Python:语法错误(+ =)

Amy*_*yth 0 python syntax-error

为什么我在这个简单的函数中使用+ =得到语法错误?

def downloadImages(urllist):
  counter = 0 
  for url in urllist:
    downloadSingleImage(url, (filename_prefix + str(counter))
    counter += 1
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Ian*_*nry 5

你的parens在线上不平衡downloadSingleImage(url, (filename_prefix + str(counter)).