[issue30641] No way to specify "File name too long" error in except statement.

[R. David Murray]

R. David Murray added the comment:

Since according to Eryk there's no way to have a reliable cross-platform 
exception class catching file name to long, I'm rejecting this feature request.

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue30641] No way to specify "File name too long" error in except statement.

[Eryk Sun]

Eryk Sun added the comment:

An exception specifically for ENAMETOOLONG would be limited to Unix systems.

The Windows CRT defines ENAMETOOLONG but doesn't use it. Windows file systems 
do not return a specific status code for a filename that's too long. Per MS-FSA 
2.1.5.1 [1], a request to open a file must fail with STATUS_OBJECT_NAME_INVALID 
if the name is invalid according to the spec in MS-FSCC 2.1.5 [2] (e.g. max 
component length cannot exceed 255 characters and may be less). This doesn't 
tell the caller why the filename is invalid. Anyway, for what it's worth, the 
Windows API translates this status code to ERROR_INVALID_NAME (0x007B), and the 
CRT in turn maps the latter to EINVAL.

Also, for versions prior to Windows 10, or Windows 10 without the 
LongPathsEnabled policy setting, the observed error is more commonly due to 
path preprocessing in user mode. In this case DOS paths are limited to MAX_PATH 
(260). The error depends on the called function -- e.g. CreateFileA vs 
SetCurrentDirectoryA, or calling the [A]NSI vs [W]ide-character version. It 
could be ERROR_PATH_NOT_FOUND (ENOENT), ERROR_FILENAME_EXCED_RANGE (ENOENT), or 
ERROR_INVALID_PARAMETER (EINVAL). 

[1]: https://msdn.microsoft.com/en-us/library/ff469536
[2]: https://msdn.microsoft.com/en-us/library/cc422524

--
nosy: +eryksun

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[issue30641] No way to specify "File name too long" error in except statement.

[Steven D'Aprano]

Steven D'Aprano added the comment:

I don't understand what you actually are requesting here.

Are you requesting a way to tell whether or not the filename is too long? 
You've already been told that the way to do that is to check errno, and you say 
that you already knew that.

exc.errno == errno.ENAMETOOLONG

Or are you asking for a way to find out ahead of time what the maximum filename 
length will be? You say:

> it's almost impossible to reliably check the allowed filename length

I think that's right: it *is* almost impossible to reliably check the allowed 
filename length, except by trying it and seeing whether or not you can create 
the file.

The difficulty here is that there's no one maximum name length, it depends on 
the file system of the particular device you're accessing. So even if we had 
this `get_maximum_filename_length` function, and you wrote:

if len(fname) < get_maximum_filename_length():
with open(fname) as f: ...

the call to open might *still* fail with ENAMETOOLONG if you happen to be 
writing to a device or disk with a shorter than expected length.

E.g. FAT32 has a limit of either 8.3 bytes or 255 double-byte Unicode 
characters depending on the implementation; NTFS and HFS+ have a limit of 255 
characters; ext4 has a limit of 255 bytes; Joliet has a limit of 64 characters; 
etc.

And then there's the question of the maximum allowed path.

So I think the first thing you should do is clarify exactly what it is that 
you're asking for:

- a way to check whether the exception is ENAMETOOLONG;

- specifically a new sub-exception for that case;

- a constant or function that returns the maximum file name length;

- or the maximum path name length;

- or something else.


But any of these (except the first, which already exists) is a new feature, not 
a bug fix, to it can only go into 3.7.

--
nosy: +steven.daprano

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[issue30641] No way to specify "File name too long" error in except statement.

[Max Staff]

Max Staff added the comment:

...at least those are the only two ways that I can think of.

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[issue30641] No way to specify "File name too long" error in except statement.

[Max Staff]

Max Staff added the comment:

Yes I know about the errno. There would be two ways to resolve this:

One way would be by introducing a new exception class which would be nice 
because it's almost impossible to reliably check the allowed filename length 
(except for trial and error) and I have quite a few functions where I would 
want the error to propagate further as long as it's not an ENAMETOOLONG.

The other way would be by introducing a new syntax feature ("except OSError as 
e if e.errno == errno.ENAMETOOLONG:") but I don't think that that approach is 
reasonable.

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[issue30641] No way to specify "File name too long" error in except statement.

[Antoine Pitrou]

Antoine Pitrou added the comment:

On Unix, you can simply check the errno value:

>>> fn = "x" * 999
>>> try: open(fn, "r")
... except OSError as e: exc = e
... 
>>> exc.errno
36
>>> exc.errno == errno.ENAMETOOLONG
True

I don't know about Windows.

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[issue30641] No way to specify "File name too long" error in except statement.

[R. David Murray]

R. David Murray added the comment:

There appears to be an errno for file name too long, so I presume you are 
making a feature request for a new exception class.  I believe Antoine tried to 
strike a balance between the utility of the sub-exceptions and their number, so 
you'll have to make an argument for its utility.

--
components: +Library (Lib) -IO
nosy: +pitrou, r.david.murray
type: behavior -> enhancement
versions:  -Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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[issue30641] No way to specify "File name too long" error in except statement.

[Max Staff]

New submission from Max Staff:

There are different ways to catch exceptions of the type "OSError": By using 
"except OSError as e:" and then checking the errno or by using "except 
FileNotFoundError e:" or "except FileExistsError e:" or whatever error one 
wants to catch. There's no such way for above mentioned error that occurs when 
a filename is too long for the filesystem/OS.

--
components: IO
messages: 295810
nosy: Max Staff
priority: normal
severity: normal
status: open
title: No way to specify "File name too long" error in except statement.
type: behavior
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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