Issue: If you try to copy a file to a directory that doesn't exist using shutil.copy, a IsADirectory error is raised saying the directory exists. 

This issue is actually caused when `open(not_a_dir, 'wb') is called on a non-existing dir.

Expected behaviour: Should instead raise NotADirectoryError
-----------------------------

Steps to reproduce:

[nav] In [1]: import os
         ...: from pathlib import Path
         ...: from shutil import copy
         ...:
         ...: tmp_file = '/tmp/some_file.txt'
         ...: Path(tmp_file).touch()
         ...: nonexistent_dir = 'not_a_dir/'
         ...: assert not os.path.exists(nonexistent_dir)
         ...: copy(tmp_file, nonexistent_dir)
---------------------------------------------------------------------------
IsADirectoryError                         Traceback (most recent call last)
<ipython-input-2-b0e0ec4f4875> in <module>
      7 nonexistent_dir = 'not_a_dir/'
      8 assert not os.path.exists(nonexistent_dir)
----> 9 copy(tmp_file, nonexistent_dir)

~/miniconda3/lib/python3.7/shutil.py in copy(src, dst, follow_symlinks)
    243     if os.path.isdir(dst):
    244         dst = os.path.join(dst, os.path.basename(src))
--> 245     copyfile(src, dst, follow_symlinks=follow_symlinks)
    246     copymode(src, dst, follow_symlinks=follow_symlinks)
    247     return dst

~/miniconda3/lib/python3.7/shutil.py in copyfile(src, dst, follow_symlinks)
    119     else:
    120         with open(src, 'rb') as fsrc:
--> 121             with open(dst, 'wb') as fdst:
    122                 copyfileobj(fsrc, fdst)
    123     return dst

IsADirectoryError: [Errno 21] Is a directory: 'not_a_dir/'

In fact, the issue seems to be coming from open() itself when opening a non-existent directory in write mode:

[nav] In [1]: import os
         ...: nonexixstent_dir = 'not_a_dir/'
         ...: assert not os.path.exists(nonexixstent_dir)
         ...: with open(nonexixstent_dir, 'wb') as fdst:
         ...:     pass
---------------------------------------------------------------------------
IsADirectoryError                         Traceback (most recent call last)
<ipython-input-1-73d4010d6f34> in <module>
      2 dir_path = 'not_a_dir/'
      3 assert not os.path.exists(nonexixstent_dir)
----> 4 with open(nonexixstent_dir, 'wb') as fdst:
      5     pass

IsADirectoryError: [Errno 21] Is a directory: 'not_a_dir/'

> IsADirectoryError: [Errno 21] Is a directory: 'not_a_dir/'

The trailing slash forces the OS to handle "not_a_dir" as a directory [1]. 

    A pathname that contains at least one non- <slash> character and that 
    ends with one or more trailing <slash> characters shall not be resolved
    successfully unless the last pathname component before the trailing 
    <slash> characters names an existing directory or a directory entry 
    that is to be created for a directory immediately after the pathname is
    resolved. 

Mode "w" corresponds to low-level POSIX open() flags O_CREAT | O_TRUNC | O_WRONLY. If write access is requested for a directory, the open() system call must fail with EISDIR [2].

    [EISDIR]
        The named file is a directory and oflag includes O_WRONLY or O_RDWR,
        or includes O_CREAT without O_DIRECTORY.

In most cases, opening a directory with O_CREAT also fails with E_ISDIR. POSIX does permit an implementation to create a directory with O_CREAT | O_DIRECTORY. In Linux, however, O_CREAT always creates a regular file, regardless of O_DIRECTORY, so open(pathname, O_CREAT | flags) always fails with EISDIR when pathname is an existing directory or names a directory by way of a trailing slash.

---
[1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html

I left this open in case someone wants to modify shutil.copy() and shutil.copy2() to raise a less misleading exception when `dst` names a non-existing directory such as 'not_a_dir/'. Failing with IsADirectoryError (errno EISDIR) is confusing since shutil.copy() and shutil.copy2() do support a destination directory.

Note that in Windows this case fails with EINVAL, which is at least less misleading than EISDIR. The EINVAL error is based on WinAPI ERROR_INVALID_NAME (123). CreateFileW() can create a directory if passed particular parameters, in which case a trailing slash is allowed. Otherwise it fails with ERROR_INVALID_NAME (123), unless it's an open-existing disposition, in which case it fails with ERROR_FILE_NOT_FOUND (2). This is specified in [MS-FSA] 2.1.5.1 [1]:

    Phase 6 -- Location of file (final path component):

        Search ParentFile.DirectoryList for a Link where Link.Name or
        Link.ShortName matches FileNameToOpen. If such a link is found:

            Set File = Link.File.
            Set Open.File to File.
            Set Open.Link to Link.

        Else:

            If (CreateDisposition == FILE_OPEN || CreateDisposition ==
            FILE_OVERWRITE), the operation MUST be failed with
            STATUS_OBJECT_NAME_NOT_FOUND.

