fix: handle ValueError from formatter.parse() on malformed format strings (#733)

[devimano2011]

Fixes #733

Problem

When a string value in a test contains an unmatched { character (e.g. from an MQTT payload field like {Explanation 1), Python's string.Formatter().parse() raises a ValueError:

ValueError: expected '}' before end of string

This caused Tavern to crash with an unhandled exception instead of continuing test execution.

Fix

As suggested by @michaelboulton in the issue, wrap formatter.parse(to_format) in a try/except ValueError. When the string cannot be parsed (due to unmatched braces), log a warning and return the original string unformatted so test execution can continue.

Changes

• tavern/_core/dict_util.py: wrap formatter.parse() in try/except ValueError in _check_and_format_values()

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Enhanced format string error handling to gracefully manage invalid syntax, preventing application errors and providing diagnostic feedback when issues occur.

[coderabbitai]

📝 Walkthrough

Walkthrough

The _check_and_format_values function in tavern/_core/dict_util.py now includes error handling for invalid format-string syntax. When string.Formatter().parse() encounters malformed format strings, it catches the ValueError, logs a warning with the problematic string, and returns the original input unmodified instead of propagating the exception.

Changes

Cohort / File(s)	Summary
Format String Error Handling tavern/_core/dict_util.py	Added try-except wrapper around string.Formatter().parse() to gracefully handle invalid format-string syntax by emitting a warning and returning unmodified input, whilst preserving validation and formatting behaviour for valid format strings.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: handling a ValueError from formatter.parse() on malformed format strings, directly addressing the issue referenced.
Linked Issues check	✅ Passed	The PR implements the required fix from issue #733: catching ValueError from formatter.parse(), logging a warning, and returning the unmodified string to allow test execution to continue gracefully.
Out of Scope Changes check	✅ Passed	The changes are narrowly focused on the specific issue in dict_util.py's _check_and_format_values() function with no extraneous modifications beyond the required error handling.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@tavern/_core/dict_util.py`:
- Around line 48-56: The _attempt_find_include() path must be protected the same
way as _check_and_format_values(): wrap the call to formatter.parse(to_format)
inside a try/except that catches ValueError, logs a warning (same message style
used in the existing block) and returns the original to_format (or otherwise
skip include processing) instead of allowing the exception to propagate;
refactor or extract the guarded-parse logic into a small helper used by both
_check_and_format_values() and _attempt_find_include() so both call the safe
parse routine rather than calling formatter.parse(...) directly.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 25effe64-e8cb-4cc4-ad35-113b3da0d6b6

📥 Commits

Reviewing files that changed from the base of the PR and between 55e5925 and 385add0.

📒 Files selected for processing (1)

• tavern/_core/dict_util.py

[coderabbitai]

⚠️ Potential issue | 🟠 Major

Also guard the !include parse path.

This try/except fixes _check_and_format_values(), but _attempt_find_include() still does a bare formatter.parse(to_format) on Line 103. A malformed string can still raise ValueError there and crash the run, so the fix is only partial. Consider reusing the same guarded parse logic in both code paths.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@tavern/_core/dict_util.py` around lines 48 - 56, The _attempt_find_include()
path must be protected the same way as _check_and_format_values(): wrap the call
to formatter.parse(to_format) inside a try/except that catches ValueError, logs
a warning (same message style used in the existing block) and returns the
original to_format (or otherwise skip include processing) instead of allowing
the exception to propagate; refactor or extract the guarded-parse logic into a
small helper used by both _check_and_format_values() and _attempt_find_include()
so both call the safe parse routine rather than calling formatter.parse(...)
directly.

[michaelboulton]

This shouldn't silently fail and send None, this would be very undesirable in the case where, for example, someone is trying to PATCH a field on an object and it sends null and wipes out a field instead of setting it.
If anything it should catch the ValueError and re-raise a specific tavern exception mentioning the unmatched curl brace.