QFileInfo vs QFile to test if a file is writable

I'm using PyQt and I noticed strange behavior when testing my application with Windows (everything is working as expected with Linux).

I have a file that I can read and write and I want to test it from the app:

>>> from PyQt4.QtCore import QFile, QFileInfo

>>> f1 = QFileInfo("C:\Users\Maxime\Desktop\script.py")
>>> f2 = QFile("C:\Users\Maxime\Desktop\script.py")

>>> f1.isWritable()
True
>>> f2.isWritable()
False

So it looks like QFile is wrong on that test case. But, on another file that is read-only:

>>> f1 = QFileInfo("C:\Program Files (x86)\MySoftware\stuff\script.py")
>>> f2 = QFile("C:\Program Files (x86)\MySoftware\stuff\script.py")

>>> f1.isWritable()
True
>>> f2.isWritable()
False

Now, this is QFileInfo which is wrong!

So I decided maybe I should use os.access instead:

>>> import os

>>> os.access("C:\Users\Maxime\Desktop\script.py")
True
>>> os.access("C:\Program Files (x86)\MySoftware\stuff\script.py")
True

So os.access is also wrong in one case and returns the same results as QFileInfo.

I have multiple questions:

• I am not familiar with Windows, is there something I'm missing?
• Using Qt, I can use QFileInfo and QFile to test if a file can be written. Should I use one instead of the other?
• In case that's just a bug in Qt (and Python??), I'd like a workaround that can also work on Linux and Mac OS.

Edit:

A very interesting comment from Frank explained that QFile::isWritable() will always return False since I haven't opened the file.

>>> f = QFile("C:\Users\Maxime\Desktop\script.py")
>>> f.open(QFile.WriteOnly)
True
>>> f.isWritable()
True

>>> f = QFile("C:\Program Files (x86)\MySoftware\stuff\script.py")
>>> f.open(QFile.WriteOnly)
False
>>> f.isWritable()
False