Decode the bytes object to produce a string:

>>> b"abcde".decode("utf-8")
'abcde'

The above example assumes that the bytes object is in UTF-8, because it is a common encoding. However, you should use the encoding your data is actually in!

It's interesting that though the question might seem reasonable, it's not that easy to figure out a practical reason why I would need to convert a StringIO into a BytesIO. Both are basically buffers and you usually need only one of them to make some additional manipulations either with the bytes or with the text.

I may be wrong, but I think your question is actually how to use a BytesIO instance when some code to which you want to pass it expects a text file.

In which case, it is a common question and the solution is codecs module.

The two usual cases of using it are the following:

Compose a File Object to Read

In [16]: import codecs, io

In [17]: bio = io.BytesIO(b'qwe\nasd\n')

In [18]: StreamReader = codecs.getreader('utf-8')  # here you pass the encoding

In [19]: wrapper_file = StreamReader(bio)

In [20]: print(repr(wrapper_file.readline()))
'qwe\n'

In [21]: print(repr(wrapper_file.read()))
'asd\n'

In [26]: bio.seek(0)
Out[26]: 0

In [27]: for line in wrapper_file:
    ...:     print(repr(line))
    ...:
'qwe\n'
'asd\n'

Compose a File Object to Write To

In [28]: bio = io.BytesIO()

In [29]: StreamWriter = codecs.getwriter('utf-8')  # here you pass the encoding

In [30]: wrapper_file = StreamWriter(bio)

In [31]: print('жаба', 'цап', file=wrapper_file)

In [32]: bio.getvalue()
Out[32]: b'\xd0\xb6\xd0\xb0\xd0\xb1\xd0\xb0 \xd1\x86\xd0\xb0\xd0\xbf\n'

In [33]: repr(bio.getvalue().decode('utf-8'))
Out[33]: "'жаба цап\\n'"