If a User Parameter ends in a backslash - such as a Windows path, then accessing that value in a PythonCaller results in extra text. Example: if a User Parameter called foldername has the value:
frog\\
then using fme.macroValues['foldername'] or FME_MacroValues['foldername'] in a PythonCaller returns:
frog GUI OPTIONAL TEXT foldername Folder name: