{"id":3608,"date":"2022-07-02T07:51:20","date_gmt":"2022-07-02T07:51:20","guid":{"rendered":"https:\/\/www.pythontutorial.net\/python-numpy\/python-stubs\/"},"modified":"2022-07-09T07:45:27","modified_gmt":"2022-07-09T07:45:27","slug":"python-stubs","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-unit-testing\/python-stubs\/","title":{"rendered":"Python Stubs"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use Python stubs to isolate parts of your program from each other for unit testing.<\/p>\n\n\n\n

Introduction to the Python stubs #<\/a><\/h2>\n\n\n\n

Stubs are test doubles that return hard-coded values. The primary purpose of stubs is to prepare a specific state of the system under test.<\/p>\n\n\n\n

Stubs are beneficial because they return consistent results, making the test easier to write. Also, you can run tests even if the components that stubs are present are not working yet.<\/p>\n\n\n\n

Suppose you need to develop an alarm system that monitors the temperature of a room like a server room. <\/p>\n\n\n\n

To do that you need to set up a temperature sensor device and use the data from that sensor to alert if the temperature is below or above a specific temperature.<\/p>\n\n\n\n

First, define a Sensor<\/code> class in the sensor.py<\/code> module:<\/p>\n\n\n

import<\/span> random\n\n\nclass<\/span> Sensor<\/span>:<\/span>\n    @property<\/span>\n    def<\/span> temperature<\/span>(self)<\/span>:<\/span>\n        return<\/span> random.randint(10<\/span>, 45<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The Sensor<\/code> class has a temperature property that returns a random temperature between 10 and 45. In the real world, the Sensor<\/code> class needs to connect to the sensor device to get the actual temperature.<\/p>\n\n\n\n
Second, define the Alarm<\/code> class that uses a Sensor<\/code> object:<\/p>\n\n\n
from<\/span> sensor import<\/span> Sensor\n\n\nclass<\/span> Alarm<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, sensor=None)<\/span> -> None<\/span>:<\/span>\n        self._low = 18<\/span>\n        self._high = 24<\/span>\n        self._sensor = sensor or<\/span> Sensor()\n        self._is_on = False<\/span>\n\n    def<\/span> check<\/span>(self)<\/span>:<\/span>\n        temperature = self._sensor.temperature\n        if<\/span> temperature < self._low or<\/span> temperature > self._high:\n            self._is_on = True<\/span>\n\n    @property<\/span>\n    def<\/span> is_on<\/span>(self)<\/span>:<\/span>\n        return<\/span> self._is_on<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
By default, the is_on<\/code> property of the Alarm<\/code> is off (False<\/code>). The check()<\/code> method turns the alarm on if the temperature is lower than 18 or higher than 42. <\/p>\n\n\n\n
Once the is_on<\/code> property of an Alarm<\/code> object is on, you can send it to the alarm device to alert accordingly.<\/p>\n\n\n\n
Because the temperature()<\/code> method of the Sensor<\/code> returns a random temperature, it’ll be difficult to test various scenarios to ensure the Alarm<\/code> class works properly.<\/p>\n\n\n\n
To resolve it, you can define a stub for the Sensor<\/code> class called TestSensor<\/code>. The TestSensor<\/code> has the temperature<\/code> property that returns a value provided when its object is initialized.<\/p>\n\n\n\n
Third, define the TestSensor<\/code> in test_sensor.py<\/code> module:<\/p>\n\n\n
class<\/span> TestSensor<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, temperature)<\/span> -> None<\/span>:<\/span>\n        self._temperature = temperature\n\n    @property<\/span>\n    def<\/span> temperature<\/span>(self)<\/span>:<\/span>\n        return<\/span> self._temperature<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The TestSensor<\/code> class is like the Sensor<\/code> class except that the temperature<\/code> property returns a value specified in the constructor.<\/p>\n\n\n\n
Fourth, define a TestAlarm<\/code> class in the test_alarm.py<\/code> test module and import the Alarm<\/code> and TestSensor<\/code> from the alarm.py<\/code> and sensor.py<\/code> modules:<\/p>\n\n\n
import<\/span> unittest\nfrom<\/span> alarm import<\/span> Alarm\nfrom<\/span> test_sensor import<\/span> TestSensor\n\n\nclass<\/span> TestAlarm<\/span>(unittest.TestCase)<\/span>:<\/span>\n   pass<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Fifth, test if the alarm is off by default:<\/p>\n\n\n
import<\/span> unittest\nfrom<\/span> alarm import<\/span> Alarm\nfrom<\/span> test_sensor import<\/span> TestSensor\n\n\nclass<\/span> TestAlarm<\/span>(unittest.TestCase)<\/span>:<\/span>\n    def<\/span> test_is_alarm_off_by_default<\/span>(self)<\/span>:<\/span>\n        alarm = Alarm()\n        self.assertFalse(alarm.is_on) <\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In the test_is_alarm_off_by_default<\/code> we create a new alarm instance and use the assertFalse()<\/code> method to check if the is_on<\/code> property of the alarm object is False<\/code>.<\/p>\n\n\n\n
Run the test:<\/p>\n\n\n
python -m unittest -v<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
test_is_alarm_off_by_default (test_alarm.TestAlarm) ... ok\n\n----------------------------------------------------------------------\nRan 1<\/span> test in<\/span> 0.000<\/span>s\n\nOK<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Sixth, test the check()<\/code> method of the Alarm<\/code> class in case the temperature is too high:<\/p>\n\n\n
import<\/span> unittest\nfrom<\/span> alarm import<\/span> Alarm\nfrom<\/span> test_sensor import<\/span> TestSensor\n\n\nclass<\/span> TestAlarm<\/span>(unittest.TestCase)<\/span>:<\/span>\n    def<\/span> test_is_alarm_off_by_default<\/span>(self)<\/span>:<\/span>\n        alarm = Alarm()\n        self.assertFalse(alarm.is_on)\n\n    def<\/span> test_check_temperature_too_high<\/span>(self)<\/span>:<\/span>\n        alarm = Alarm(TestSensor(25<\/span>))\n        alarm.check()\n        self.assertTrue(alarm.is_on)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In the test_check_temperature_too_high()<\/code> test method: <\/p>\n\n\n\n