swev-id: astropy__astropy-14309 Fix is_fits identifier guard

[casey-brooks]

Summary

• guard astropy.io.fits is_fits for read/write origin semantics
• prevent write-mode identify_format calls from touching absent args and rely on extensions
• add regression tests covering new detection logic and signature failures

Testing

• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/pytest astropy/io/fits/tests/test_connect.py -k is_fits
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/flake8 astropy/io/fits/connect.py astropy/io/fits/tests/test_connect.py

Refs #106

[casey-brooks]

Local Validation

• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/pytest astropy/io/fits/tests/test_connect.py -k is_fits (5 passed, 154 deselected)
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/flake8 astropy/io/fits/connect.py astropy/io/fits/tests/test_connect.py (no issues)

[rowan-stein]

Requesting review for PR #108.

Summary:

• Fixes IndexError in identify_format due to unconditional args[0] access in FITS is_fits.
• Makes is_fits origin-aware; guards args access; clarifies return semantics (True/False/None) based on path/fileobj/signature/HDU presence.

Focus areas:

• Origin-aware logic for write vs read.
• Guards around args and avoiding IndexError.
• Return semantics compatibility with io registry (identify_format) expectations.
• Regression tests in astropy/io/fits/tests/test_connect.py cover write-path handling and signature checks.

Refs: Issue #106.}

[noa-lucent]

Thanks for tightening up the logic—the read/write branching and signature handling look solid. The remaining gap is test coverage for the HDU object path that should still identify FITS when an HDUList/TableHDU instance is supplied via args. Could you add that regression test before we merge?

[casey-brooks]

Follow-up Validation

• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/pytest astropy/io/fits/tests/test_connect.py -k "is_fits or identify_format" (6 passed, 154 deselected)
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/zmjqwzhgl9hwr8xnk8raj8d50lzkql66-gcc-14.3.0-lib/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/flake8 astropy/io/fits/tests/test_connect.py (no issues)

[noa-lucent]

Thanks for adding the HDU-list regression coverage—the identify_format path now exercises the guarded args branch and passes. With that in place, the origin-aware is_fits checks look solid, and the test suite covers the key read/write scenarios. Good to merge from my side.