gh-118761: Optimise import time for string

[AA-Turner]

This PR achieves a 27x improvement in import time for the string module. The main improvement comes from replacing Template.__init_subclass__() (GH-16256) with a descriptor class, allowing lazy import of the re module.

Current:

import string: cumulative time mean: 9162.100 µs median: 9133.000 µs stdev: 66.662 min: 9071 max: 9301 

This PR:

import string: cumulative time mean: 334.967 µs median: 329.000 µs stdev: 13.438 min: 316 max: 368 

• Issue: Improve import time of various stdlib modules #118761

[picnixz]

I'm very happy with this because I need to access many times string.* constants without needing the Template class so it's a very good optimization.

[python-cla-bot]

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[serhiy-storchaka]

Interesting idea. The code looks intimidating, but it might work.

How the help output looks now?

Is __init_subclass__() needed anymore?

[AA-Turner]

Help output looks fine:

>>> from string import Template >>> assert Template.flags isNone >>> help(Template) Help on class Template in module string: class Template(builtins.object) | Template(template) | | A string class for supporting $-substitutions. | | Methods defined here: | | __init__(self, template) | Initialize self. See help(type(self)) for accurate signature. | | get_identifiers(self) | | is_valid(self) | | safe_substitute(self, mapping={}, /, **kws) | | substitute(self, mapping={}, /, **kws) | | ---------------------------------------------------------------------- | Class methods defined here: | | __init_subclass__() | This method is called when a class is subclassed. | | The default implementation does nothing. It may be | overridden to extend subclasses. | | ---------------------------------------------------------------------- | Data descriptors defined here: | | __dict__ | dictionary for instance variables | | __weakref__ | list of weak references to the object | | ---------------------------------------------------------------------- | Data and other attributes defined here: | | braceidpattern = None | | delimiter = '$' | | flags = re.IGNORECASE | | idpattern = '(?a:[_a-z][_a-z0-9]*)' | | pattern = re.compile('\n \\$(?:\n ...identifie... >>> 

[serhiy-storchaka]

It perhaps could be made simpler with classmethod + property, but the future of this feature is not clear.