Question

According to the 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.

Meaning hashing is not used for this purpose.

Now, lets assume that internally during an equality test Python first checks the length of the two strings and proceeds with the lexicographical comparison, if both are of same length (I suppose it does the same with all the other comparisons too).

Well, if that is so then why does the following two, different, comparisons consume different amounts of time?

>>> 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

In both the comparisons str1 and str2 are of the same length. In the first one both differ only by the last character and in the second by none.

PS: Used a long strings and large iterations to make the difference appreciable.

Answer 1

There is some pool of string literals in Python, as in many languages, in the second case both strings are, in fact, the same object, so they are compared by references, not by actual values.

Typical string comparison function in reference-based languages:

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