8000 bpo-8978: improve tarfile.open error message when lzma / bz2 are missing by asottile · Pull Request #24850 · python/cpython · GitHub
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bpo-8978: improve tarfile.open error message wh 8000 en lzma / bz2 are missing #24850

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Merged
merged 1 commit into from
Apr 27, 2021
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bpo-8978: improve tarfile.open error message when lzma / bz2 are missing #24850

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7 changes: 5 additions & 2 deletions Lib/tarfile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1604,17 +1604,20 @@ def open(cls, name=None, mode="r", fileobj=None, bufsize=RECORDSIZE, **kwargs):
# Find out which *open() is appropriate for opening the file.
def not_compressed(comptype):
return cls.OPEN_METH[comptype] == 'taropen'
error_msgs = []
for comptype in sorted(cls.OPEN_METH, key=not_compressed):
func = getattr(cls, cls.OPEN_METH[comptype])
if fileobj is not None:
saved_pos = fileobj.tell()
try:
return func(name, "r", fileobj, **kwargs)
except (ReadError, CompressionError):
except (ReadError, CompressionError) as e:
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I am ok with this, but this is going to create a hell of a lot of cycles, so I prefer to manually delete the list with all the exceptions once error_msgs is done to alleviate this a bit.

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I actually thought about the reference cycle problem -- that's why these are strings and not exception objects -- I don't think there's actually any cycles added by this PR ? unless you're hinting at something I don't know about!

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I don't think there's actually any cycles added by this PR ?

Oh, my bad, I was reading this as keeping the errors in the list instead of the string representation. Maybe renaming errors to error_messages? If you prefer not to do that is fine, I think we can go as it is as well.

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ah yeah that would be better -- but then what to call the joined variable 🤔 (to avoid having a variable change type)

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the joined variable

final_error_msg ?

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Notice that even if it contains the summary of multiple error messages is a single error message at the end.

Maybe you can call it summary_error_msg?

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went with error_msgs and error_msgs_summary -- thanks for the suggestion!

error_msgs.append(f'- method {comptype}: {e!r}')
if fileobj is not None:
fileobj.seek(saved_pos)
continue
raise ReadError("file could not be opened successfully")
error_msgs_summary = '\n'.join(error_msgs)
raise ReadError(f"file could not be opened successfully:\n{error_msgs_summary}")

elif ":" in mode:
filemode, comptype = mode.split(":", 1)
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12 changes: 12 additions & 0 deletions Lib/test/test_tarfile.py
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Expand Up @@ -2283,6 +2283,18 @@ def test__all__(self):
'SubsequentHeaderError', 'ExFileObject', 'main'}
support.check__all__(self, tarfile, not_exported=not_exported)

def test_useful_error_message_when_modules_missing(self):
fname = os.path.join(os.path.dirname(__file__), 'testtar.tar.xz')
with self.assertRaises(tarfile.ReadError) as excinfo:
error = tarfile.CompressionError('lzma module is not available'),
with unittest.mock.patch.object(tarfile.TarFile, 'xzopen', side_effect=error):
tarfile.open(fname)

self.assertIn(
"\n- method xz: CompressionError('lzma module is not available')\n",
str(excinfo.exception),
)


class CommandLineTest(unittest.TestCase):

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2021-03-13-14-02-07.bpo-8978.CRxG-O.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve error message for :func:`tarfile.open` when :mod:`lzma` / :mod:`bz2`
are unavailable. Patch by Anthony Sottile.
0