大师兄的Python源码学习笔记(四）: 字符串对象

大师兄的Python源码学习笔记(三）: 整数对象
大师兄的Python源码学习笔记(五）: List对象

一、关于字符串对象

• Python2和Python3的字符串对象有显著差别,Python3中的文本总是unicode,而二进制数据是bytes:

Python2对应类型	Python3对应类型
str	bytes
unicode	str

• 8-bit字符串对象在Python2中为PyStringObject, Python3后改由PyBytesObject实现。
• unicode字符串对象在Python3中由PyUnicodeObject实现。

1 PyBytesObject对象

• PyBytesObject属于变长对象(PyVarObject)中的不可变对象(immutable)。

比如"Hello"和"World!"是两个不同的PyBytesObject，他们所占的内存空间不同。
但当创建后，他们内部维护的字符串就不能改变了。

1.1 PyBytesObject的结构

include/bytesobject.h

typedef struct {
    PyObject_VAR_HEAD
    Py_hash_t ob_shash;
    char ob_sval[1];

    /* Invariants:
     *     ob_sval contains space for 'ob_size+1' elements.
     *     ob_sval[ob_size] == 0.
     *     ob_shash is the hash of the string or -1 if not computed yet.
     */
} PyBytesObject;

• 由以上代码可以发现对象头部是一个PyObject_VAR_HEAD，为变长对象的公共部分。
• ob_shash是这个对象存储的字节数组的哈希值，如果哈希值没有计算则为-1。
• ob_sval指向内存中的真实的位置，这段内存保存实际的字符串。
• PyObject_VAR_HEAD中还包含ob_size，记录ob_sval的可变长度大小。
• ob_sval指向的字符串必须满足ob_sval[ob_size] = '\0'。

1.2 计算哈希值

• PyBytesObject中的哈希值保存在ob_shash中，为了避免每一次都需要重新计算哈希值。
• 在计算哈希值时，使用以下算法：

objects/bytesobject.c

static Py_hash_t
bytes_hash(PyBytesObject *a)
{
    if (a->ob_shash == -1) {
        /* Can't fail */
        a->ob_shash = _Py_HashBytes(a->ob_sval, Py_SIZE(a));
    }
    return a->ob_shash;
}

1.3 类型对象

• PyBytesObject对应的类型对象为PyBytes_Type:

objects/bytesobject.c

PyTypeObject PyBytes_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "bytes",
    PyBytesObject_SIZE,
    sizeof(char),
    bytes_dealloc,                      /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    (reprfunc)bytes_repr,                       /* tp_repr */
    &bytes_as_number,                           /* tp_as_number */
    &bytes_as_sequence,                         /* tp_as_sequence */
    &bytes_as_mapping,                          /* tp_as_mapping */
    (hashfunc)bytes_hash,                       /* tp_hash */
    0,                                          /* tp_call */
    bytes_str,                                  /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    &bytes_as_buffer,                           /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
        Py_TPFLAGS_BYTES_SUBCLASS,              /* tp_flags */
    bytes_doc,                                  /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    (richcmpfunc)bytes_richcompare,             /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    bytes_iter,                                 /* tp_iter */
    0,                                          /* tp_iternext */
    bytes_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 */
    bytes_new,                                  /* tp_new */
    PyObject_Del,                               /* tp_free */
};

• 在类型中，tp_itemsize被设置为一个字节大小sizeof(char),指明了变长对象元素的单位长度。

 &bytes_as_number,                           /* tp_as_number */
 &bytes_as_sequence,                         /* tp_as_sequence */
 &bytes_as_mapping,                          /* tp_as_mapping */

• 此外，tp_as_number、tp_as_sequence、tp_as_mapping全都设置了，表示支持数值操作、序列操作和映射操作。

2. PyUnicodeObject对象

2.1 PyUnicodeObject对象结构

• 基本就是各种字符map的集合。

Include\unicodeobject.h

typedef struct {
    PyCompactUnicodeObject _base;
    union {
        void *any;
        Py_UCS1 *latin1;
        Py_UCS2 *ucs2;
        Py_UCS4 *ucs4;
    } data;                     /* Canonical, smallest-form Unicode buffer */
} PyUnicodeObject;

