StringIO and compatibility with ‘with’ statement (context manager)

A StringIO instance is an open file already. The open command, on the other hand, only takes filenames, to return an open file. A StringIO instance is not suitable as a filename.

Also, you don’t need to close a StringIO instance, so there is no need to use it as a context manager either. While closing an instance frees the memory allocated, so does simply letting the garbage collector reap the object. At any rate, the contextlib.closing() context manager could take care of closing the object if you want to ensure freeing the memory while still holding a reference to the object.

If all your legacy code can take is a filename, then a StringIO instance is not the way to go. Use the tempfile module to generate a temporary filename instead.

Here is an example using a contextmanager to ensure the temp file is cleaned up afterwards:

import os
import tempfile
from contextlib import contextmanager

@contextmanager
def tempinput(data):
    temp = tempfile.NamedTemporaryFile(delete=False)
    temp.write(data)
    temp.close()
    try:
        yield temp.name
    finally:
        os.unlink(temp.name)

with tempinput('Some data.\nSome more data.') as tempfilename:
    processFile(tempfilename)

You can also switch to the newer Python 3 infrastructure offered by the io module (available in Python 2 and 3), where io.BytesIO is the more robust replacement for StringIO.StringIO / cStringIO.StringIO. This object does support being used as a context manager (but still can’t be passed to open()).