[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[John Mark Vandenberg]

John Mark Vandenberg added the comment:

It seems like there is already sufficient detection of invalid stack levels in 
warnings.warn, and one of the code paths does `module = ""` and later 
another does `filename = module`, so `filename` can be intentionally junk data, 
which will be passed to `linecache`.

I expect this could be satisfactorily resolved by warn() setting filename = 
'', and `formatwarning` not invoking linecache when the 
filename is '', '', etc., or at least ignoring the 
exception from linecache when the filename is .

Looking forward, why not let Python 3.6 warn() behave 'better' when the 
stacklevel is invalid.

e.g. it could raise ValueError (ouch, but 'correct'), or it could reset the 
stacklevel to 1 (a sensible fallback) and issue an auxillary SyntaxWarning to 
inform everyone that the stacklevel requested was incorrect.

--
nosy: +John.Mark.Vandenberg

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Christoph Reiter]

Christoph Reiter added the comment:

To add some info why we hit that bug:

https://bugs.python.org/issue24305 changed the warning stacklevel needed for 
import warnings from 8 to 2. As a result any code doing import warnings will 
likely hit that edge case when run with 3.5 and openafs installed. But I'm not 
sure if there is much code out there doing import warnings.. so ymmv.


For anyone with the same problem looking for a workaround:

I went through all Python versions and searched for the needed stacklevel.

def get_import_stacklevel(import_hook):
"""Returns the stacklevel value for warnings.warn() for when the warning
gets emitted by an imported module, but the warning should point at the
code doing the import.

Pass import_hook=True if the warning gets generated by an import hook
(warn() gets called in load_module(), see PEP302)
"""

py_version = sys.version_info[:2]
if py_version <= (3, 2):
# 2.7 included
return 4 if import_hook else 2
elif py_version == (3, 3):
return 8 if import_hook else 10
elif py_version == (3, 4):
return 10 if import_hook else 8
else:
# fixed again in 3.5+, see https://bugs.python.org/issue24305
return 4 if import_hook else 2

--
nosy: +lazka

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Brett Cannon]

Brett Cannon added the comment:

So I'm going to close this as "won't fix" because linecache is horribly 
over-engineered and I'm afraid trying to fix this will break code that actually 
worked previously. This is obviously an odd edge case that I think we can live 
with.

--
resolution:  -> wont fix
status: open -> closed

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[R. David Murray]

R. David Murray added the comment:

Hmm.  I remember fixing problems with linecache and inspect.  I wonder if this 
is a variation on that?  I don't remember the issue number.

--
nosy: +r.david.murray

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The "sys" file is not imported. It is read by linecache.

warnings.warn() tries to determine the file name by looking at __file__ in 
module's globals. If it fails, it falls back to the module name. Definitely it 
fails also for builtin modules, binary extensions and zipimported modules.

--
nosy: +serhiy.storchaka

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis :


--
nosy: +Arfrever

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Brett Cannon]

Brett Cannon added the comment:

I'll have to dig into this to figure out what's going on, but since the sys 
module is built into Python it makes it so import won't accidentally import 
some file named sys. the real question is who thinks the 'sys' file should be 
considered.

--
assignee:  -> brett.cannon

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[R. David Murray]

Changes by R. David Murray :


--
nosy: +brett.cannon

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[issue25493] warnings.warn: wrong stacklevel causes import of local file "sys"

[Michael Laß]

New submission from Michael Laß:

When there is a file called "sys" in the local directory of a python script and 
warning.warn is called with an invalid stacklevel, python tries to import that 
file and throws an error like the following:

>>> import warnings
>>> warnings.warn("foo", Warning, stacklevel=2)
Traceback (most recent call last):
  File "/usr/lib/python3.5/tokenize.py", line 392, in find_cookie
line_string = line.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 24: 
invalid start byte

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/lib/python3.5/warnings.py", line 18, in showwarning
file.write(formatwarning(message, category, filename, lineno, line))
  File "/usr/lib/python3.5/warnings.py", line 26, in formatwarning
line = linecache.getline(filename, lineno) if line is None else line
  File "/usr/lib/python3.5/linecache.py", line 16, in getline
lines = getlines(filename, module_globals)
  File "/usr/lib/python3.5/linecache.py", line 47, in getlines
return updatecache(filename, module_globals)
  File "/usr/lib/python3.5/linecache.py", line 136, in updatecache
with tokenize.open(fullname) as fp:
  File "/usr/lib/python3.5/tokenize.py", line 456, in open
encoding, lines = detect_encoding(buffer.readline)
  File "/usr/lib/python3.5/tokenize.py", line 433, in detect_encoding
encoding = find_cookie(first)
  File "/usr/lib/python3.5/tokenize.py", line 397, in find_cookie
raise SyntaxError(msg)
SyntaxError: invalid or missing encoding declaration for 'sys'

In this case "sys" is a binary that belongs to openafs (/usr/bin/sys) and of 
course it is no valid python.

"import sys" produces no error though, so typically python is able to 
distinguish between its sys module and this file.

A workaround is to run python with the "-I" parameter.

Expected output:

>>> import warnings
>>> warnings.warn("foo", Warning, stacklevel=2)
sys:1: Warning: foo

This bug was spotted in Gnome's pygobject bindings. Here is the corresponding 
bug report:
https://bugzilla.gnome.org/show_bug.cgi?id=757184

--
components: Interpreter Core
messages: 253551
nosy: bevan-bi-co
priority: normal
severity: normal
status: open
title: warnings.warn: wrong stacklevel causes import of local file "sys"
type: behavior
versions: Python 3.5

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