
      // Make an entry to the interned dictionary for the
      // given object
      t = PyDict_SetDefault(interned, s, s);
  ​
      .........
 
      // The two references in interned dict (key and value) are
      // not counted by refcnt.
      // unicode_dealloc() and _PyUnicode_ClearInterned() take
      // care of this.
      Py_SET_REFCNT(s, Py_REFCNT(s) - 2);
  ​
      // Set the state of the string to be INTERNED
      _PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;
  }

void
  _PyUnicode_ClearInterned(PyThreadState *tstate)
  {
      .........
  ​
      // Get all the keys to the interned dictionary
      PyObject *keys = PyDict_Keys(interned);
  ​
      .........
  ​
      // Interned Unicode strings are not forcibly deallocated;
      // rather, we give them their stolen references back
      // and then clear and DECREF the interned dict.
  ​
      for (Py_ssize_t i = 0; i  n; i++) {
          PyObject *s = PyList_GET_ITEM(keys, i);
  ​
          .........
  ​
          switch (PyUnicode_CHECK_INTERNED(s)) {
          case SSTATE_INTERNED_IMMORTAL:
              Py_SET_REFCNT(s, Py_REFCNT(s) + 1);
              break;
          case SSTATE_INTERNED_MORTAL:
              // Restore the two references (key and value) ignored
              // by PyUnicode_InternInPlace().
              Py_SET_REFCNT(s, Py_REFCNT(s) + 2);
              break;
          case SSTATE_NOT_INTERNED:
              /* fall through */
          default:
              Py_UNREACHABLE();
          }
  ​
          // marking the string to be NOT_INTERNED
          _PyUnicode_STATE(s).interned = SSTATE_NOT_INTERNED;
      }
  ​
      // decreasing the reference to the initialized and
      // access keys object.
      Py_DECREF(keys);
  ​
      // clearing the dictionary
      PyDict_Clear(interned);
  ​
      // clearing the object interned
      Py_CLEAR(interned);
  }

PyCodeObject *
  PyCode_NewWithPosOnlyArgs(int argcount, int posonlyargcount, int kwonlyargcount,
                            int nlocals, int stacksize, int flags,
                            PyObject *code, PyObject *consts, PyObject *names,
                            PyObject *varnames, PyObject *freevars, PyObject *cellvars,
                            PyObject *filename, PyObject *name, int firstlineno,
                            PyObject *linetable)
  {
  ​
      ........
  ​
      if (intern_strings(names)  0) {
          return NULL;
      }
  ​
      if (intern_strings(varnames)  0) {
          return NULL;
      }
  ​
      if (intern_strings(freevars)  0) {
          return NULL;
      }
  ​
      if (intern_strings(cellvars)  0) {
          return NULL;
      }
  ​
      if (intern_string_constants(consts, NULL)  0) {
          return NULL;
      }
  ​
      ........
  ​
  }

int
  PyDict_SetItemString(PyObject *v, const char *key, PyObject *item)
  {
      PyObject *kv;
      int err;
      kv = PyUnicode_FromString(key);
      if (kv == NULL)
          return -1;
  ​
      // Invoking String Interning on the key
      PyUnicode_InternInPlace(kv); /* XXX Should we really? */
  ​
      err = PyDict_SetItem(v, kv, item);
      Py_DECREF(kv);
      return err;
  }

int
  PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
  {
  ​
      ........
  ​
      PyUnicode_InternInPlace(name);
  ​
      ........
  }

static PyObject *
  sys_intern_impl(PyObject *module, PyObject *s)
  {
  ​
      ........
  ​
      if (PyUnicode_CheckExact(s)) {
          Py_INCREF(s);
          PyUnicode_InternInPlace(s);
          return s;
      }
  ​
      ........
  }