id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35250	Stop URL system checks from compiling regular expressions	Adam Johnson	Adam Johnson	"Continuing my project to optimize the system checks, I found some good optimizations under  `django.core.checks.urls.check_url_config()`, which showed up as quite expensive in profiling.

Looking down the call tree, it seems the most expensive part of this process is compiling the each URL pattern’s regular expression. This is unnecessary work though, as the checks only need *uncompiled* regular expression patterns. Using the compiled versions “undoes” the lazy-compile optimization that `LocaleRegexDescriptor` was created for in #27453 / 6e222dae5636f875c19ec66f730a4241abe33faa, at least for any process that runs checks.

The checks were fetching the uncompiled pattern with `self.regex.pattern`, which makse `LocaleRegexDescriptor`  compile the pattern only to then read the uncompiled pattern from [https://docs.python.org/3.12/library/re.html#re.Pattern.pattern its pattern attribute].

Additionally, `RoutePattern` was calling `_route_to_regex()` twice to fetch its two result variables in different places: once in `__init__()` and again in `_compile()` (in the non-translated case). This function has non-trivial cost so avoiding double execution is worth it.

Before optimization stats:

* `check_url_config` took 67ms, or ~10% of the time for checks.
* `LocaleRegexDescriptor.__get__()` showed 965 calls taking ~60ms, ~9% of the total runtime of checks.
* `re.compile()` showed 741 calls for 94ms.
* `_route_to_regex()` had 1900 calls taking 18ms (~2.6% of the total runtime).

After optimization:

* `check_url_config()` took 5ms, ~0.9% of the new total runtime.
* The calls to `LocaleRegexDescriptor.__get__` are gone.
* `re.compile()` drops to 212 calls from other sites, for a total of 51ms.
* `_route_to_regex()` drops to the expected 950 calls, taking half the time at 9ms.
"	Cleanup/optimization	assigned	Core (System checks)	dev	Normal				Unreviewed	1	0	0	0	0	0