The IC module returns a path too long error for file:/localhost type URLs. Here is an example:

# this works:
>>> webbrowser.open("file:///Users/rowen/PythonRO/TUI/TUIHelp/Telescope.html#CoordSys")

# this fails, though URLs of this form are accepted on unix:
>>> webbrowser.open("file:/Users/rowen/PythonRO/TUI/TUIHelp/Telescope.html#CoordSys")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/Library/Frameworks/Python.framework/Versions/2.2/lib/python2.2/webbrowser.py", line 43, in open
    get().open(url, new, autoraise)
  File "/Library/Frameworks/Python.framework/Versions/2.2/lib/python2.2/webbrowser.py", line 275, in open
    ic.launchurl(url)
  File "/Library/Frameworks/Python.framework/Versions/2.2/Mac/Lib/ic.py", line 231, in launchurl
    return _dft_ic.launchurl(url, hint)
  File "/Library/Frameworks/Python.framework/Versions/2.2/Mac/Lib/ic.py", line 194, in launchurl
    self.ic.ICLaunchURL(hint, url, 0, len(url))
MacOS.Error: (-2110, 'pathTooLongErr')

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I think this is actually an Apple bug, but I've implemented a workaround in ic.py rev. 1.4: URLs of the second form are converted to the first form before being passed to ICLaunchURL.