Question

And get the bytes of that StringIO object?

Answer 1

StringIO objects implement the file API, so you can get their size in exactly the same way as you can with a file object: seek to the end and see where it goes.

from StringIO import StringIO
import os
s = StringIO()
s.write("abc")
pos = s.tell()
s.seek(0, os.SEEK_END)
print s.tell()
s.seek(pos)

As Kimvais mentions, you can also use the len, but note that that’s specific to StringIO objects. In general, a major reason to use these objects in the first place is to use them with code that expects a file-like object. When you’re dealing with a generic file-like object, you generally want to do the above to get its length, since that works with any file-like object.