3.11.0b1 regression: re.template removed without a deprecation period

[hroncok]

Bug report

In Python 3.10, the re.template function exists and is not deprecated:

>>> import re
>>> re.template('', re.I)
re.compile('', re.TEMPLATE|re.IGNORECASE)

In Python 3.11.0b1 this function is gone:

>>> import re
>>> re.template('', re.I)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 're' has no attribute 'template'

This was removed in b09184bf05 without a depreciation period.
Despite being undocumented, projects actually use this function, see e.g. rpm-software-management/dnf#1827

Please restore this function and deprecate it in 3.11 and 3.12 if you want to remove it in 3.13. (That is how I understand PEP 387, section Making Incompatible Changes.)

Your environment

• CPython versions tested on: 3.11.0b1
• Operating system and architecture: All

[hroncok]

I intend to send a PR later today.

However, I am not sure if I can revert the removal + deprecate after the feature freeze, or if I must only revert the removal in 3.11 and deprecate it in 3.12+. Will start with a PR to main anyway.

EDIT: I won't send a PR before there is a consensus.

[vstinner]

Removal discussed at #91367

[vstinner]

The discussion started in this larger issue about re "reorganization": #91308

[vstinner]

I don't understand the purpose of the re.template() function and the re.TEMPLATE flag. It doesn't seem to disable backtracking:

>>> re.compile(r"((a)b\2)", re.IGNORECASE).match("aba").groups()
('aba', 'a')
>>> re.template(r"((a)b\2)", re.IGNORECASE).match("aba").groups()
('aba', 'a')

The only thing which seems to be changed is that repetition is not supported by TEMPLATE:

>>> re.template(r"a+")
re.error: internal: unsupported template operator MAX_REPEAT
>>> re.template(r"a*")
re.error: internal: unsupported template operator MAX_REPEAT

[vstinner]

Code search for "TEMPLATE" in re/sre source code files of Python 3.10:

Lib/re.py:    TEMPLATE = T = sre_compile.SRE_FLAG_TEMPLATE # disable backtracking
Lib/sre_compile.py:            if flags & SRE_FLAG_TEMPLATE:
Lib/sre_constants.py:SRE_FLAG_TEMPLATE = 1 # template mode (disable backtracking)
Lib/sre_constants.py:        f.write("#define SRE_FLAG_TEMPLATE %d\n" % SRE_FLAG_TEMPLATE)
Lib/sre_parse.py:    "t": SRE_FLAG_TEMPLATE,
Lib/sre_parse.py:GLOBAL_FLAGS = SRE_FLAG_DEBUG | SRE_FLAG_TEMPLATE
Modules/sre_constants.h:#define SRE_FLAG_TEMPLATE 1

[vstinner]

cc @serhiy-storchaka

[hroncok]

To be honest, I don't understand the purpose of the re.template() function either and I think that deprecating and removing it is a good thing.

[vstinner]

If we decide to not revert the change, I suggest to document the removal in What's New in Python 3.11 and explain how to port impacted applications. For example, replace re.template() with re.compile().