werkzeug里的safe_str_cmp函数意义是什么？

thepoy

代码如下：

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def safe_str_cmp(a, b):

    if isinstance(a, str):
        a = a.encode("utf-8")
    if isinstance(b, str):
        b = b.encode("utf-8")

    if len(a) != len(b):
        return False

    rv = 0
    for x, y in zip(a, b):
        rv |= x ^ y

    return rv == 0

和 is 或 == 有什么不同？
注释为：

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"""This function compares strings in somewhat constant time.  This
requires that the length of at least one string is known in advance.

Returns `True` if the two strings are equal, or `False` if they are not.

.. versionadded:: 0.7
"""

在注释里没有看出来有对字符串进行什么特别的比较，请问大佬们，这个函数的意义何在呢？