Fixing the double logging issue with a logging.filter

22 May 2025 10:22

I finally fixed a very annoying issue with frontmatter-check.

The problem

frontmatter-check currently has two levels:

  • WARN - generates a warning if a frontmatter key is missing
  • ERROR - generates an error instead

To break in this we actually have a few logging handlers:

  • stdout - Pass Warnings to stdout
  • stderr - Pass Errors to stderr
  • MemoryHandler - pass Errors to MemoryHandler so that those errors can be used in frontmatter-check itself.

The problem here lived in stdout. Logging operates on levels:

  • 10 DEBUG
  • 20 INFO
  • 30 WARNING
  • 40 ERROR

Any level higher than the level set will also show. This meant that ERROR messages were appearing in both stdout and stderr which is why Errors would show twice.

❯ pre-commit try-repo ../frontmatter-check
[INFO] Initializing environment for ../frontmatter-check.
===============================================================================
Using config:
===============================================================================
repos:
-   repo: ../frontmatter-check
    rev: 258bcb39385a76ac54610121bf89abb04c9b904e
    hooks:
    -   id: frontmatter-check
===============================================================================
[INFO] Installing environment for ../frontmatter-check.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
frontmatter check........................................................Failed
- hook id: frontmatter-check
- exit code: 1

Checking File: content/test-content-fm-check.md
ERROR - Missing field: 'description'
ERROR - Missing field: 'description'
ERROR - Missing field: 'date'
ERROR - Missing field: 'date'

The solution

I added a filter to the stdout_handler.

class WarningOnlyFilter(logging.Filter):
    def filter(self, record):
        return record.levelno == logging.WARNING

stdout_handler = logging.StreamHandler(sys.stdout)
stdout_handler.setLevel(logging.WARNING)
stdout_handler.setFormatter(formatter)
stdout_handler.addFilter(WarningOnlyFilter())

This means that ONLY WARNING messages will pass through stdout.

Considerations

The only concern I have here is that if you are trying to specifically capture message you have to capture BOTH stdout and stderr.

You can see this change and more in PR #42.

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