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finish translate Python unittest - assertTrue is truthy - assertFalse is falsy
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wenchunyang
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Translated by chunyang-wen
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Python unittest: assertTrue is truthy, assertFalse is falsy
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===========================
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In this post, I explore the differences between the unittest boolean assert methods assertTrue and assertFalse and the assertIs identity assertion.
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### Definitions
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Here’s what the [unittest module documentation][1] currently notes about assertTrue and assertFalse, with the appropriate code highlighted:
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>assertTrue(expr, msg=None)
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>assertFalse(expr, msg=None)
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>>Test that expr is true (or false).
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>>Note that this is equivalent to
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>>bool(expr) is True
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>>and not to
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>>expr is True
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>>(use assertIs(expr, True) for the latter).
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[Mozilla Developer Network defines truthy][2] as:
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> A value that translates to true when evaluated in a Boolean context.
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In Python this is equivalent to:
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```
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bool(expr) is True
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```
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Which exactly matches what assertTrue is testing for.
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Therefore the documentation already indicates assertTrue is truthy and assertFalse is falsy. These assertion methods are creating a bool from the received value and then evaluating it. It also suggests that we really shouldn’t use assertTrue or assertFalse for very much at all.
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### What does this mean in practice?
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Let’s use a very simple example - a function called always_true that returns True. We’ll write the tests for it and then make changes to the code and see how the tests perform.
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Starting with the tests, we’ll have two tests. One is “loose”, using assertTrue to test for a truthy value. The other is “strict” using assertIs as recommended by the documentation:
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```
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import unittest
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from func import always_true
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class TestAlwaysTrue(unittest.TestCase):
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def test_assertTrue(self):
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"""
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always_true returns a truthy value
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"""
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result = always_true()
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self.assertTrue(result)
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def test_assertIs(self):
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"""
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always_true returns True
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"""
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result = always_true()
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self.assertIs(result, True)
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```
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Here’s the code for our simple function in func.py:
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```
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def always_true():
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"""
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I'm always True.
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Returns:
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bool: True
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"""
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return True
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```
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When run, everything passes:
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```
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always_true returns True ... ok
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always_true returns a truthy value ... ok
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----------------------------------------------------------------------
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Ran 2 tests in 0.004s
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OK
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```
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Happy days!
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Now, “someone” changes always_true to the following:
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```
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def always_true():
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"""
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I'm always True.
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Returns:
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bool: True
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"""
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return 'True'
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```
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Instead of returning True (boolean), it’s now returning string 'True'. (Of course this “someone” hasn’t updated the docstring - we’ll raise a ticket later.)
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This time the result is not so happy:
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```
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always_true returns True ... FAIL
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always_true returns a truthy value ... ok
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======================================================================
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FAIL: always_true returns True
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----------------------------------------------------------------------
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Traceback (most recent call last):
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File "/tmp/assertttt/test.py", line 22, in test_is_true
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self.assertIs(result, True)
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AssertionError: 'True' is not True
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----------------------------------------------------------------------
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Ran 2 tests in 0.004s
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FAILED (failures=1)
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```
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Only one test failed! This means assertTrue gave us a false-positive. It passed when it shouldn’t have. It’s lucky we wrote the second test with assertIs.
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Therefore, just as we learned from the manual, to keep the functionality of always_true pinned tightly the stricter assertIs should be used rather than assertTrue.
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### Use assertion helpers
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Writing out assertIs to test for True and False values is not too lengthy. However, if you have a project in which you often need to check that values are exactly True or exactly False, then you can make yourself the assertIsTrue and assertIsFalse assertion helpers.
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This doesn’t save a particularly large amount of code, but it does improve readability in my opinion.
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```
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def assertIsTrue(self, value):
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self.assertIs(value, True)
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def assertIsFalse(self, value):
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self.assertIs(value, False)
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```
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### Summary
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In general, my recommendation is to keep tests as tight as possible. If you mean to test for the exact value True or False, then follow the [documentation][3] and use assertIs. Do not use assertTrue or assertFalse unless you really have to.
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If you are looking at a function that can return various types, for example, sometimes bool sometimes int, then consider refactoring. This is a code smell and in Python, that False value for an error would probably be better raised as an exception.
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In addition, if you really need to assert the return value from a function under test is truthy, there might be a second code smell - is your code correctly encapsulated? If assertTrue and assertFalse are asserting that function return values will trigger if statements correctly, then it might be worth sense-checking you’ve encapsulated everything you intended in the appropriate place. Maybe those if statements should be encapsulated within the function under test.
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Happy testing!
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--------------------------------------------------------------------------------
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via: http://jamescooke.info/python-unittest-asserttrue-is-truthy-assertfalse-is-falsy.html
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作者：[James Cooke][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: http://jamescooke.info/pages/hello-my-name-is-james.html
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[1]:https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertTrue
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[2]:https://developer.mozilla.org/en-US/docs/Glossary/Truthy
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[3]:https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertTrue
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