python - 为什么两个相同长度的字符串之间的两个不同的相等性测试需要不同的时间

Question

根据Python reference manual :

Strings are compared lexicographically using the numeric equivalents (the result of the built-in function ord()) of their characters. Unicode and 8-bit strings are fully interoperable in this behavior.

意思是散列不用于此目的。

现在，假设在相等性测试期间，Python 首先在内部检查两个字符串的长度，如果两个字符串的长度相同，则继续进行字典顺序比较(我想它对所有其他比较也是如此)。

好吧，如果是这样，那么为什么以下两个不同的比较会消耗不同的时间？

>>> str1 = "foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge1"
>>> 
>>> str2 = "foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge2"
>>> 
>>> def compare():
     t1 = time.time()
     for x in xrange(100000000):
         str1 == str2
     print time.time() - t1
 
>>> 
>>> compare()
13.001019001
>>> 
>>> str2 = "foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge_foo_bar_cruft_kludge1"
>>> 
>>> compare()
7.41645097733

在两个比较中，str1 和 str2 的长度相同。在第一个中，两者仅在 last 字符上有所不同，而在第二个中则没有。

PS:使用长字符串和大迭代使差异变得明显。

Answer 1

Python 中有一些字符串文字池，就像在许多语言中一样，在第二种情况下，两个字符串实际上是同一个对象，因此它们是通过引用而不是实际值进行比较的。

基于引用的语言中典型的字符串比较函数:

if (ref(a) == ref(b)) return true;
if (len(a) != len(b)) return false;
return compare_actual_data(a, b);