Can I redirect the stdout in python into some sort of string buffer?

Question

I\'m using python\'s ftplib to write a small FTP client, but some of the functions in the package don\'t return string output, but print to stdout.

Answer 1

Here's another take on this. contextlib.redirect_stdout with io.StringIO() as documented is great, but it's still a bit verbose for every day use. Here's how to make it a one-liner by subclassing contextlib.redirect_stdout:

import sys
import io
from contextlib import redirect_stdout

class capture(redirect_stdout):

    def __init__(self):
        self.f = io.StringIO()
        self._new_target = self.f
        self._old_targets = []  # verbatim from parent class

    def __enter__(self):
        self._old_targets.append(getattr(sys, self._stream))  # verbatim from parent class
        setattr(sys, self._stream, self._new_target)  # verbatim from parent class
        return self  # instead of self._new_target in the parent class

    def __repr__(self):
        return self.f.getvalue()  

Since __enter__ returns self, you have the context manager object available after the with block exits. Moreover, thanks to the __repr__ method, the string representation of the context manager object is, in fact, stdout. So now you have,

with capture() as message:
    print('Hello World!')
print(str(message)=='Hello World!\n')  # returns True

Answer 2

Just to add to Ned's answer above: you can use this to redirect output to any object that implements a write(str) method.

This can be used to good effect to "catch" stdout output in a GUI application.

Here's a silly example in PyQt:

import sys
from PyQt4 import QtGui

class OutputWindow(QtGui.QPlainTextEdit):
    def write(self, txt):
        self.appendPlainText(str(txt))

app = QtGui.QApplication(sys.argv)
out = OutputWindow()
sys.stdout=out
out.show()
print "hello world !"

Answer 3

from cStringIO import StringIO # Python3 use: from io import StringIO
import sys

old_stdout = sys.stdout
sys.stdout = mystdout = StringIO()

# blah blah lots of code ...

sys.stdout = old_stdout

# examine mystdout.getvalue()