>>> import string
>>> class MyTemplate(string.Template):
...   delimiter = '.'
... 
>>> MyTemplate.delimiter = 'x'
>>> mt = MyTemplate('.field xfield')
>>> mt.substitute(field=None)
'None xfield'
>>> mt.delimiter
'x'
>>>

If I want to change the pattern string by any delimiter, I should create a new class for every delimiter

I would change the delimiter either in the object or in the class at any time

3.1 is in security fixes-only mode

You are correct that this is a feature request as written. I think there is also a doc bug (the docs should probably mention that changing delimiter after class creation is a no-op).

To add this feature the computation of the regex would need to be moved from the metaclass init to the object init. The question is, was doing this in a metaclass a design decision (perhaps a micro-optimization?), or a "cute trick" that turns out to cause unintuitive consequences? I think Barry was involved in adding the Template feature, so I'm adding him as nosy (I don't have a clone handy to check hg annotate).

See 629200d880ea for answers.

I don't know that anybody relies on the current behavior and computers are now a bazillion times faster than they were in 2004, so who needs that micro optimization any more?

As this issue has been open for a long time, and I don't think it's worth changing the implementation, I am changing this to a documentation bug and will fix it along with the rewrites for bpo-19824 and bpo-20314

New changeset 9f74deb by Barry Warsaw in branch 'master':
Improve the documentation for template strings (#856)
9f74deb