Jinja2: catch and log warnings

[MetRonnie]

Follow-on from #7325 (comment)

Partly addresses #6850

Example

#!jinja2
# flow.cylc
[scheduler]
    allow implicit tasks = True
[scheduling]
    [[graph]]
        R1 = {{ 'foo' | myfilter }}

# Jinja2Filters/myfilter.py
from jinja2 import escape

def myfilter(task):
    return escape(task)

$ cylc val .
WARNING - The following warnings were raised during Jinja2 preprocessing:
    ~/cylc-run/j2/Jinja2Filters/myfilter.py:5: DeprecationWarning: 'jinja2.escape' is
    deprecated and will be removed in Jinja 3.1. Import 'markupsafe.escape' instead.
      return escape(task)
    
Valid for cylc-8.6.6.dev

Whereas before, no warning would be seen.

Note

It seems warnings that are already emitted by the time the config file is processed do not get re-emitted, so the current sre_constants warning from isodatetime will not be logged (even if the user imports sre_constants themselves. Not sure how to get around this so I'm just going to leave it as a mixed blessing, unless anyone has any ideas)

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Add changelog for jinja2 warnings cylc-doc#954
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[Copilot]

Pull request overview

This PR improves observability of Jinja2 preprocessing by capturing Python warnings raised during templating and emitting them via Cylc’s logger, helping users see deprecations (e.g. Jinja2/MarkupSafe changes) that were previously silent.

Changes:

• Capture warnings during jinja2process(...) in read_and_proc and log them after preprocessing.
• Modernize/clarify typing annotations (move away from typing as t, use built-in generics and | unions).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
cylc/flow/parsec/fileparse.py	Adds warnings capture around Jinja2 preprocessing and logs captured warnings; updates typing/imports.
cylc/flow/parsec/jinja2support.py	Updates typing annotations/import formatting (no behavioral change).

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[oliver-sanders]

👍

Asserted that the built-in filters do not raise warnings:

#!jinja2

# {{ 'P1D' | duration_as('s') }}
# {{ 1 | pad(5) }}
# {{ '10661004T08+01' | strftime('%H') }}

[oliver-sanders]

Should tailor this message to make it clearer that this is a call to action on the part of the user.

[MetRonnie]

How about?

Suggested change

	"The following warnings were raised during Jinja2 "
	"preprocessing:\n"
	"The following warnings were raised during Jinja2 "
	"preprocessing (if not addressed, your workflow may break "
	"in future versions of Cylc):\n"

or

Suggested change

	"The following warnings were raised during Jinja2 "
	"preprocessing:\n"
	"The following warnings were raised during Jinja2 "
	"preprocessing (please address at the earliest "
	"opportunity):\n"

[oliver-sanders]

It's a hard one to phrase. We really want to say, "this will break at Cylc 8.7.0", but that might not be true.

[oliver-sanders]

Perhaps something like:

Any Jinja2 v3.1 deprecations will break at Cylc 8.7.0

[samuel-denton]

If you are putting in version specific text anyway, would it be appropriate to detect 'v3.1' or something like that in the warning and alter the text to be more specific?

[MetRonnie]

Effort 😅

[samuel-denton]

LGTM

[oliver-sanders]

That aught to do it.

Can remove the codicil at 8.7.0 - #7368

[MetRonnie]

(Tutorial tests failing for unrelated HTTP 404, all other tests passing. Merging with 2 approvals and only minor test fix in last force push)