stream - Capturing stdout within the same process in Python

7
votes

I've got a python script that calls a bunch of functions, each of which writes output to stdout. Sometimes when I run it, I'd like to send the output in an e-mail (along with a generated file). I'd like to know how I can capture the output in memory so I can use the email module to build the e-mail.

My ideas so far were:

  • use a memory-mapped file (but it seems like I have to reserve space on disk for this, and I don't know how long the output will be)
  • bypass all this and pipe the output to sendmail (but this may be difficult if I also want to attach the file)
4
pythonstream

4 Answers

4
votes

You said that your script "calls a bunch of functions" so I'm assuming that they're python functions accessible from your program. I'm also assuming you're using print to generate the output in all these functions. If that's the case, you can just replace sys.stdout with a StringIO.StringIO which will intercept all the stuff you're writing. Then you can finally call the .getValue method on your StringIO to get everything that has been sent to the output channel. This will also work for external programs using the subprocess module which write to sys.stdout.

This is a cheap way. I'd recommend that you do your output using the logging module. You'll have much more control over how it does it's output and you can control it more easily as well.

14
votes

I modified None's answer to make it a context manager:

import sys, StringIO, contextlib

class Data(object):
    pass

@contextlib.contextmanager
def capture_stdout():
    old = sys.stdout
    capturer = StringIO.StringIO()
    sys.stdout = capturer
    data = Data()
    yield data
    sys.stdout = old
    data.result = capturer.getvalue()

Usage:

with capture_stdout() as capture:
    print 'Hello'
    print 'Goodbye'
assert capture.result == 'Hello\nGoodbye\n'
8
votes

It's pretty simple to capture output.

import sys, StringIO
old_stdout = sys.stdout
capturer = StringIO.StringIO()
sys.stdout = capturer
#call functions
print "Hi"
#end functions
sys.stdout = old_stdout
output = capturer.getvalue()
2
votes

And I modified Gary Robinson's answer to make sure that stdout is always restored, even if there's an exception:

import sys, StringIO, contextlib

class Data(object):
    pass

@contextlib.contextmanager
def capture_stdout():
    old = sys.stdout
    capturer = StringIO.StringIO()
    data = Data()
    try:
        sys.stdout = capturer
        yield data
    finally:
        sys.stdout = old
        data.result = capturer.getvalue()