5. Python3源碼—字符串（str）對象

5.1. 字符串對象

字符串對象是「變長對象」。

5.1.1. Python中的建立

Python中字符串（strs）對象最重要的建立方法爲PyUnicode_DecodeUTF8Stateful，以下Python語句最終會調用到PyUnicode_DecodeUTF8Stateful：

a = 'hello
b = str('world')

5.1.2. PyUnicode_DecodeUTF8Stateful的C調用棧

詞法解析，最終調到PyUnicode_DecodeUTF8Stateful，調用順序以下：

// ast.c
ast_for_expr
=>ast_for_power
=>ast_for_atom_expr
=>ast_for_atom (case STRING)
=>parsestrplus
=>parsestr

// unicodeobject.c
=> PyUnicode_DecodeUTF8Stateful

5.1.3. PyUnicode_DecodeUTF8Stateful源碼

// unicodeobject.c
PyObject *
PyUnicode_DecodeUTF8Stateful(const char *s,
                             Py_ssize_t size,
                             const char *errors,
                             Py_ssize_t *consumed)
{
    _PyUnicodeWriter writer;
    const char *starts = s;
    const char *end = s + size;

    Py_ssize_t startinpos;
    Py_ssize_t endinpos;
    const char *errmsg = "";
    PyObject *error_handler_obj = NULL;
    PyObject *exc = NULL;
    _Py_error_handler error_handler = _Py_ERROR_UNKNOWN;

    if (size == 0) {
        if (consumed)
            *consumed = 0;
        _Py_RETURN_UNICODE_EMPTY();
    }

    /* ASCII is equivalent to the first 128 ordinals in Unicode. */
    if (size == 1 && (unsigned char)s[0] < 128) {
        if (consumed)
            *consumed = 1;
        return get_latin1_char((unsigned char)s[0]);
    }

    _PyUnicodeWriter_Init(&writer);
    writer.min_length = size;
    if (_PyUnicodeWriter_Prepare(&writer, writer.min_length, 127) == -1)
        goto onError;

    writer.pos = ascii_decode(s, end, writer.data);
    s += writer.pos;
    while (s < end) {
        // ascii解碼後的size小於傳入的size
    }

End:
    if (consumed)
        *consumed = s - starts;

    Py_XDECREF(error_handler_obj);
    Py_XDECREF(exc);
    return _PyUnicodeWriter_Finish(&writer);

onError:
    Py_XDECREF(error_handler_obj);
    Py_XDECREF(exc);
    _PyUnicodeWriter_Dealloc(&writer);
    return NULL;
}

能夠看到：