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    <p>Here's a start on the win32 half of a portable implementation, that does not need a seperate locking mechanism.</p> <p>Requires the <a href="http://python.net/crew/mhammond/win32/" rel="nofollow noreferrer">Python for Windows Extensions</a> to get down to the win32 api, but that's pretty much mandatory for python on windows already, and can alternatively be done with <a href="http://www.python.org/doc/lib/module-ctypes.html" rel="nofollow noreferrer">ctypes</a>. The code could be adapted to expose more functionality if it's needed (such as allowing <code>FILE_SHARE_READ</code> rather than no sharing at all). See also the MSDN documentation for the <a href="http://msdn.microsoft.com/en-us/library/aa363858.aspx" rel="nofollow noreferrer"><code>CreateFile</code></a> and <a href="http://msdn.microsoft.com/en-us/library/aa365747.aspx" rel="nofollow noreferrer"><code>WriteFile</code></a> system calls, and the <a href="http://msdn.microsoft.com/en-us/library/aa363874.aspx" rel="nofollow noreferrer">article on Creating and Opening Files</a>.</p> <p>As has been mentioned, you can use the standard <a href="http://www.python.org/doc/lib/module-fcntl.html" rel="nofollow noreferrer">fcntl</a> module to implement the unix half of this, if required.</p> <pre><code>import winerror, pywintypes, win32file class LockError(StandardError): pass class WriteLockedFile(object): """ Using win32 api to achieve something similar to file(path, 'wb') Could be adapted to handle other modes as well. """ def __init__(self, path): try: self._handle = win32file.CreateFile( path, win32file.GENERIC_WRITE, 0, None, win32file.OPEN_ALWAYS, win32file.FILE_ATTRIBUTE_NORMAL, None) except pywintypes.error, e: if e[0] == winerror.ERROR_SHARING_VIOLATION: raise LockError(e[2]) raise def close(self): self._handle.close() def write(self, str): win32file.WriteFile(self._handle, str) </code></pre> <p>Here's how your example from above behaves:</p> <pre><code>&gt;&gt;&gt; path = "C:\\scr.txt" &gt;&gt;&gt; file1 = WriteLockedFile(path) &gt;&gt;&gt; file2 = WriteLockedFile(path) #doctest: +IGNORE_EXCEPTION_DETAIL Traceback (most recent call last): ... LockError: ... &gt;&gt;&gt; file1.write("111") &gt;&gt;&gt; file1.close() &gt;&gt;&gt; print file(path).read() 111 </code></pre>
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    1. POWhat is the best way to open a file for exclusive access in Python?
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    1. COOP wanted writes to be fully exclusive, so I think win32file.FILE_SHARE_READ should be replaced with 0 (SHARE_NONE).
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    2. COHm... yes, and wants a read-locking version too. Have made a couple of edits, but will leave more detailed mapping of python-file-semantics to win32-api-semantics as exercise for any reader who wants to go down this route.
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