<p>My code processes all the files in a folder on a Windows box. I want to offer the user (who happens to also be me) the option to select the folder to be processed, using the standard File Chooser dialog.</p>

<p>I am trying to use the <a href="http://msdn.microsoft.com/en-us/library/ms646927(VS.85).aspx" rel="nofollow noreferrer" title="MSDN Page for GetOpenFileName">GetOpenFileName</a> function to make this happen. (I am actually calling it from Python via <a href="http://docs.activestate.com/activepython/2.5/pywin32/win32gui__GetOpenFileNameW_meth.html" rel="nofollow noreferrer" title="PyWin32 Manual on GetOpenFileNameW">pywin32</a>, but that shouldn't be relevant.)</p>

<p>I can get it to select a particular file, but I can't see any options to let the user select a folder instead.</p>

<p>Is there a flag combination I haven't understood, am I calling the wrong function entirely or is there another idiom for doing this?</p>

Unfortunately, I think you're calling the wrong function entirely. It's been a while since I've done Win32 gui stuff.. but I seem to recall that there was a different way of bringing up a "select directory" box. It wasn't a straight Win32 function, but was some shell API function that required a whole heap of code to do the simplest thing.
Update: A bit of google reveals the
SHBrowseForFolder
function.
Update two: And here is an
example for Python
.