有三个因素在起作用，综合起来，产生了这种令人惊讶的行为。

首先:in操作符使用了一个快捷方式，在检查相等性(x == y)之前检查身份(x是y):

>>> n = float('nan')
>>> n in (n, )
True
>>> n == n
False
>>> n is n
True

第二:由于Python的字符串实习，在"x" in ("x"，)中的"x"将是相同的:

>>> "x" is "x"
True

(警告:这是特定于实现的行为!Is永远不应该用来比较字符串，因为它有时会给出令人惊讶的答案;例如“x”* 100是“x”* 100 ==>错误)

第三，正如Veedrac奇妙的答案中详细描述的那样，元组。__contains__(x in (y，)大致相当于(y，).__contains__(x))比str.__eq__(同样，x == y大致相当于x.__eq__(y))更快地执行身份检查。

你可以看到这样的证据，因为(y，)中的x比逻辑上等价的x == y要慢得多:

In [18]: %timeit 'x' in ('x', )
10000000 loops, best of 3: 65.2 ns per loop

In [19]: %timeit 'x' == 'x'    
10000000 loops, best of 3: 68 ns per loop

In [20]: %timeit 'x' in ('y', ) 
10000000 loops, best of 3: 73.4 ns per loop

In [21]: %timeit 'x' == 'y'    
10000000 loops, best of 3: 56.2 ns per loop

(y，)情况中的x比较慢，因为在is比较失败后，in操作符会退回到正常的相等性检查(即使用==)，因此比较所花费的时间与==相同，导致整个操作变慢，因为创建元组、遍历其成员等开销。

还要注意(b，)中的a只有当a是b时才会更快:

In [48]: a = 1             

In [49]: b = 2

In [50]: %timeit a is a or a == a
10000000 loops, best of 3: 95.1 ns per loop

In [51]: %timeit a in (a, )      
10000000 loops, best of 3: 140 ns per loop

In [52]: %timeit a is b or a == b
10000000 loops, best of 3: 177 ns per loop

In [53]: %timeit a in (b, )      
10000000 loops, best of 3: 169 ns per loop

(为什么a在(b，)中比a在b或a == b中快?我猜会有更少的虚拟机指令- a在(b，)中只有~3条指令，其中a是b或a == b将是相当多的虚拟机指令)

Veedrac的答案——https://stackoverflow.com/a/28889838/71522——更详细地介绍了在==和in期间发生的事情，非常值得一读。

正如我向David Wolever提到的那样，事情远比看上去的要复杂;两种方法都调度为;你可以通过实践来证明这一点

min(Timer("x == x", setup="x = 'a' * 1000000").repeat(10, 10000))
#>>> 0.00045456900261342525

min(Timer("x == y", setup="x = 'a' * 1000000; y = 'a' * 1000000").repeat(10, 10000))
#>>> 0.5256857610074803

第一种只能这么快，因为它是通过身份进行检查的。

为了找出为什么一个会比另一个花费更长的时间，让我们跟踪执行过程。

它们都从ceval.c开始，从COMPARE_OP开始，因为这是所涉及的字节码

TARGET(COMPARE_OP) {
    PyObject *right = POP();
    PyObject *left = TOP();
    PyObject *res = cmp_outcome(oparg, left, right);
    Py_DECREF(left);
    Py_DECREF(right);
    SET_TOP(res);
    if (res == NULL)
        goto error;
    PREDICT(POP_JUMP_IF_FALSE);
    PREDICT(POP_JUMP_IF_TRUE);
    DISPATCH();
}

这将从堆栈中弹出值(技术上它只弹出一个)

PyObject *right = POP();
PyObject *left = TOP();

并运行比较:

PyObject *res = cmp_outcome(oparg, left, right);

Cmp_outcome是这样的:

static PyObject *
cmp_outcome(int op, PyObject *v, PyObject *w)
{
    int res = 0;
    switch (op) {
    case PyCmp_IS: ...
    case PyCmp_IS_NOT: ...
    case PyCmp_IN:
        res = PySequence_Contains(w, v);
        if (res < 0)
            return NULL;
        break;
    case PyCmp_NOT_IN: ...
    case PyCmp_EXC_MATCH: ...
    default:
        return PyObject_RichCompare(v, w, op);
    }
    v = res ? Py_True : Py_False;
    Py_INCREF(v);
    return v;
}

这就是路径分裂的地方。PyCmp_IN分支可以

int
PySequence_Contains(PyObject *seq, PyObject *ob)
{
    Py_ssize_t result;
    PySequenceMethods *sqm = seq->ob_type->tp_as_sequence;
    if (sqm != NULL && sqm->sq_contains != NULL)
        return (*sqm->sq_contains)(seq, ob);
    result = _PySequence_IterSearch(seq, ob, PY_ITERSEARCH_CONTAINS);
    return Py_SAFE_DOWNCAST(result, Py_ssize_t, int);
}

注意，元组被定义为

static PySequenceMethods tuple_as_sequence = {
    ...
    (objobjproc)tuplecontains,                  /* sq_contains */
};

PyTypeObject PyTuple_Type = {
    ...
    &tuple_as_sequence,                         /* tp_as_sequence */
    ...
};

所以这个分支

if (sqm != NULL && sqm->sq_contains != NULL)

