[Python-Dev] Why not using the hash when comparing strings? (original) (raw)

Victor Stinner victor.stinner at gmail.com
Fri Oct 19 14:36:39 CEST 2012

2012/10/19 Benjamin Peterson <benjamin at python.org>:

It would be interesting to see how common it is for strings which have their hash computed to be compared.

I implemented a quick hack. When running "./python -m test test_os": Python calls PyUnicode_RichCompare() 15206 times with Py_EQ or Py_NE operator. In 41.4% (6295 calls), the hash of the two operands is known. In 41.2% (6262 times on 15206), the hash of the two operands are known and are different!

The hit rate may depend since when the process was started. For example, in a fresh interpreter: the hit rate is only 7% (189 hit / 2703 calls).

When running the test suite, the hit rate is around 80% (hashs are known in 90%) after running 70 tests. At the same time, the average of string length is 4.1 characters and quite all strings are pure ASCII.

I create the issue http://bugs.python.org/issue16286 to discuss this optimization.

Victor

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