[Python-Dev] Question about the current implementation of str

Victor Stinner victor.stinner at gmail.com
Sat Apr 9 05:09:37 EDT 2016

2016-04-09 9:52 GMT+02:00 Victor Stinner <victor.stinner at gmail.com>:
> But the hash is used as an heuristic to decide if a string is "immutable" or
> not, the refcount is also used by the heuristic. If the string is immutable,
> an operation like resize must create a new string.

I'm talking about this private function:

static int
unicode_modifiable(PyObject *unicode)
    if (Py_REFCNT(unicode) != 1)
        return 0;
    if (_PyUnicode_HASH(unicode) != -1)
        return 0;
    if (PyUnicode_CHECK_INTERNED(unicode))
        return 0;
    if (!PyUnicode_CheckExact(unicode))
        return 0;
#ifdef Py_DEBUG
    /* singleton refcount is greater than 1 */
    return 1;


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