Include\unicodeobject.h

typedef struct {
    PyASCIIObject _base;
    Py_ssize_t utf8_length;     /* Number of bytes in utf8, excluding the
                                 * terminating \0. */
    char *utf8;                 /* UTF-8 representation (null-terminated) */
    Py_ssize_t wstr_length;     /* Number of code points in wstr, possible
                                 * surrogates count as two code points. */
} PyCompactUnicodeObject;

2.2 PyUnicodeObject类型对象

• PyUnicodeObject对应的类型为PyUnicode_Type,与PyBytes_Type类似。

object\unicodeobject.c

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 */
};

二、创建PyBytesObject对象

• 从C的原生字符串创建PyBytesObject有两条路径：_PyBytes_FromString和PyBytes_FromStringAndSize。

1. _PyBytes_FromString

• 此方法的参数str必须是指向一个固定长度的空终止字符串。

objects/bytesobject.c

PyObject *
PyBytes_FromString(const char *str)
{
    size_t size;
    PyBytesObject *op;

    assert(str != NULL);
    size = strlen(str);
    if (size > PY_SSIZE_T_MAX - PyBytesObject_SIZE) {
        PyErr_SetString(PyExc_OverflowError,
            "byte string is too long");
        return NULL;
    }
    if (size == 0 && (op = nullstring) != NULL) {
#ifdef COUNT_ALLOCS
        null_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }
    if (size == 1 && (op = characters[*str & UCHAR_MAX]) != NULL) {
#ifdef COUNT_ALLOCS
        one_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }

    /* Inline PyObject_NewVar */
    op = (PyBytesObject *)PyObject_MALLOC(PyBytesObject_SIZE + size);
    if (op == NULL)
        return PyErr_NoMemory();
    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
    op->ob_shash = -1;
    memcpy(op->ob_sval, str, size+1);
    /* share short strings */
    if (size == 0) {
        nullstring = op;
        Py_INCREF(op);
    } else if (size == 1) {
        characters[*str & UCHAR_MAX] = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;
}

• 此方法首先判断传入指针指向的str长度，如果超出PY_SSIZE_T_MAX - PyBytesObject_SIZE长度则报错，PY_SSIZE_T_MAX的值与平台相关，在win32系统下约为2GB，64位系统约为4GB。

 size_t size;
     PyBytesObject *op;

 assert(str != NULL);
    size = strlen(str);
    if (size > PY_SSIZE_T_MAX - PyBytesObject_SIZE) {
        PyErr_SetString(PyExc_OverflowError,
            "byte string is too long");
        return NULL;
    }

• 之后，会检查str是不是空串，如果是第一次在一个空字符串基础上创建PyBytesObject对象，会为这个空字符串建立一个对象，并通过intern机制共享。

if (size == 0 && (op = nullstring) != NULL) {
#ifdef COUNT_ALLOCS
        null_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }

• 如果不是第一次，则直接引用。

 if (size == 1 && (op = characters[*str & UCHAR_MAX]) != NULL) {
#ifdef COUNT_ALLOCS
        one_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }

• 如果不是创建空字符串对象，则申请内存创建PyBytesObject对象。

op = (PyBytesObject *)PyObject_MALLOC(PyBytesObject_SIZE + size);
    if (op == NULL)
        return PyErr_NoMemory();
    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
    op->ob_shash = -1;
    memcpy(op->ob_sval, str, size+1);
    /* share short strings */
    if (size == 0) {
        nullstring = op;
        Py_INCREF(op);
    } else if (size == 1) {
        characters[*str & UCHAR_MAX] = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;

• 新创建的PyBytesObject布局:

  
  

2. PyBytes_FromStringAndSize

• 与PyBytes_FromString类似，但是不要求传入的str指针必须是空终止字符串，因为另一个参数size可以确定字符个数。

objects/bytesobject.c

PyObject *
PyBytes_FromStringAndSize(const char *str, Py_ssize_t size)
{
    PyBytesObject *op;
    if (size < 0) {
        PyErr_SetString(PyExc_SystemError,
            "Negative size passed to PyBytes_FromStringAndSize");
        return NULL;
    }
    if (size == 1 && str != NULL &&
        (op = characters[*str & UCHAR_MAX]) != NULL)
    {
#ifdef COUNT_ALLOCS
        one_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }

