When parsing something like '<a>x</a><a>y</a>' with xml.etree.ElementTree, or '{}{}' with json, these parser throw exceptions instead of reading a single element of the kind they understand off the stream (or throwing an exception if there is no element they understand) and leaving the stream in a sane state.

So I should be able to write

import xml.etree.ElementTree as et
import StringIO
s = StringIO.StringIO("<a>x</a><a>y</a>")
elem1 = et.parse(s)
elem2 = et.parse(s)

and have elem1 correspond to "<a>x</a>" and elem2 correspond to "<a>y</a>".

At the very least, if the parsers refuse to parse partial streams, they should at least not destroy the streams.