flask The `template_filter` decorator doesn't work if you

The Fix

Upgrade to version 3.1.2 or later.

Based on closed pallets/flask issue #5729 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -5,6 +5,8 @@ Unreleased
 -   Drop support for Python 3.9. :pr:`5730`
 -   Remove previously deprecated code: ``__version__``. :pr:`5648`
+-   ``template_filter``, ``template_test``, and ``template_global`` decorators
+    can be used without parentheses. :issue:`5729`

repro.py

from flask import Flask, render_template_string


app = Flask(__name__)


@app.template_filter
def double(x):
    return x * 2


@app.route("/")
def index():
    return render_template_string("2 times 2 is {{ 2 | double }}")

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 3.1.2 or later.\nWhen NOT to use: This fix is not applicable if the decorator's behavior without parentheses is intended.\n\n

Why This Fix Works in Production

Trigger: jinja2.exceptions.TemplateAssertionError: No filter named 'double'.
Mechanism: The template_filter decorator fails to register without parentheses
Why the fix works: Enhances the App.template_filter decorator to support being called without parentheses, while maintaining backward compatibility. (first fixed release: 3.1.2).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.11.11 in real deployments (not just unit tests).
The template_filter decorator fails to register without parentheses
Surfaces as: jinja2.exceptions.TemplateAssertionError: No filter named 'double'.

Proof / Evidence

GitHub issue: #5729
Fix PR: https://github.com/pallets/flask/pull/5736
First fixed release: 3.1.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“@alexwlchan is it okay if i take up this issue? I understand from https://palletsprojects.com/contributing that I don't have to ask first but I thought it…”

@Gable-github · 2025-05-16 · source

“Not the author of the issue (not sure why you pinged him) but a maintainer: Yes sure, feel free to work on it. PRs to…”

@ThiefMaster · 2025-05-16 · source

“Hi there! I'm keen to start contributing to Flask and this issue caught my eye”

@lekuG12 · 2025-05-22 · source

“There's already a draft PR for this... why do you want to start working on yet another one while there's an open PR? :)”

@ThiefMaster · 2025-05-22 · source

Failure Signature (Search String)

jinja2.exceptions.TemplateAssertionError: No filter named 'double'.

Error Message

Stack trace

error.txt

Error Message
-------------
jinja2.exceptions.TemplateAssertionError: No filter named 'double'.

Minimal Reproduction

repro.py

from flask import Flask, render_template_string


app = Flask(__name__)


@app.template_filter
def double(x):
    return x * 2


@app.route("/")
def index():
    return render_template_string("2 times 2 is {{ 2 | double }}")

Environment

Python: 3.11.11

What Broke

Users encounter a TemplateAssertionError when using the filter without parentheses.

Why It Broke

The template_filter decorator fails to register without parentheses

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 3.1.2 or later.

When NOT to use: This fix is not applicable if the decorator's behavior without parentheses is intended.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pallets/flask/pull/5736

First fixed release: 3.1.2

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if the decorator's behavior without parentheses is intended.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
3.1.2	Fixed

Related Issues

No related fixes found.

Cluster: flask:configuration-error All hubs All clusters

Related clusters: Data consistency Other Duplicate processing

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

flask The `template_filter` decorator doesn't work if you don't pass an argument (Fix)