    op = (PyBytesObject *)_PyBytes_FromSize(size, 0);
    if (op == NULL)
        return NULL;
    if (str == NULL)
        return (PyObject *) op;

    memcpy(op->ob_sval, str, size);
    /* share short strings */
    if (size == 1) {
        characters[*str & UCHAR_MAX] = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;
}

objects/bytesobject.c

static PyObject *
_PyBytes_FromSize(Py_ssize_t size, int use_calloc)
{
    PyBytesObject *op;
    assert(size >= 0);

    if (size == 0 && (op = nullstring) != NULL) {
#ifdef COUNT_ALLOCS
        null_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }

    if ((size_t)size > (size_t)PY_SSIZE_T_MAX - PyBytesObject_SIZE) {
        PyErr_SetString(PyExc_OverflowError,
                        "byte string is too large");
        return NULL;
    }

    /* Inline PyObject_NewVar */
    if (use_calloc)
        op = (PyBytesObject *)PyObject_Calloc(1, PyBytesObject_SIZE + size);
    else
        op = (PyBytesObject *)PyObject_Malloc(PyBytesObject_SIZE + size);
    if (op == NULL)
        return PyErr_NoMemory();
    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
    op->ob_shash = -1;
    if (!use_calloc)
        op->ob_sval[size] = '\0';
    /* empty byte string singleton */
    if (size == 0) {
        nullstring = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;
}

三、字符串驻留机制

• 与数值类型一样，字符串都是一样的不可变对象，不需要用不同的对象来区分。
• 字符串驻留(intern)机制将值同样的字符串对象分享共用，避免频繁创建和销毁内存，提升效率。
• 由于与unicode字符相关，intern机制是在PyUnicodeObject对象中实现的：

objects\unicodeobject.c

void
PyUnicode_InternInPlace(PyObject **p)
{
    PyObject *s = *p;
    PyObject *t;
#ifdef Py_DEBUG
    assert(s != NULL);
    assert(_PyUnicode_CHECK(s));
#else
    if (s == NULL || !PyUnicode_Check(s))
        return;
#endif
    /* If it's a subclass, we don't really know what putting
       it in the interned dict might do. */
    if (!PyUnicode_CheckExact(s))
        return;
    if (PyUnicode_CHECK_INTERNED(s))
        return;
    if (interned == NULL) {
        interned = PyDict_New();
        if (interned == NULL) {
            PyErr_Clear(); /* Don't leave an exception */
            return;
        }
    }
    Py_ALLOW_RECURSION
    t = PyDict_SetDefault(interned, s, s);
    Py_END_ALLOW_RECURSION
    if (t == NULL) {
        PyErr_Clear();
        return;
    }
    if (t != s) {
        Py_INCREF(t);
        Py_SETREF(*p, t);
        return;
    }
    /* The two references in interned are not counted by refcnt.
       The deallocator will take care of this */
    Py_REFCNT(s) -= 2;
    _PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;
}

• 首先检查传入的是否是PyUnicodeObject对象：

#ifdef Py_DEBUG
    assert(s != NULL);
    assert(_PyUnicode_CHECK(s));
#else
    if (s == NULL || !PyUnicode_Check(s))
        return;
#endif

• 再检查传入的对象是否已经被intern机制处理过,如果没有处理过则返回:

  if (!PyUnicode_CheckExact(s))
        return;
    if (PyUnicode_CHECK_INTERNED(s))
        return;
    if (interned == NULL) {
        interned = PyDict_New();
        if (interned == NULL) {
            PyErr_Clear(); /* Don't leave an exception */
            return;
        }
    }

• intern实际上指向一个包含所有unicode字符串的字典。

objects\unicodeobject.c

/* This dictionary holds all interned unicode strings.  Note that references
   to strings in this dictionary are *not* counted in the string's ob_refcnt.
   When the interned string reaches a refcnt of 0 the string deallocation
   function will delete the reference from this dictionary.

