前言

Python3中有六个标准的数据类型：

• Number
• String
• List
• Tuple
• Set
• Dictionary

本章主要介绍String数据类型

String

类型对象

string在python中，也是一个类型对象，其定义在Python/bltinmodule.c文件中:

    ...
    SETBUILTIN("staticmethod",          &PyStaticMethod_Type);
    SETBUILTIN("str",                   &PyUnicode_Type);
    SETBUILTIN("super",                 &PySuper_Type);
    ...

也就是对应着PyUnicode_Type，其定义在Include/unicodeobject.h:

PyTypeObject PyUnicode_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "str",              /* tp_name */
    sizeof(PyUnicodeObject),        /* tp_size */
    0,                  /* tp_itemsize */
    /* Slots */
    (destructor)unicode_dealloc,    /* tp_dealloc */
    0,                  /* tp_print */
    0,                  /* tp_getattr */
    0,                  /* tp_setattr */
    0,                  /* tp_reserved */
    unicode_repr,           /* tp_repr */
    &unicode_as_number,         /* tp_as_number */
    &unicode_as_sequence,       /* tp_as_sequence */
    &unicode_as_mapping,        /* tp_as_mapping */
    (hashfunc) unicode_hash,        /* tp_hash*/
    0,                  /* tp_call*/
    (reprfunc) unicode_str,     /* tp_str */
    PyObject_GenericGetAttr,        /* tp_getattro */
    0,                  /* tp_setattro */
    0,                  /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
    Py_TPFLAGS_UNICODE_SUBCLASS,    /* tp_flags */
    unicode_doc,            /* tp_doc */
    0,                  /* tp_traverse */
    0,                  /* tp_clear */
    PyUnicode_RichCompare,      /* tp_richcompare */
    0,                  /* tp_weaklistoffset */
    unicode_iter,           /* tp_iter */
    0,                  /* tp_iternext */
    unicode_methods,            /* tp_methods */
    0,                  /* tp_members */
    0,                  /* tp_getset */
    &PyBaseObject_Type,         /* tp_base */
    0,                  /* tp_dict */
    0,                  /* tp_descr_get */
    0,                  /* tp_descr_set */
    0,                  /* tp_dictoffset */
    0,                  /* tp_init */
    0,                  /* tp_alloc */
    unicode_new,            /* tp_new */
    PyObject_Del,           /* tp_free */
};

字符串对象的创建

字符串对象的创建通过unicode_new方法实现，通用定义在Include/unicodeobject.h:

static PyObject *
unicode_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *x = NULL;
    static char *kwlist[] = {"object", "encoding", "errors", 0};
    char *encoding = NULL;
    char *errors = NULL;

    if (type != &PyUnicode_Type)
        return unicode_subtype_new(type, args, kwds);
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|Oss:str",
                                     kwlist, &x, &encoding, &errors))
        return NULL;
    if (x == NULL)
        _Py_RETURN_UNICODE_EMPTY();
    if (encoding == NULL && errors == NULL)
        return PyObject_Str(x);
    else
        return PyUnicode_FromEncodedObject(x, encoding, errors);
}

字符的创建涉及到编码的问题，最终调用了PyUnicode_FromEncodedObject函数：

PyObject *
PyUnicode_FromEncodedObject(PyObject *obj,
                            const char *encoding,
                            const char *errors)
{
    Py_buffer buffer;
    PyObject *v;

    if (obj == NULL) {
        PyErr_BadInternalCall();
        return NULL;
    }

    /* Decoding bytes objects is the most common case and should be fast */
    if (PyBytes_Check(obj)) {
        if (PyBytes_GET_SIZE(obj) == 0)
            _Py_RETURN_UNICODE_EMPTY();
        v = PyUnicode_Decode(
                PyBytes_AS_STRING(obj), PyBytes_GET_SIZE(obj),
                encoding, errors);
        return v;
    }

    if (PyUnicode_Check(obj)) {
        PyErr_SetString(PyExc_TypeError,
                        "decoding str is not supported");
        return NULL;
    }

    /* Retrieve a bytes buffer view through the PEP 3118 buffer interface */
    if (PyObject_GetBuffer(obj, &buffer, PyBUF_SIMPLE) < 0) {
        PyErr_Format(PyExc_TypeError,
                     "decoding to str: need a bytes-like object, %.80s found",
                     Py_TYPE(obj)->tp_name);
        return NULL;
    }

    if (buffer.len == 0) {
        PyBuffer_Release(&buffer);
        _Py_RETURN_UNICODE_EMPTY();
    }

    v = PyUnicode_Decode((char*) buffer.buf, buffer.len, encoding, errors);
    PyBuffer_Release(&buffer);
    return v;
}