def generate_temporary_file(folder='/tmp', prefix='helpme', ext='json'):
    '''write a temporary file, in base directory with a particular extension.
      
       Parameters
       ==========
       folder: the base directory to write in. 
       prefix: the prefix to use
       ext: the extension to use.

    '''        
    tmp = next(tempfile._get_candidate_names())
    return '%s/%s.%s.%s' %(folder, prefix, tmp, ext)
write a temporary file, in base directory with a particular extension.
      
       Parameters
       ==========
       folder: the base directory to write in. 
       prefix: the prefix to use
       ext: the extension to use.