Issue 17441: Do not cache re.compile

Ezio proposed in issue16389 to not cache re.compile. Caching of re.compile has no sense and only pollutes the cache.

Here is a patch.

I'm not sure I agree. I've seen plenty of code that called re.compile() over and over again -- or called it with a computed string that would have only a small number of possible values.

I think we could happily call such code buggy or at least suboptimal. The docs don't even mention that re.compile() actually uses a cache.

> The docs don't even mention that re.compile() actually uses a cache.

Actually it does:
"""
re.compile(pattern, flags=0)

Note The compiled versions of the most recent patterns passed to re.match(), re.search() or re.compile() are cached, so programs that use only a few regular expressions at a time needn’t worry about compiling regular expressions.
"""

Now, I agree that it's definitely suboptimal...

So what is a decision?

> So what is a decision?

Is there a clear performance benefit in removing the caching?

If the problem is cache pollution perhaps re.compile can get a separate cache :-)

I don't like the idea that you want to remove a feature without a deprecation period.

I'm surprised, but perhaps performance benefit actually exists (check this on other computers).

### regex_effbot ###
Min: 0.333525 -> 0.325349: 1.03x faster
Avg: 0.342451 -> 0.331665: 1.03x faster
Significant (t=12.13)
Stddev: 0.00606 -> 0.00651: 1.0738x larger

However the main benefit is that non-cached constructor (re.compile is a constructor of regular expression object) is expected to be non-cached, while other module level functions can be cached. See for example struct.Struct. We can cache non-cached constructor explicitly (as in fnmatch), but when a constructor already cached, adding new cache can only add overhead.

Can this be closed.  Caching of regexes has been around for a very long time and I expect that a lot of code depends on it.  This should not be washed away without considerable discussion.

If we were doing it today, maybe we wouldn't cache them, but, as you say, it's been like that for a long time. (The regex module also caches them, because the re module does.) Unless someone can demonstrate that it's a problem, I'd say just leave it as it is.

JFYI.
regex library now supports avoiding cache:

https://bitbucket.org/mrabarnett/mrab-regex/issues/387/compilaton-flag-to-avoid-storing-compiled#comment-59117125

stage: needs patch -> patch review