Mock Python's built in print function

Question

I\'ve tried

from mock import Mock
import __builtin__

__builtin__.print = Mock()

But that raises a syntax error. I\'ve also tried patching

Answer 1

First, the module is called __builtins__ and you don't need to import it.

Now, in Python 2 print is a keyword so you can't use it as an attribute name directly. You can use setattr/getattr to workaround it:

getattr(__builtins__, "print")

Another option is to use from __future__ import print_function which changes how Python parses the module to Python 3 syntax.

Answer 2

As lcq says, print is a keyword. So, think about what it would mean if you were actually successful in patching/mocking print under Python 2.7.3. You would have code like this:

print "Hi."

turning into:

<MagicMock id='49489360'> "Hi."

MagicMock objects cannot be accessed this way, so you would get a syntax error.

So... Yeah. You can only mock the Python3 print function or sys.stdout.

Answer 3

print is a keyword in python 2.x, using it as attribute raises a SyntaxError. You can avoid that by using from __future__ import print_function in the beginning of the file.

Note: you can't simply use setattr, because the print function you modified doesn't get invoked unless the print statement is disabled.

Edit: you also need to from __future__ import print_function in every file you want your modified print function to be used, or it will be masked by the print statement.

Answer 4

import mock
import sys

mock_stdout = mock.Mock()
sys.stdout = mock_stdout
print 'Hello!'
sys.stdout = sys.__stdout__

print mock_stdout.mock_calls
[call.write('Hello!'), call.write('\n')]

Answer 5

This Python 3 example builds upon the Python 2 answer by Krzysztof. It uses unittest.mock. It uses a reusable helper method for making the assertion.

import io
import unittest
import unittest.mock

from .solution import fizzbuzz


class TestFizzBuzz(unittest.TestCase):

    @unittest.mock.patch('sys.stdout', new_callable=io.StringIO)
    def assert_stdout(self, n, expected_output, mock_stdout):
        fizzbuzz(n)
        self.assertEqual(mock_stdout.getvalue(), expected_output)

    def test_only_numbers(self):
        self.assert_stdout(2, '1\n2\n')