IOError: [Errno 13] Permission denied when trying to open hidden file in "w" mode

Question

I want to replace the contents of a hidden file, so I attempted to open it in w mode so it would be erased/truncated:

>>> import os
>>> ini_path = '.picasa.ini'
>>> os.path.exists(ini_path)
True
>>> os.access(ini_path, os.W_OK)
True
>>> ini_handle = open(ini_path, 'w')

But this resulted in a traceback:

IOError: [Errno 13] Permission denied: '.picasa.ini'

However, I was able to achieve the intended result with r+ mode:

>>> ini_handle = open(ini_path, 'r+')
>>> ini_handle.truncate()
>>> ini_handle.write(ini_new)
>>> ini_handle.close()

Q. What is the difference between the w and r+ modes, such that one has "permission denied" but the other works fine?

UPDATE: I am on win7 x64 using Python 2.6.6, and the target file has its hidden attribute set. When I tried turning off the hidden attribute, w mode succeeds. But when I turn it back on, it fails again.

Q. Why does w mode fail on hidden files? Is this known behaviour?

Answer 1

It's just how the Win32 API works. Under the hood, Python's open function is calling the CreateFile function, and if that fails, it translates the Windows error code into a Python IOError.

The r+ open mode corresponds to a dwAccessMode of GENERIC_READ|GENERIC_WRITE and a dwCreationDisposition of OPEN_EXISTING. The w open mode corresponds to a dwAccessMode of GENERIC_WRITE and a dwCreationDisposition of CREATE_ALWAYS.

If you carefully read the remarks in the CreateFile documentation, it says this:

If CREATE_ALWAYS and FILE_ATTRIBUTE_NORMAL are specified, CreateFile fails and sets the last error to ERROR_ACCESS_DENIED if the file exists and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To avoid the error, specify the same attributes as the existing file.

So if you were calling CreateFile directly from C code, the solution would be to add in FILE_ATTRIBUTE_HIDDEN to the dwFlagsAndAttributes parameter (instead of just FILE_ATTRIBUTE_NORMAL). However, since there's no option in the Python API to tell it to pass in that flag, you'll just have to work around it by either using a different open mode or making the file non-hidden.