Caching css_to_xpath()'s recently used patterns to improve efficiency

I profiled the scrapy-bench spider which uses response.css() for extracting information.

The profiling results are here. The function css_to_xpath() takes 5% of the total time.

When response.xpath()(profiling result) was used, the items extracted per second (benchmark result) was higher.

Hence, I’m proposing caching for the recently used patterns, so that the function takes lesser time. I’m working on a prototype for the same and will add the results for it too.

[eliasdorneles]

What @lopuhin said! 👍

Though I believe this is the stable backport for functools: https://github.com/MiCHiLU/python-functools32 It would need to conditionally add functools32 to install_requires in setup.py

[kmike]

This is fixed by https://github.com/scrapy/parsel/pull/109.