How to test Python function which writes to file

Question

I have a Python function that takes a list as an argument and writes it to a file:

def write_file(a):
    try:
        f = open('testfile', 'w')
        for i in a:
            f.write(str(i))

    finally:
        f.close()

How do I test this function ?

def test_write_file(self):
    a = [1,2,3]
    #what next ?

Answer 1

Call the write_file function and check whether testfile is created with expected content.

def test_write_file(self):
    a = [1,2,3]
    write_file(a)
    with open('testfile') as f:
        assert f.read() == '123' # Replace this line with the method
                                 #   provided by your testing framework.

If you don't want test case write to actual filesystem, use something like mock.mock_open.

Answer 2

First solution: rewrite you function to accept a writable file-like object. You can then pass a StringIO instead and test the StringIO's value after the call.

Second solution: use some mock library that let you patch builtins.