Fixing the double logging issue with a logging.filter

Thu, 22 May 2025 10:22:46 -0000

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.