blocks with multiline style breaking in 2.5.5

[kjaymiller]

After upgrading to the latest version of markdown2 (v2.5.5). I began to get an AssertionError.

AI Attestation: I troubleshot this using Claude Code Opus4.6. Claude Code helped to write this summary but I did verify the issue and the working fix

The error occurs in _hash_html_block_sub at line 927:

File "markdown2.py", line 927, in _hash_html_block_sub
    assert m is not None
           ^^^^^^^^^^^^^
AssertionError

The file in question has a <div> tag with a style attribute that wraps across lines:

<div style="position: relative; width: 100%; height: 0; padding-top: 56.2500%;
 padding-bottom: 0; box-shadow: 0 2px 8px 0 rgba(63,69,81,0.16); margin-top: 1.6em; margin-bottom: 0.9em; overflow: hidden;
 border-radius: 8px; will-change: transform;">
  <iframe loading="lazy" title="2024 Year in Review Presentation" style="position: absolute; width: 100%; height: 100%; top: 0; left: 0; border: none; padding: 0;margin: 0;"
    src="https://www.canva.com/design/DAGbnweUoGY/-WaS6yuP3nwUW_lEH4TNKA/view?embed" allowfullscreen="allowfullscreen" allow="fullscreen">
  </iframe>
</div>

With some AI troubleshooting, the fix was deemed to be to put the <div> style attribute on a single line:

<div style="position: relative; width: 100%; height: 0; padding-top: 56.2500%; padding-bottom: 0; box-shadow: 0 2px 8px 0 rgba(63,69,81,0.16); margin-top: 1.6em; margin-bottom: 0.9em; overflow: hidden; border-radius: 8px; will-change: transform;">
  <iframe loading="lazy" title="2024 Year in Review Presentation" style="position: absolute; width: 100%; height: 100%; top: 0; left: 0; border: none; padding: 0;margin: 0;"
    src="https://www.canva.com/design/DAGbnweUoGY/-WaS6yuP3nwUW_lEH4TNKA/view?embed" allowfullscreen="allowfullscreen" allow="fullscreen">
  </iframe>
</div>

How We Verified the fix

I created a test script that runs content against multiple markdown2 versions:

"""Test a markdown string against multiple markdown2 versions."""

import subprocess
import sys

versions = ['2.5.5', '2.5.4', '2.5.3', '2.5.2', '2.5.1', '2.5.0', '2.4.13', '2.4.12', '2.4.11', '2.4.10']

# Multiline attribute on <div> - triggers the bug
failing_content = '''<div style="position: relative; width: 100%; height: 0; padding-top: 56.2500%;
 padding-bottom: 0; box-shadow: 0 2px 8px 0 rgba(63,69,81,0.16); margin-top: 1.6em; margin-bottom: 0.9em; overflow: hidden;
 border-radius: 8px; will-change: transform;">
  <iframe loading="lazy" title="2024 Year in Review Presentation" style="position: absolute; width: 100%; height: 100%; top: 0; left: 0; border: none; padding: 0;margin: 0;"
    src="https://www.canva.com/design/DAGbnweUoGY/-WaS6yuP3nwUW_lEH4TNKA/view?embed" allowfullscreen="allowfullscreen" allow="fullscreen">
  </iframe>
</div>'''

# Single-line attribute on <div> - the fix
fixed_content = '''<div style="position: relative; width: 100%; height: 0; padding-top: 56.2500%; padding-bottom: 0; box-shadow: 0 2px 8px 0 rgba(63,69,81,0.16); margin-top: 1.6em; margin-bottom: 0.9em; overflow: hidden; border-radius: 8px; will-change: transform;">
  <iframe loading="lazy" title="2024 Year in Review Presentation" style="position: absolute; width: 100%; height: 100%; top: 0; left: 0; border: none; padding: 0;margin: 0;"
    src="https://www.canva.com/design/DAGbnweUoGY/-WaS6yuP3nwUW_lEH4TNKA/view?embed" allowfullscreen="allowfullscreen" allow="fullscreen">
  </iframe>
</div>'''

for label, content in [("FAILING (multiline attribute)", failing_content), ("FIXED (single-line attribute)", fixed_content)]:
    print(f"=== {label} ===")
    for v in versions:
        r = subprocess.run(
            ["uv", "run", "--no-project", "--python", "3.13", "--with", f"markdown2=={v}",
             "python", "-c",
"""
import markdown2, sys
markdown2.markdown(sys.argv[1])
print('OK')
""", content],
            capture_output=True, text=True, timeout=30
        )
        out = r.stdout.strip()
        if not out:
            if 'AssertionError' in r.stderr:
                out = 'FAIL (AssertionError)'
            else:
                out = r.stderr.strip().split('\n')[-1][:100]
        print(f'{v}: {out}')
    print()

Results:

=== FAILING (multiline attribute) ===
2.5.5: FAIL (AssertionError)
2.5.4: OK
2.5.3: OK
2.5.2: OK
2.5.1: OK
2.5.0: OK
2.4.13: OK
2.4.12: OK
2.4.11: OK
2.4.10: OK

=== FIXED (single-line attribute) ===
2.5.5: OK
2.5.4: OK
2.5.3: OK
2.5.2: OK
2.5.1: OK
2.5.0: OK
2.4.13: OK
2.4.12: OK
2.4.11: OK
2.4.10: OK

I also tried this with a simple div with multiline styling and a p tag. I got the same issue, so I can rule out this is a problem with just iframes.

Conclusion

The bug was introduced in 2.5.5 — all prior versions handle multiline attributes without issue.