Mock file with string in Python

_mock_file_with_string.md

Mock file with string in Python

This is an example on how to mock a file with string.

Use case

Testing a function which opens a file and does some kind of operation on it.

About the example

Let us make a function which takes json from file and returns some data. In this example, we want to take data from this API and return name of each planet and its surface temperature in a list.

Instead of creating a mock file, we can limit ourselves to short string with only necessary information.

Example with @mock_patch decorator and corresponding function are shown.

Sources

• mock_open
• Michele d'Amico's answer on: How do I mock an open used in a with statement (using the Mock framework in Python)?

function.py

# Python 3.7
import json
from typing import List


def celestial_bodies_temperatures(filename: str) -> List(List(str, int)):
    return_list = []
    with open(filename) as f:
        data = json.load(f)
        for each_planet in data:
            return_list.append(
                [each_planet["name"],
                 int(each_planet["surfaceTemperature"])]
            )
    return return_list

tests.py

# Python 3.7
import unittest
import unittest.mock

from .function import celestial_bodies_temperatures


class TestFunction(unittest.TestCase):
    mock_file_content = """
    [{"name": "Earth","surfaceTemperature": 288},
    {"name": "Mars", "surfaceTemperature": 260}]
    """

    def test_celestial_bodies_temperatures_returns_list_correctly_with(self):
        with unittest.mock.patch(
                'builtins.open',
                new=unittest.mock.mock_open(read_data=self.mock_file_content),
                create=True
        ) as file_mock:
            self.assertEqual(
                celestial_bodies_temperatures('/dev/null'),
                [["Earth", 288], ["Mars", 260]]
            )

    @unittest.mock.patch(
        'builtins.open',
        new=unittest.mock.mock_open(read_data=mock_file_content),
        create=True
    )
    def test_celestial_bodies_temperatures_returns_list_correctly_decorator(self):
        self.assertEqual(
            celestial_bodies_temperatures('/dev/null'),
            [["Earth", 288], ["Mars", 260]]
        )