[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[Eryk Sun]

Eryk Sun  added the comment:

I can help, but in this case there isn't much to do. Just replace os.rename() 
with os.replace(), make a minor doc change, and maybe add a test that ensures a 
junction can be moved over an existing file on the same filesystem. For example:

>>> os.mkdir('temp')
>>> _winapi.CreateJunction('temp', 'src')
>>> os.lstat('src').st_reparse_tag == stat.IO_REPARSE_TAG_MOUNT_POINT
True
>>> open('dst', 'w').close()

The current implementation tries copytree() on the junction mountpoint and 
fails to create a new directory named "dst":

>>> try: shutil.move('src', 'dst')
... except FileExistsError as e: e
...
FileExistsError(17, 'Cannot create a file when that file already exists')

But move() should simply replace "dst" with the junction via os.replace():

>>> os.replace('src', 'dst')
>>> os.lstat('dst').st_reparse_tag == stat.IO_REPARSE_TAG_MOUNT_POINT
True

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[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Eryk Sun, do you mind to create a PR?

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nosy: +serhiy.storchaka

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[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[Eryk Sun]

Change by Eryk Sun :


--
components: +Windows
nosy: +paul.moore, steve.dower, tim.golden, zach.ware
versions: +Python 3.10

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[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[Eryk Sun]

Eryk Sun  added the comment:

shutil.move() has always fallen back on copy/unlink if os.rename() fails. This 
used to be clearly documented to account for the behavior of os.rename() in 
Windows, but the details were obscured over multiple rewrites. Currently the 
documentation makes it seem that copy/unlink are only used if the destination 
is on another filesystem.

Anyway, falling back on copy/unlink is sub-optimal as a move operation [1] and 
should be avoided wherever possible. It would be better to use os.replace() 
instead of os.rename(). This expands support for an atomic move within a 
filesystem, and it avoids the need to modify the documentation, except to 
reference os.replace() instead of os.rename().

---

[1] As implemented, a copy will not retain the source file's alternate data 
streams, security descriptor (i.e. owner and access/audit control), file 
attributes, or extended file attributes. There are additional problems with 
reparse points other than symlinks, but that's a difficult problem. We don't 
support raw copying of reparse points, or even high-level support for creating 
mountpoint (aka junction) reparse points. There's _winapi.CreateJunction, but 
the current implementation is just for testing purposes.

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nosy: +eryksun

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[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[fireattack]

fireattack  added the comment:

Sorry, I should link https://docs.python.org/3/library/shutil.html#shutil.move 
for latest doc. But the content is the same.

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[issue42929] On Windows, shutil.move doesn't raise FileExistsError if dst exists like os.rename

[fireattack]

New submission from fireattack :

According to https://docs.python.org/3.8/library/shutil.html#shutil.move

"If the destination already exists but is not a directory, it may be 
overwritten depending on os.rename() semantics."

I interpret "depending on os.rename() semantics" to mean it will follow 
os.rename()'s behavior.

According to https://docs.python.org/3/library/os.html#os.rename

"On Windows, if dst exists a FileExistsError is always raised."

However, their behaviors are not the same.

For os.rename, it does raise FileExistsError if dst exists.
For shutil.move, it silently overwrites dst.

It's either a bug in behavior of shutil.move, or the documentation need to be 
updated.

--
components: Library (Lib)
messages: 385083
nosy: fireattack
priority: normal
severity: normal
status: open
title: On Windows, shutil.move doesn't raise FileExistsError if dst exists like 
os.rename
type: behavior
versions: Python 3.9

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