How to capture stdout output from a Python function call?
Question: I'm using a Python library that does something to an object

    
    
    do_something(my_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.,

    
    
    start_capturing()
    do_something(my_object)
    out = end_capturing()
    

?


Answer: 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):
            self.extend(self._stringio.getvalue().splitlines())
            sys.stdout = self._stdout
    

Usage:

    
    
    with Capturing() as output:
        do_something(my_object)
    

`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
    

Output:

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

