How to capture stdout output from a Python function call?

I'm using a Python library that does something to an object


and changes it. While doing so, it prints some statistics to stdout, and I'd like to get a grip on this information. The proper solution would be to change do_something() to return the relevant information,

    out = do_something(my_object)

but it will be a while before the devs of do_something() get to this issue. As a workaround, I thought about parsing whatever do_something() writes to stdout.

How can I capture stdout output between two points in the code, e.g.,

    out = end_capturing()


Try this context manager:

    from cStringIO import StringIO
    import sys

    class Capturing(list):
        def __enter__(self):
            self._stdout = sys.stdout
            sys.stdout = self._stringio = StringIO()
            return self
        def __exit__(self, *args):
            del self._stringio    # free up some memory
            sys.stdout = self._stdout


    with Capturing() as output:

output is now a list containing the lines printed by the function call.

Advanced usage:

What may not be obvious is that this can be done more than once and the results concatenated:

    with Capturing() as output:
        print 'hello world'

    print 'displays on screen'

    with Capturing(output) as output:  # note the constructor argument
        print 'hello world2'

    print 'done'
    print 'output:', output


    displays on screen                     
    output: ['hello world', 'hello world2']

Update : They added redirect_stdout() to contextlib in Python 3.4 (along with redirect_stderr()). So you could use io.StringIO with that to achieve a similar result (though Capturing being a list as well as a context manager is arguably more convenient).