[issue29657] os.symlink: FileExistsError shows wrong message

[Yonatan Goldschmidt]

Yonatan Goldschmidt  added the comment:

Just reached this issue independently (spent a few minutes debugging an error 
message like "FileExistsError: [Errno 17] File exists: 'a' -> 'b'", where 'a' 
didn't exist...)

I agree with Rich on this - for me, the source of confusion was that the way 
Python uses the arrow notation is just swapped from how ls(1) uses it.

Stage is "patch review" but I couldn't find any PR for it; if we're good with 
Larry's solution then I'm happy to post a PR implementing it.

--
nosy: +Yonatan Goldschmidt

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[issue29657] os.symlink: FileExistsError shows wrong message

[Kamil Turek]

Change by Kamil Turek :


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nosy: +kamilturek

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[issue29657] os.symlink: FileExistsError shows wrong message

[Eryk Sun]

Change by Eryk Sun :


--
components: +Extension Modules, Interpreter Core -Library (Lib)
stage: patch review -> 
versions: +Python 3.10, Python 3.8, Python 3.9 -Python 3.5, Python 3.6, Python 
3.7

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[issue29657] os.symlink: FileExistsError shows wrong message

[Rich Jones]

Rich Jones  added the comment:

@Larry - that would be an acceptable solution!

I'm here because I encountered this error independently. I explain why the  
arrow is a problem here: https://bugs.python.org/issue35367

The issue is that the '->' notation is already used by the standard operating 
system utilities in this context, so for Python overload this semantically in 
this case is the source of all the confusion.

It would avoid the scare that we've all encountered if it just said 'src'/'dst' 
rather than '->'.

Thanks!
R

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[issue29657] os.symlink: FileExistsError shows wrong message

[Larry Hastings]

Larry Hastings added the comment:

"Special cases aren't special enough to break the rules."  I want the error 
message to mirror the API, which it currently does.  If we swapped them, the 
error message would now contradict the API.  So no, I don't support swapping 
"src" and "dst" in the error message only when the error pertains to os.symlink.

If literally every time the two-filename version of OSError is used inside 
Python, the two filenames are "src" and "dst", then we could consider making it 
slightly more explicit, e.g.

FileExistsError: [Errno 17] File exists: src='a', dst='a_link'

I think I'd want the source code to reflect this (e.g. thinking about "src" and 
"dst" rather than "filename" and "filename2").

Would OP et al consider this change to the error message an improvement, or is 
it not interesting?

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[issue29657] os.symlink: FileExistsError shows wrong message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

See issue20517 for the discussion about current implementation. Are there any 
ideas about clearer error messages?

Added Larry as the author of the original idea and implementation.

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nosy: +larry

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[issue29657] os.symlink: FileExistsError shows wrong message

[Xiang Zhang]

Xiang Zhang added the comment:

I agree with Eryk. The error message is misleading. When I see it, I take it as 
source -> destination. I think we should make the error message clearer, or 
document it in the OSError documentation.

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[issue29657] os.symlink: FileExistsError shows wrong message

[Eryk Sun]

Eryk Sun added the comment:

To me the error message is in the model of a source -> destination operation, 
in which the arrow indicates the operation's information flow (e.g. of the 
target path or inode number) from the source file to the destination file. I've 
never viewed it superficially as just an ordering of parameter1 -> parameter2.

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[issue29657] os.symlink: FileExistsError shows wrong message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

This is not about how a symbolic link is represented. This is about how the 
operation of creating a symbolic link is represented. The first filename is the 
first argument of os.symlink(), the second filename is the second argument. Try 
to run

os.symlink('a', 'a_link')
os.symlink('b', 'a_link')

You should get an error:

FileExistsError: [Errno 17] File exists: 'b' -> 'a_link'

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[issue29657] os.symlink: FileExistsError shows wrong message

[Eryk Sun]

Eryk Sun added the comment:

A symbolic link is typically represented the other way around, from the point 
of view of the link pointing at the target. However, Python conceptualizes 
linking as something like a copy or rename operation, with source and 
destination filenames, and the current error message represents this point of 
view.

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nosy: +eryksun

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[issue29657] os.symlink: FileExistsError shows wrong message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The current error message looks good to me. What is wrong with it?

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nosy: +serhiy.storchaka

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[issue29657] os.symlink: FileExistsError shows wrong message

[Terry J. Reedy]

Changes by Terry J. Reedy :


--
stage:  -> patch review
type:  -> behavior

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[issue29657] os.symlink: FileExistsError shows wrong message

[Andrew Nester]

Andrew Nester added the comment:

I've been investigating this issue and did not come up with some easy solution.

So the problem is:
os_symlink uses `path_error2` to throw exception.
the order of file arguments now is src then dest. For provided example src is 
`a` and dest is `sym_link`. As a result `src` -> `dest` is generated here 
https://github.com/python/cpython/blob/master/Objects/exceptions.c#L1059

If we change order of arguments passed to `path_error2`, error message will be 
generated properly but OSError.filename will be incorrect (a_link instead of a) 
and following test will fail for `link`/`symlink`

https://github.com/python/cpython/blob/master/Lib/test/test_os.py#L2901

Not sure if it's OK or not, so it definitely needs some input from Python core 
developers.

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nosy: +andrewnester

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[issue29657] os.symlink: FileExistsError shows wrong message

[Sayan Chowdhury]

Changes by Sayan Chowdhury :


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nosy: +sayanchowdhury

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[issue29657] os.symlink: FileExistsError shows wrong message

[Xiang Zhang]

Xiang Zhang added the comment:

I concur the current message is misleading.

OSError makes the string "file1 -> file2". This also affects other methods 
calling `path_error2()` such as os.link().

--
nosy: +xiang.zhang
versions: +Python 3.6, Python 3.7

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[issue29657] os.symlink: FileExistsError shows wrong message

[Wolfgang Rohdewald]

New submission from Wolfgang Rohdewald:

execute the attached script. It should return

FileExistsError: [Errno 17] File exists: 'a_link' -> 'a'

but it returns

FileExistsError: [Errno 17] File exists: 'a' -> 'a_link'

--
components: Library (Lib)
files: x.py
messages: 288591
nosy: wrohdewald
priority: normal
severity: normal
status: open
title: os.symlink: FileExistsError shows wrong message
versions: Python 3.5
Added file: http://bugs.python.org/file46671/x.py

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