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]

All commit authors signed the Contributor License Agreement.

[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.