   Another way to look at this is that to say that the actual reference
   count of a string is:  s->ob_refcnt + (s->state ? 2 : 0)
*/
static PyObject *interned = NULL;

• 如果处理过，则将指针指向字符串地址并计数。

Py_ALLOW_RECURSION
    t = PyDict_SetDefault(interned, s, s);
    Py_END_ALLOW_RECURSION
    if (t == NULL) {
        PyErr_Clear();
        return;
    }
    if (t != s) {
        Py_INCREF(t);
        Py_SETREF(*p, t);
        return;
    }
    /* The two references in interned are not counted by refcnt.
       The deallocator will take care of this */
    Py_REFCNT(s) -= 2;
    _PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;

• 释放interned string:

objects\unicodeobject.c

void
_Py_ReleaseInternedUnicodeStrings(void)
{
    PyObject *keys;
    PyObject *s;
    Py_ssize_t i, n;
    Py_ssize_t immortal_size = 0, mortal_size = 0;

    if (interned == NULL || !PyDict_Check(interned))
        return;
    keys = PyDict_Keys(interned);
    if (keys == NULL || !PyList_Check(keys)) {
        PyErr_Clear();
        return;
    }

    /* Since _Py_ReleaseInternedUnicodeStrings() is intended to help a leak
       detector, interned unicode strings are not forcibly deallocated;
       rather, we give them their stolen references back, and then clear
       and DECREF the interned dict. */

    n = PyList_GET_SIZE(keys);
    fprintf(stderr, "releasing %" PY_FORMAT_SIZE_T "d interned strings\n",
            n);
    for (i = 0; i < n; i++) {
        s = PyList_GET_ITEM(keys, i);
        if (PyUnicode_READY(s) == -1) {
            Py_UNREACHABLE();
        }
        switch (PyUnicode_CHECK_INTERNED(s)) {
        case SSTATE_NOT_INTERNED:
            /* XXX Shouldn't happen */
            break;
        case SSTATE_INTERNED_IMMORTAL:
            Py_REFCNT(s) += 1;
            immortal_size += PyUnicode_GET_LENGTH(s);
            break;
        case SSTATE_INTERNED_MORTAL:
            Py_REFCNT(s) += 2;
            mortal_size += PyUnicode_GET_LENGTH(s);
            break;
        default:
            Py_FatalError("Inconsistent interned string state.");
        }
        _PyUnicode_STATE(s).interned = SSTATE_NOT_INTERNED;
    }
    fprintf(stderr, "total size of all interned strings: "
            "%" PY_FORMAT_SIZE_T "d/%" PY_FORMAT_SIZE_T "d "
            "mortal/immortal\n", mortal_size, immortal_size);
    Py_DECREF(keys);
    PyDict_Clear(interned);
    Py_CLEAR(interned);
}

• PyUnicodeObject根据intern处理状态可分为三类:

/* Interning state. */
#define SSTATE_NOT_INTERNED 0
#define SSTATE_INTERNED_MORTAL 1
#define SSTATE_INTERNED_IMMORTAL 2

四、字符缓冲池

• 字符缓冲池是Python为PyBytesObject的每一个字节的字符对应的对象设计的对象池:

objects/bytesobject.c

static PyBytesObject *characters[UCHAR_MAX + 1];

• 字符缓冲池以静态变量的形式存在，Python初始化完成后，缓冲池中的所有PyBytesObject指针为空。
• 在创建PyBytesObject时，无论调用哪种方法，如果字符串实际是一个字符，则会进行如下操作：

objects/bytesobject.c

PyBytes_FromString(const char *str)
{
  ... ...
if (size == 1 && (op = characters[*str & UCHAR_MAX]) != NULL) {
#ifdef COUNT_ALLOCS
        one_strings++;
#endif
        Py_INCREF(op);
        return (PyObject *)op;
    }
  
   /* Inline PyObject_NewVar */
    op = (PyBytesObject *)PyObject_MALLOC(PyBytesObject_SIZE + size);
    if (op == NULL)
        return PyErr_NoMemory();
    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
    op->ob_shash = -1;
    memcpy(op->ob_sval, str, size+1);
    /* share short strings */
    if (size == 0) {
        nullstring = op;
        Py_INCREF(op);
    } else if (size == 1) {
        characters[*str & UCHAR_MAX] = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;
}

