[issue38572] Misleading AttributeError accessing fileno attribute in tarfile

[Zackery Spytz]

Change by Zackery Spytz :


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nosy: +ZackerySpytz
nosy_count: 4.0 -> 5.0
pull_requests: +21244
pull_request: https://github.com/python/cpython/pull/22178

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[issue38572] Misleading AttributeError accessing fileno attribute in tarfile

[PCManticore]

PCManticore  added the comment:

Just submitted a PR for this issue, hope I got it right. Curious to see if 
there are any backwards compatibility concerns with this approach.

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[issue38572] Misleading AttributeError accessing fileno attribute in tarfile

[PCManticore]

Change by PCManticore :


--
keywords: +patch
pull_requests: +16930
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17449

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[issue38572] Misleading AttributeError accessing fileno attribute in tarfile

[PCManticore]

Change by PCManticore :


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nosy: +Claudiu.Popa

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[issue38572] Misleading AttributeError accessing fileno attribute in tarfile

[Karthikeyan Singaravelan]

New submission from Karthikeyan Singaravelan :

This came up during https://github.com/psf/requests/issues/5229 . While fileno 
returns True as an attribute the actual implementation uses self.raw.fileno 
where the AttributeError is raised at 
https://github.com/python/cpython/blob/96b06aefe23521b61e4e9cdd44f5d30b00c7eb95/Lib/_pyio.py#L878
 (Python implementation) . I guess it should be raising UnsupportedOperation 
when self.raw has no fileno so that the callers only need to catch 
UnsupportedOperation. Something like below for fileno implementation that will 
give a better exception.

   def fileno(self):
if hasattr(self.raw, 'fileno'):
return self.raw.fileno()
else:
self._unsupported("fileno")

# tarfile_fileno.py

import tarfile
import os

for f in ["foo.txt", "foo.tar"]:
try:
os.unlink(f)
except OSError:
pass

with open("foo.txt", "w") as f:
f.write("foo.txt")
with tarfile.open("foo.tar", "w:gz") as tarfile_obj:
tarfile_obj.add(f.name)

with tarfile.open("foo.tar", "r:gz") as f:
member = f.extractfile("foo.txt")
print(f"Has attr : {hasattr(member, 'fileno')}")
print(member.fileno())

# Current exception

python.exe .\tarfile_fileno.py
Has attr : True
Traceback (most recent call last):
  File ".\tarfile_fileno.py", line 18, in 
print(member.fileno())
AttributeError: '_FileInFile' object has no attribute 'fileno'

# Proposed exception (Patching tarfile to use _pyio and modifying fileno as 
above) 

python.exe .\tarfile_fileno.py
Has attr : True
Traceback (most recent call last):
  File ".\tarfile_fileno.py", line 11, in 
print(member.fileno())
  File 
"C:\Users\kasingar\AppData\Local\Programs\Python\Python37-32\lib\_pyio.py", 
line 831, in fileno
self._unsupported("fileno")
  File 
"C:\Users\kasingar\AppData\Local\Programs\Python\Python37-32\lib\_pyio.py", 
line 320, in _unsupported
(self.__class__.__name__, name))
io.UnsupportedOperation: ExFileObject.fileno() not supported

--
components: Library (Lib)
messages: 355283
nosy: lars.gustaebel, serhiy.storchaka, xtreak
priority: normal
severity: normal
status: open
title: Misleading AttributeError accessing fileno attribute in tarfile
type: behavior
versions: Python 3.9

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