*sqm->sq_contains，即函数(objobjproc) tuplcontains。

这并

static int
tuplecontains(PyTupleObject *a, PyObject *el)
{
    Py_ssize_t i;
    int cmp;

    for (i = 0, cmp = 0 ; cmp == 0 && i < Py_SIZE(a); ++i)
        cmp = PyObject_RichCompareBool(el, PyTuple_GET_ITEM(a, i),
                                           Py_EQ);
    return cmp;
}

.．.等等，那个PyObject_RichCompareBool不是另一个分支取的吗?不，那是PyObject_RichCompare。

这条代码路径很短，所以它可能归结为这两个的速度。让我们来比较一下。

int
PyObject_RichCompareBool(PyObject *v, PyObject *w, int op)
{
    PyObject *res;
    int ok;

    /* Quick result when objects are the same.
       Guarantees that identity implies equality. */
    if (v == w) {
        if (op == Py_EQ)
            return 1;
        else if (op == Py_NE)
            return 0;
    }

    ...
}

PyObject_RichCompareBool中的代码路径几乎立即终止。对于PyObject_RichCompare，确实如此

PyObject *
PyObject_RichCompare(PyObject *v, PyObject *w, int op)
{
    PyObject *res;

    assert(Py_LT <= op && op <= Py_GE);
    if (v == NULL || w == NULL) { ... }
    if (Py_EnterRecursiveCall(" in comparison"))
        return NULL;
    res = do_richcompare(v, w, op);
    Py_LeaveRecursiveCall();
    return res;
}

Py_EnterRecursiveCall/Py_LeaveRecursiveCall组合不在前面的路径中，但它们是相对快速的宏，在增加或减少一些全局变量后会短路。

do_richcompare:

static PyObject *
do_richcompare(PyObject *v, PyObject *w, int op)
{
    richcmpfunc f;
    PyObject *res;
    int checked_reverse_op = 0;

    if (v->ob_type != w->ob_type && ...) { ... }
    if ((f = v->ob_type->tp_richcompare) != NULL) {
        res = (*f)(v, w, op);
        if (res != Py_NotImplemented)
            return res;
        ...
    }
    ...
}

这做了一些快速检查，调用v->ob_type->tp_richcompare

PyTypeObject PyUnicode_Type = {
    ...
    PyUnicode_RichCompare,      /* tp_richcompare */
    ...
};

这

PyObject *
PyUnicode_RichCompare(PyObject *left, PyObject *right, int op)
{
    int result;
    PyObject *v;

    if (!PyUnicode_Check(left) || !PyUnicode_Check(right))
        Py_RETURN_NOTIMPLEMENTED;

    if (PyUnicode_READY(left) == -1 ||
        PyUnicode_READY(right) == -1)
        return NULL;

    if (left == right) {
        switch (op) {
        case Py_EQ:
        case Py_LE:
        case Py_GE:
            /* a string is equal to itself */
            v = Py_True;
            break;
        case Py_NE:
        case Py_LT:
        case Py_GT:
            v = Py_False;
            break;
        default:
            ...
        }
    }
    else if (...) { ... }
    else { ...}
    Py_INCREF(v);
    return v;
}

也就是说，这个快捷键在左==右…但只有在

    if (!PyUnicode_Check(left) || !PyUnicode_Check(right))

    if (PyUnicode_READY(left) == -1 ||
        PyUnicode_READY(right) == -1)

所有的路径看起来就像这样(手动递归内联，展开和修剪已知的分支)

POP()                           # Stack stuff
TOP()                           #
                                #
case PyCmp_IN:                  # Dispatch on operation
                                #
sqm != NULL                     # Dispatch to builtin op
sqm->sq_contains != NULL        #
*sqm->sq_contains               #
                                #
cmp == 0                        # Do comparison in loop
i < Py_SIZE(a)                  #
v == w                          #
op == Py_EQ                     #
++i                             # 
cmp == 0                        #
                                #
res < 0                         # Convert to Python-space
res ? Py_True : Py_False        #
Py_INCREF(v)                    #
                                #
Py_DECREF(left)                 # Stack stuff
Py_DECREF(right)                #
SET_TOP(res)                    #
res == NULL                     #
DISPATCH()                      #

vs

POP()                           # Stack stuff
TOP()                           #
                                #
default:                        # Dispatch on operation
                                #
Py_LT <= op                     # Checking operation
op <= Py_GE                     #
v == NULL                       #
w == NULL                       #
Py_EnterRecursiveCall(...)      # Recursive check
                                #
v->ob_type != w->ob_type        # More operation checks
f = v->ob_type->tp_richcompare  # Dispatch to builtin op
f != NULL                       #
                                #
!PyUnicode_Check(left)          # ...More checks
!PyUnicode_Check(right))        #
PyUnicode_READY(left) == -1     #
PyUnicode_READY(right) == -1    #
left == right                   # Finally, doing comparison
case Py_EQ:                     # Immediately short circuit
Py_INCREF(v);                   #
                                #
res != Py_NotImplemented        #
                                #
Py_LeaveRecursiveCall()         # Recursive check
                                #
Py_DECREF(left)                 # Stack stuff
Py_DECREF(right)                #
SET_TOP(res)                    #
res == NULL                     #
DISPATCH()                      #

现在，PyUnicode_Check和PyUnicode_READY非常便宜，因为它们只检查两个字段，但应该很明显，最上面的是一个更小的代码路径，它有更少的函数调用，只有一个开关 声明，只是有点薄。

TL; diana:

if (left_pointer == right_pointer);不同之处在于他们要做多少工作才能达到目标。只是做得少。