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 is None
>>> 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.