The Fix
Upgrade to version 1.0.3 or later.
Based on closed pallets/flask issue #3163 · 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%.
@@ -20,6 +20,8 @@ Unreleased
- :func:`send_file` supports :class:`~io.BytesIO` partial content.
(`#2957`_)
+- :func:`open_resource` accepts the "rt" file mode. This still does
+ the same thing as "r". (:issue:`3163`)
from flask import Flask
app = Flask(__name__)
with app.open_resource('README.md', mode='rt') as f:
print(f.read())
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Option A — Upgrade to fixed release\nUpgrade to version 1.0.3 or later.\nWhen NOT to use: Do not use this fix if compatibility with Python 2 is required.\n\n
Why This Fix Works in Production
- Trigger: with app.open_resource('README.md', mode='rt') as f:
- Mechanism: Flask disallows mode='rt' for open_resource() despite it being valid in Python 3
- Why the fix works: Allows the use of mode='rt' in the open_resource() helper, providing compatibility with Python 3. (first fixed release: 1.0.3).
- 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 3.6 in real deployments (not just unit tests).
- Flask disallows mode='rt' for open_resource() despite it being valid in Python 3
- Surfaces as: Traceback (most recent call last):
Proof / Evidence
- GitHub issue: #3163
- Fix PR: https://github.com/pallets/flask/pull/3174
- First fixed release: 1.0.3
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.59
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“In Python 3, mode='rt' is synonymous with mode='r' but Flask disallows it thinking it is not a valid mode for reading. While this is true in Python 2, Flask could (at least) provide a more meaningful error message, if not just allow it. A s”
Failure Signature (Search String)
- with app.open_resource('README.md', mode='rt') as f:
Error Message
Stack trace
Error Message
-------------
Traceback (most recent call last):
File "mwe.py", line 5, in <module>
with app.open_resource('README.md', mode='rt') as f:
File "/home/goodmami/repos/OMW/py3env/lib/python3.6/site-packages/flask/helpers.py", line 1002, in open_resource
raise ValueError('Resources can only be opened for reading')
ValueError: Resources can only be opened for reading
Minimal Reproduction
from flask import Flask
app = Flask(__name__)
with app.open_resource('README.md', mode='rt') as f:
print(f.read())
Environment
- Python: 3.6
What Broke
Misleading exception raised when attempting to open a resource in text mode.
Why It Broke
Flask disallows mode='rt' for open_resource() despite it being valid in Python 3
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 1.0.3 or later.
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/3174
First fixed release: 1.0.3
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if compatibility with Python 2 is required.
Verify Fix
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 |
|---|---|
| 1.0.3 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.