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Shouldn't it be checking whether `file` (or `filename`) is a directory [1]? For example:
 except PermissionError:
 # This exception is thrown when a directory with
 # the chosen name already exists on windows.
 if _os.name == 'nt' and _os.path.isdir(file):
 continue
 # If the directory allows write access, continue 
 # trying names. On Windows, currently this test 
 # doesn't work. The implementation assumes all 
 # directories allow write access, but it really 
 # depends on the directory's discretionary and 
 # system access control lists (DACL & SACL).
 elif _os.access(dir, _os.W_OK):
 continue
 else:
 raise
---
[1]: Windows sets the last error to ERROR_ACCESS_DENIED when a system
 call returns the NTSTATUS code STATUS_FILE_IS_A_DIRECTORY. This
 status code is returned by NtCreateFile and NtOpenFile when the
 target file is a directory, but the caller asked for anything 
 but a directory (i.e. CreateOptions contains 
 FILE_NON_DIRECTORY_FILE).