    Phase 7 -- Type of stream to open:

        If PathName contains a trailing backslash:

            If StreamTypeToOpen is DataStream or
            CreateOptions.FILE_NON_DIRECTORY_FILE is TRUE, the operation
            MUST be failed with STATUS_OBJECT_NAME_INVALID.

NTAPI STATUS_OBJECT_NAME_NOT_FOUND and STATUS_OBJECT_NAME_INVALID map to WinAPI ERROR_FILE_NOT_FOUND and ERROR_INVALID_NAME.

---
[1] https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fsa/8ada5fbe-db4e-49fd-aef6-20d54b748e40

I've opened the PR here:
https://github.com/python/cpython/pull/27049/files

.. but can someone trigger builds for other OSes like Aix, FreeBSD, etc? I'm not sure what errors they would use for this case so the unit test may have to be updated to skip some of them.

I'm not sure if shutil module docs should be updated or not, as this change applies to a narrow corner case.

Note that someone else ran into this confusion here: https://bugs.python.org/issue43153 .

New changeset 248173cc0483a9ad9261353302f1234cf9eb2ebe by andrei kulakov in branch 'main':
bpo-43219: shutil.copyfile, raise a less confusing exception instead of IsADirectoryError (GH-27049)
https://github.com/python/cpython/commit/248173cc0483a9ad9261353302f1234cf9eb2ebe

New changeset 1577259cc51d0d46ad676798ce0a130039acf956 by Miss Islington (bot) in branch '3.10':
bpo-43219: shutil.copyfile, raise a less confusing exception instead of IsADirectoryError (GH-27049)
https://github.com/python/cpython/commit/1577259cc51d0d46ad676798ce0a130039acf956

New changeset c89f0b2587eb0b16175a0bbb12d0b86314ff9320 by Miss Islington (bot) in branch '3.9':
[3.9] bpo-43219: shutil.copyfile, raise a less confusing exception instead of IsADirectoryError (GH-27049) (GH-27082)
https://github.com/python/cpython/commit/c89f0b2587eb0b16175a0bbb12d0b86314ff9320

On Solaris (I checked this on Oracle and SmartOS), the error is:

  NotADirectoryError: [Errno 20] Not a directory: 'not_a_dir/'

which I think belongs to the 'errors are not confusing' category with Windows and macOS.

New changeset 6564656495d456a1bcc1aaa06abfc696209f37b2 by Jakub Kulík in branch 'main':
bpo-43219: skip Solaris in the test as well (GH-27257)
https://github.com/python/cpython/commit/6564656495d456a1bcc1aaa06abfc696209f37b2

New changeset dae4928dd07109db69e090b1c8193a023ce695cd by Miss Islington (bot) in branch '3.9':
[3.9] bpo-43219: skip Solaris in the test as well (GH-27257) (GH-27267)
https://github.com/python/cpython/commit/dae4928dd07109db69e090b1c8193a023ce695cd

New changeset 574da4633b44b4048f74c93da496ed2a3ead99dd by Miss Islington (bot) in branch '3.10':
[3.10] bpo-43219: skip Solaris in the test as well (GH-27257) (GH-27268)
https://github.com/python/cpython/commit/574da4633b44b4048f74c93da496ed2a3ead99dd

Thanks for reporting Jakub, and for patching Gregory!

The changelog wrongfully links to https://bugs.python.org/issue41928 instead of this issue.

Also the fix introduced a regression: Trying to copy a directory now raises a FileNotFoundError

New changeset b7eac52b466f697d3e89f47508e0df0196a98970 by andrei kulakov in branch 'main':
bpo-45234: Fix FileNotFound exception raised instead of IsADirectoryError in shutil.copyfile() (GH-28421)
https://github.com/python/cpython/commit/b7eac52b466f697d3e89f47508e0df0196a98970

New changeset 41d48bc038b254cc4a78a2d840097196b9545a84 by Miss Islington (bot) in branch '3.10':
bpo-45234: Fix FileNotFound exception raised instead of IsADirectoryError in shutil.copyfile() (GH-28421) (GH-28508)
https://github.com/python/cpython/commit/41d48bc038b254cc4a78a2d840097196b9545a84

New changeset 09390c837a0bf73e213db2fbde34d756fa77a837 by Miss Islington (bot) in branch '3.9':
bpo-45234: Fix FileNotFound exception raised instead of IsADirectoryError in shutil.copyfile() (GH-28421) (GH-28507)
https://github.com/python/cpython/commit/09390c837a0bf73e213db2fbde34d756fa77a837

New changeset e0b61b28641bdd20cfeff6d9878f1318b711ca19 by Pablo Galindo (Miss Islington (bot)) in branch '3.10':
bpo-45234: Fix FileNotFound exception raised instead of IsADirectoryError in shutil.copyfile() (GH-28421) (GH-28508)
https://github.com/python/cpython/commit/e0b61b28641bdd20cfeff6d9878f1318b711ca19