Register a Python written function to for error reporting.
       The function is called back as f(ctx, error).

def registerErrorHandler(f, ctx):
    """Register a Python written function to for error reporting.
       The function is called back as f(ctx, error). """
    import sys
    if 'libxslt' not in sys.modules:
        # normal behaviour when libxslt is not imported
        ret = libxml2mod.xmlRegisterErrorHandler(f,ctx)
    else:
        # when libxslt is already imported, one must
        # use libxst's error handler instead
        import libxslt
        ret = libxslt.registerErrorHandler(f,ctx)
    return ret