• 如果不是空字符，在创建PyBytesObject后，会将其添加到池子中。
• 如果字符已经在池子中，则直接返回池子中的对象。

五、字符串对象的效率问题

• 由于PyUnicodeObject是不可变对象，当在Python中使用"+"对两个字符串拼接时，实际上会调用PyUnicode_Concat，从而创建一个新的PyUnicodeObject。
• 如果用"+"拼接n个PyUnicodeObject对象，意味着需要进行n-1次内存申请和搬运工作，严重影响效率。

objects\unicodeobject.c

/* Concat to string or Unicode object giving a new Unicode object. */

PyObject *
PyUnicode_Concat(PyObject *left, PyObject *right)
{
    PyObject *result;
    Py_UCS4 maxchar, maxchar2;
    Py_ssize_t left_len, right_len, new_len;

    if (ensure_unicode(left) < 0)
        return NULL;

    if (!PyUnicode_Check(right)) {
        PyErr_Format(PyExc_TypeError,
                     "can only concatenate str (not \"%.200s\") to str",
                     right->ob_type->tp_name);
        return NULL;
    }
    if (PyUnicode_READY(right) < 0)
        return NULL;

    /* Shortcuts */
    if (left == unicode_empty)
        return PyUnicode_FromObject(right);
    if (right == unicode_empty)
        return PyUnicode_FromObject(left);

    left_len = PyUnicode_GET_LENGTH(left);
    right_len = PyUnicode_GET_LENGTH(right);
    if (left_len > PY_SSIZE_T_MAX - right_len) {
        PyErr_SetString(PyExc_OverflowError,
                        "strings are too large to concat");
        return NULL;
    }
    new_len = left_len + right_len;

    maxchar = PyUnicode_MAX_CHAR_VALUE(left);
    maxchar2 = PyUnicode_MAX_CHAR_VALUE(right);
    maxchar = Py_MAX(maxchar, maxchar2);

    /* Concat the two Unicode strings */
    result = PyUnicode_New(new_len, maxchar);
    if (result == NULL)
        return NULL;
    _PyUnicode_FastCopyCharacters(result, 0, left, 0, left_len);
    _PyUnicode_FastCopyCharacters(result, left_len, right, 0, right_len);
    assert(_PyUnicode_CheckConsistency(result, 1));
    return result;
}

• Python官方推荐的做法是通过PyUnicodeObject的PyUnicode_Join操作来对存储在list或tuple中的一组对象进行连接操作。
• PyUnicode_Join会首先统计出在list中一共有多少个PyUnicodeObject对象，并统计这些对象维护的字符串一共多长，然后申请内存，并将list中的所有字符串拷贝到新开辟的内存空间中，只需要一次内存空间申请。

objects\unicodeobject.c

PyObject *
PyUnicode_Join(PyObject *separator, PyObject *seq)
{
    PyObject *res;
    PyObject *fseq;
    Py_ssize_t seqlen;
    PyObject **items;

    fseq = PySequence_Fast(seq, "can only join an iterable");
    if (fseq == NULL) {
        return NULL;
    }

    /* NOTE: the following code can't call back into Python code,
     * so we are sure that fseq won't be mutated.
     */

    items = PySequence_Fast_ITEMS(fseq);
    seqlen = PySequence_Fast_GET_SIZE(fseq);
    res = _PyUnicode_JoinArray(separator, items, seqlen);
    Py_DECREF(fseq);
    return res;
}

PyObject *
_PyUnicode_JoinArray(PyObject *separator, PyObject *const *items, Py_ssize_t seqlen)
{
    PyObject *res = NULL; /* the result */
    PyObject *sep = NULL;
    Py_ssize_t seplen;
    PyObject *item;
    Py_ssize_t sz, i, res_offset;
    Py_UCS4 maxchar;
    Py_UCS4 item_maxchar;
    int use_memcpy;
    unsigned char *res_data = NULL, *sep_data = NULL;
    PyObject *last_obj;
    unsigned int kind = 0;

    /* If empty sequence, return u"". */
    if (seqlen == 0) {
        _Py_RETURN_UNICODE_EMPTY();
    }

