feat: MetaflowTaggingError formatting and add test coverage for tagging_util

[Vaishnav88sk]

The validate_tag function was passing arguments incorrectly to MetaflowTaggingError, causing str() to fail when the exception was rendered. This also adds comprehensive test coverage for is_utf8_encodable, is_utf8_decodable, validate_tag, and validate_tags including edge cases for type validation, length limits, and tag set size constraints.

Fixes error message formatting for non-string tag types.

PR Type

• Bug fix
• New feature
• Core Runtime change (higher bar -- see CONTRIBUTING.md)
• Docs / tooling
• Refactoring

Summary

Fix error message formatting bug in validate_tag and add comprehensive unit test coverage for tagging_util.py (30 tests).

Issue

Fixes #3162

Context / Motivation

The validate_tag function passed two positional arguments to MetaflowTaggingError, but the exception class expects msg, lineno, source_file as positional args. This caused TypeError: %d format: a real number is required, not str when rendering the exception for non-string tag types.

Additionally, tagging_util.py had zero test coverage despite being
critical infrastructure for run/step tagging validation.

Reproduction

Runtime:

Commands to run:

# paste exact commands

Where evidence shows up:

Before (error / log snippet)

paste here

After (evidence that fix works)

paste here

Root Cause

Changes Made

• Fixed MetaflowTaggingError call to use string formatting instead of multiple positional arguments
• Added 30 tests covering:
  • is_utf8_encodable / is_utf8_decodable functions
  • validate_tag with valid inputs and all error conditions
  • validate_tags including tag set size limits and remediation logic

Why This Fix Is Correct

• All 30 new tests pass
• Tests cover edge cases: empty tags, oversized tags, non-string types, UTF-8 encoding/decoding, tag deduplication, and size limits

Failure Modes Considered

Tests

• Unit tests added/updated
• Reproduction script provided (required for Core Runtime)
• CI passes
• If tests are impractical: explain why below and provide manual evidence above

Non-Goals

AI Tool Usage

• No AI tools were used in this contribution
• AI tools were used (describe below)

For formatting

[greptile-apps]

Greptile Summary

This PR fixes a genuine bug in validate_tag: passing str(type(tag)) as the second positional argument to MetaflowTaggingError silently set lineno to a string, causing TypeError: %d format: a real number is required, not str in MetaflowException.__str__ whenever the error was rendered. The fix pre-formats the message string before passing it, which is correct. The new test file is comprehensive and all test assertions are logically sound.

Confidence Score: 5/5

Safe to merge — minimal, targeted fix with solid test coverage and no regressions.

The only change to production code is a one-line fix that correctly pre-evaluates a format string. All 30 new tests are logically correct. No security, data, or runtime concerns.

No files require special attention.

Important Files Changed

Filename	Overview
metaflow/tagging_util.py	One-line fix: replaces two-argument MetaflowTaggingError call with a pre-formatted string, correctly preventing TypeError when str tried to use the tag type as a %d lineno.
test/unit/test_tagging_util.py	New test file with 30 tests covering is_utf8_encodable, is_utf8_decodable, validate_tag, and validate_tags including edge cases; one test name is slightly misleading but logic is correct.

_{Reviews (4): Last reviewed commit: "Merge branch 'master' into feat/add-tagg..." | Re-trigger Greptile}