Python pytest + stdout: Capture messages sent to stdout / stderr from your functions with pytest capsys

Pytest - Capture print statements in Pytest modules from stdout using capsys parameter

# test.py

# Print statements to pytest
# Capture messages sent to stdout / stderr from your functions, 
# and test the output with a pytest unit test.

# tl;dr - use the capsys parameter in your test arguments
# extract the messages sent to stdout/stderr with capsys.readouterr()



import sys
import pytest

message = 'The meaning of life is not actually 42\n'


def print_greeting():
    """print 42 to stdout"""

    print(message) 
    # write to stdout



def message_to_stdout():
    """print message directly to stdout"""

    # print to stdout
    sys.stdout.write(message)         


def message_to_stderr():
    """print error message to stderr"""

    # print error message to stderr
    sys.stderr.write('An error has happened!')


def return_greeting():
    """ return 42  """

    return message 



class TestClass():


    def setup_method(self, function):
        """ Setup before each function """
        print('\n\n[+] Test: "{}"'.format( function.__name__))
        print('    ' + function.__doc__ )



    def test_messagetostdout(self, capsys):
        """assert '42' was printed to stdout"""

        # Run my function
        message_to_stdout()


        # catch what was set to stdout, stderr
        out, err = capsys.readouterr()

        assert '42' in out


    def test_messagetosterr(self, capsys):
        """assert 'error' was printed to stderr"""

        # Run my function
        message_to_stderr()


        # catch what was set to stdout, stderr
        out, err = capsys.readouterr()

        assert 'error' in err


    def test_printgreeting(self, capsys):
        """assert '42' was printed to console"""

        # capture stdout / stderr

        # Run my function
        print_greeting()

        out, err = capsys.readouterr()
        # 42 not in stdout from sys.stdout.write
        assert '42' in out


    def test_returngreeting(self):
        """assert '42' returned from function"""

        # Run my function
        greeting = return_greeting()

        # 42 is in returned from function
        assert '42' in greeting