From e317c630a60fc5925bd10368c1c412b5fda62577 Mon Sep 17 00:00:00 2001
From: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>
Date: Fri, 17 Jul 2026 17:14:45 +0100
Subject: [PATCH 1/2] Improve logging of warnings from Jinja2 lexing

---
 changes.d/7365.feat.md        | 2 +-
 cylc/flow/parsec/fileparse.py | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/changes.d/7365.feat.md b/changes.d/7365.feat.md
index ae8ff76376e..10f6ef284ce 100644
--- a/changes.d/7365.feat.md
+++ b/changes.d/7365.feat.md
@@ -1 +1 @@
-Python warnings raised in users' custom jinja2 filters, globals and tests are now logged.
+Python warnings raised during Jinja2 preprocessing are now logged.
diff --git a/cylc/flow/parsec/fileparse.py b/cylc/flow/parsec/fileparse.py
index 849723136e7..be0f5873f0c 100644
--- a/cylc/flow/parsec/fileparse.py
+++ b/cylc/flow/parsec/fileparse.py
@@ -513,6 +513,13 @@ def read_and_proc(
                 flines = jinja2process(
                     fpath, flines, fdir, template_vars
                 )
+            for w in warns:
+                if w.filename.endswith(os.path.join('jinja2', 'lexer.py')):
+                    # Warning originating from lexing of a jinja2 template.
+                    # Unfortunately, we can't know exactly where it originated,
+                    # so just set the config filename and vague line "number":
+                    w.filename = fpath
+                    w.lineno = '<jinja2 template>'  # type: ignore
             if warns:
                 LOG.warning(
                     "The following warnings were raised during Jinja2 "

From 1219755ffa0c1ac421dc8431bb9b3703d913ce2e Mon Sep 17 00:00:00 2001
From: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>
Date: Tue, 4 Aug 2026 17:20:26 +0100
Subject: [PATCH 2/2] Catch additional jinja2 filepath that warnings can come
 from during parsing of Cylc config

---
 cylc/flow/parsec/fileparse.py       |  6 ++++--
 tests/unit/parsec/test_fileparse.py | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/cylc/flow/parsec/fileparse.py b/cylc/flow/parsec/fileparse.py
index be0f5873f0c..35c4be616ec 100644
--- a/cylc/flow/parsec/fileparse.py
+++ b/cylc/flow/parsec/fileparse.py
@@ -125,6 +125,8 @@
     TEMPLATING_DETECTED: None
 }
 
+_J2_WARN_LOCS = re.compile(rf'jinja2{re.escape(os.sep)}(lexer|runtime)\.py$')
+
 
 def get_cylc_env_vars() -> dict[str, str]:
     """Return a restricted dict of CYLC_ environment variables for templating.
@@ -514,8 +516,8 @@ def read_and_proc(
                     fpath, flines, fdir, template_vars
                 )
             for w in warns:
-                if w.filename.endswith(os.path.join('jinja2', 'lexer.py')):
-                    # Warning originating from lexing of a jinja2 template.
+                if _J2_WARN_LOCS.search(w.filename):
+                    # Warning originating from processing of a jinja2 template.
                     # Unfortunately, we can't know exactly where it originated,
                     # so just set the config filename and vague line "number":
                     w.filename = fpath
diff --git a/tests/unit/parsec/test_fileparse.py b/tests/unit/parsec/test_fileparse.py
index d0190246a7c..79caa7439a9 100644
--- a/tests/unit/parsec/test_fileparse.py
+++ b/tests/unit/parsec/test_fileparse.py
@@ -20,6 +20,7 @@
 import re
 import sqlite3
 from tempfile import NamedTemporaryFile
+from textwrap import dedent
 from types import SimpleNamespace
 import warnings
 
@@ -483,6 +484,28 @@ def mock_jinja2process(fpath, *a, **k):
     )
 
 
+def test_read_and_proc_jinja2_warnings_invalid_escape(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+):
+    """When a warning comes from a j2 template expression, it should be logged
+    with the cylc config filepath."""
+    (fpath := tmp_path / 'flow.cylc').write_text(
+        dedent(r"""
+            #!jinja2
+            {% from 'warnings' import warn %}
+            {% do warn('Mock warning') %}
+        """).lstrip()
+    )
+    read_and_proc(
+        fpath=str(fpath),
+        viewcfg={'jinja2': True, 'contin': False, 'inline': False},
+    )
+
+    assert (
+        f"{fpath}:<jinja2 template>: UserWarning: Mock warning" in caplog.text
+    )
+
+
 def test_parse_keys_only_singleline():
     with NamedTemporaryFile() as of, NamedTemporaryFile() as tf:
         fpath = tf.name