    /* If singleton sequence with an exact Unicode, return that. */
    last_obj = NULL;
    if (seqlen == 1) {
        if (PyUnicode_CheckExact(items[0])) {
            res = items[0];
            Py_INCREF(res);
            return res;
        }
        seplen = 0;
        maxchar = 0;
    }
    else {
        /* Set up sep and seplen */
        if (separator == NULL) {
            /* fall back to a blank space separator */
            sep = PyUnicode_FromOrdinal(' ');
            if (!sep)
                goto onError;
            seplen = 1;
            maxchar = 32;
        }
        else {
            if (!PyUnicode_Check(separator)) {
                PyErr_Format(PyExc_TypeError,
                             "separator: expected str instance,"
                             " %.80s found",
                             Py_TYPE(separator)->tp_name);
                goto onError;
            }
            if (PyUnicode_READY(separator))
                goto onError;
            sep = separator;
            seplen = PyUnicode_GET_LENGTH(separator);
            maxchar = PyUnicode_MAX_CHAR_VALUE(separator);
            /* inc refcount to keep this code path symmetric with the
               above case of a blank separator */
            Py_INCREF(sep);
        }
        last_obj = sep;
    }

    /* There are at least two things to join, or else we have a subclass
     * of str in the sequence.
     * Do a pre-pass to figure out the total amount of space we'll
     * need (sz), and see whether all argument are strings.
     */
    sz = 0;
#ifdef Py_DEBUG
    use_memcpy = 0;
#else
    use_memcpy = 1;
#endif
    for (i = 0; i < seqlen; i++) {
        size_t add_sz;
        item = items[i];
        if (!PyUnicode_Check(item)) {
            PyErr_Format(PyExc_TypeError,
                         "sequence item %zd: expected str instance,"
                         " %.80s found",
                         i, Py_TYPE(item)->tp_name);
            goto onError;
        }
        if (PyUnicode_READY(item) == -1)
            goto onError;
        add_sz = PyUnicode_GET_LENGTH(item);
        item_maxchar = PyUnicode_MAX_CHAR_VALUE(item);
        maxchar = Py_MAX(maxchar, item_maxchar);
        if (i != 0) {
            add_sz += seplen;
        }
        if (add_sz > (size_t)(PY_SSIZE_T_MAX - sz)) {
            PyErr_SetString(PyExc_OverflowError,
                            "join() result is too long for a Python string");
            goto onError;
        }
        sz += add_sz;
        if (use_memcpy && last_obj != NULL) {
            if (PyUnicode_KIND(last_obj) != PyUnicode_KIND(item))
                use_memcpy = 0;
        }
        last_obj = item;
    }

    res = PyUnicode_New(sz, maxchar);
    if (res == NULL)
        goto onError;

    /* Catenate everything. */
#ifdef Py_DEBUG
    use_memcpy = 0;
#else
    if (use_memcpy) {
        res_data = PyUnicode_1BYTE_DATA(res);
        kind = PyUnicode_KIND(res);
        if (seplen != 0)
            sep_data = PyUnicode_1BYTE_DATA(sep);
    }
#endif
    if (use_memcpy) {
        for (i = 0; i < seqlen; ++i) {
            Py_ssize_t itemlen;
            item = items[i];

            /* Copy item, and maybe the separator. */
            if (i && seplen != 0) {
                memcpy(res_data,
                          sep_data,
                          kind * seplen);
                res_data += kind * seplen;
            }

            itemlen = PyUnicode_GET_LENGTH(item);
            if (itemlen != 0) {
                memcpy(res_data,
                          PyUnicode_DATA(item),
                          kind * itemlen);
                res_data += kind * itemlen;
            }
        }
        assert(res_data == PyUnicode_1BYTE_DATA(res)
                           + kind * PyUnicode_GET_LENGTH(res));
    }
    else {
        for (i = 0, res_offset = 0; i < seqlen; ++i) {
            Py_ssize_t itemlen;
            item = items[i];

            /* Copy item, and maybe the separator. */
            if (i && seplen != 0) {
                _PyUnicode_FastCopyCharacters(res, res_offset, sep, 0, seplen);
                res_offset += seplen;
            }

            itemlen = PyUnicode_GET_LENGTH(item);
            if (itemlen != 0) {
                _PyUnicode_FastCopyCharacters(res, res_offset, item, 0, itemlen);
                res_offset += itemlen;
            }
        }
        assert(res_offset == PyUnicode_GET_LENGTH(res));
    }

    Py_XDECREF(sep);
    assert(_PyUnicode_CheckConsistency(res, 1));
    return res;

  onError:
    Py_XDECREF(sep);
    Py_XDECREF(res);
    return NULL;
}