Python: Write unittest for console print

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You can easily capture standard output by just temporarily redirecting sys.stdout to a StringIO object, as follows:

import StringIO
import sys

def foo(inStr):
    print "hi"+inStr

def test_foo():
    capturedOutput = StringIO.StringIO()          # Create StringIO object
    sys.stdout = capturedOutput                   #  and redirect stdout.
    foo('test')                                   # Call unchanged function.
    sys.stdout = sys.__stdout__                   # Reset redirect.
    print 'Captured', capturedOutput.getvalue()   # Now works as before.

test_foo()

The output of this program is:

Captured hitest

showing that the redirection successfully captured the output and that you were able to restore the output stream to what it was before you began the capture.

Note that the code above in for Python 2.7, as the question indicates. Python 3 is slightly different:

import io
import sys

def foo(inStr):
    print ("hi"+inStr)

def test_foo():
    capturedOutput = io.StringIO()                  # Create StringIO object
    sys.stdout = capturedOutput                     #  and redirect stdout.
    foo('test')                                     # Call function.
    sys.stdout = sys.__stdout__                     # Reset redirect.
    print ('Captured', capturedOutput.getvalue())   # Now works as before.

test_foo()

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