Python import cStringIO ImportError: No module named 'cStringIO'

From Python 3.0 changelog;
The StringIO and cStringIO modules are gone. Instead, import the io module and use io.StringIO or io.BytesIO for text and data respectively.
From the Python 3 email documentation it can be seen that io.StringIO should be used instead:

from io import StringIO
from email.generator import Generator
fp = StringIO()
g = Generator(fp, mangle_from_=True, maxheaderlen=60)
g.flatten(msg)
text = fp.getvalue()

从Python 3.0开始，StringIO和cStringIO模块已经取消。通过import io模块代替，分别使用io.String或io.BytesIO处理文本和数据。从Python 3邮件流文档能看到相关实现StringIO的代码为：

from io import StringIO
from email.generator import Generator
fp = StringIO()
g = Generator(fp, mangle_from_=True, maxheaderlen=60)
g.flatten(msg)
text = fp.getvalue()