Python: Problem trying catching an output during a unit test - Unittest library

Question

I'm trying to catching an output to be tested, I'm using unittest and unittest.mock libraries. I have taken a look in many others related questions (like this: How to assert output with nosetest/unittest in python? among others) and all they suggest something like which a have in my code, but I'm getting only an empty string, and can't find where I'm mistaking.

My function do something like that:

def my_func():
  a = int(input("put a 2 digit number"))
  b = [1,2,3,4,5,6,7,8,9,0]
  if a in b:
    print("wrong number")

And this is the related test:

import io
import sys
import unittest
from unittest.mock import patch
from io import StringIO
from somemodule import my_func

def test_entering_wrong_value(self):

        user_input = ['2']

        with patch('builtins.input', side_effect=user_input):
            self.my_func()
            capturedOutput = io.StringIO()
            sys.stdout = capturedOutput
            output = capturedOutput.getvalue().strip()
            self.assertEqual('wrong number', output)

As I undestood this should capture the outputed string and compared with the test value, but can't capturing the string, I'm just getting an empty string and this error:

AssertionError: 'wrong number' != ''

Where am I messing up?

Error - Syntactical Remorse · Accepted Answer · 2020-06-09 18:44:14Z

1

You have to redirect stdout BEFORE you do your prints. You can't redirect after and expect to get the prints that weren't being redirected:

import io
import sys
import unittest
from unittest.mock import patch
from somemodule import my_func

def test_entering_wrong_value(self):

        user_input = ['2']

        with patch('builtins.input', side_effect=user_input):
            capturedOutput = io.StringIO()
            sys.stdout = capturedOutput
            self.my_func()
            output = capturedOutput.getvalue().strip()
            sys.stdout = sys.__stdout__ # Restore stdout so prints work normally
            self.assertEqual('wrong number', output)

edited Jun 9, 2020 at 18:44

answered May 28, 2019 at 19:28

Error - Syntactical Remorse

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1 Comment

Elephant Over a year ago

Restoring stdout (sys.stdout = sys.__stdout__) causes error AttributeError: '_io.TextIOWrapper' object has no attribute 'getvalue' when exiting the test. Added this code into the tearDownClass method.

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