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Handle missing uncompresspy in fitsinfo without traceback#19869

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mokashang:fix/fitsinfo-fitsdiff-uncompresspy
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Handle missing uncompresspy in fitsinfo without traceback#19869
mokashang wants to merge 2 commits into
astropy:mainfrom
mokashang:fix/fitsinfo-fitsdiff-uncompresspy

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@mokashang mokashang commented Jun 5, 2026

Fixes #19852.

Problem

Running fitsinfo on an LZW-compressed .Z file currently crashes with an uncaught ModuleNotFoundError traceback when the optional uncompresspy dependency is not installed:

$ fitsinfo astropy/io/fits/tests/data/lzw.fits.Z
Traceback (most recent call last):
  File ".../fitsinfo", line 6, in <module>
    sys.exit(main())
  ...
  File ".../astropy/io/fits/file.py", line 614, in _try_read_compressed
    raise ModuleNotFoundError(
ModuleNotFoundError: The optional package uncompresspy is necessary for reading LZW compressed files (.Z extension).

fitsinfo.fitsinfo() only catches OSError, so the ModuleNotFoundError raised by _BaseHDU._readfrom → _File.__init__ → _try_read_compressed propagates up uncaught.

Fix

Broaden the except in fitsinfo() to also catch ImportError and log a clean error message, matching the behaviour of fitsdiff (which already handles this case — FITSDiff.__init__ wraps any exception from fitsopen in OSError, so its existing except OSError happens to catch this).

After the fix:

$ fitsinfo astropy/io/fits/tests/data/lzw.fits.Z
ERROR: The optional package uncompresspy is necessary for reading LZW compressed files (.Z extension). [astropy.io.fits.scripts.fitsinfo]

Scope

This PR only addresses fitsinfo. The issue mentions "fitsinfo (and probably fitsdiff too)", but I verified fitsdiff already produces a clean Warning: failed to open ... message on the same input (covered by the existing test_warning_unreadable_file test in test_fitsdiff.py, added in #18882).

Testing

  • Added test_missing_optional_dependency in astropy/io/fits/tests/test_fitsinfo.py, modelled on the existing test_warning_unreadable_file for fitsdiff. It monkeypatches HAS_UNCOMPRESSPY to False and asserts that running fitsinfo on lzw.fits.Z logs an error mentioning uncompresspy rather than raising.
  • Verified the new test fails on main (with the original ModuleNotFoundError traceback) and passes with this fix.
  • Full astropy/io/fits/tests/ suite passes locally (1078 passed, 29 skipped — the skipped tests are the ones that require uncompresspy to actually be installed).

Changelog

Added docs/changes/io.fits/19852.bugfix.rst. Happy to rename it to the PR number once assigned.

@mokashang
BUG: handle missing optional dependency in fitsinfo
0382630 
When the optional dependency uncompresspy is not installed, running
fitsinfo on an LZW-compressed .Z file currently raises an
uncaught ModuleNotFoundError and prints a long traceback.  Catch
ImportError alongside the existing OSError so the user instead
sees a clean error message, matching the behaviour of fitsdiff
(which already catches this case via FITSDiff.__init__).

Includes a regression test that mirrors the existing fitsdiff test by
monkeypatching HAS_UNCOMPRESSPY to False and running the script
against the bundled lzw.fits.Z data file.

Fixes astropy#19852
@mokashang mokashang requested a review from saimn as a code owner June 5, 2026 16:21
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github-actions Bot commented Jun 5, 2026

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@pllim pllim added this to the v8.1.0 milestone Jun 5, 2026
@pllim pllim added the Bug label Jun 5, 2026
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@saimn saimn Awaiting requested review from saimn saimn is a code owner

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BUG: fitsinfo/fitsdiff crashes when uncompresspy is not installed

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