def lookup_module(filename):
    """Helper function for break/clear parsing -- may be overridden.

    lookupmodule() translates (possibly incomplete) file or module name
    into an absolute file name.
    """
    import os
    import sys
    if os.path.isabs(filename) and os.path.exists(filename):
        return filename
    f = os.path.join(sys.path[0], filename)
    if os.path.exists(f):
        return f
    root, ext = os.path.splitext(filename)
    if ext == '':
        filename = filename + '.py'
    if os.path.isabs(filename):
        return filename
    for dirname in sys.path:
        while os.path.islink(dirname):
            dirname = os.readlink(dirname)
        fullname = os.path.join(dirname, filename)
        if os.path.exists(fullname):
            return fullname
    return None
