jlengrand.github.io/_posts/2011-12-02-pythonunittest-assertraises-raises-error.markdown

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layout	status	published	title	author	author_login	author_email	author_url	wordpress_id	wordpress_url	date	categories	tags	comments
post	publish	true	Python/Unittest : assertRaises raises Error	Julien Lengrand-Lambert	jlengrand	julien@lengrand.fr	http://www.lengrand.fr	327	http://www.lengrand.fr/?p=327	2011-12-02 13:49:49.000000000 +01:00	Python	tippy unittest unit tests agile TDD test driven development	id author author_email author_url date date_gmt content 828 md md@pmcd-cons.com MjAxMi0wNC0xOSAyMTo1MzowNSArMDIwMA== MjAxMi0wNC0xOSAyMDo1MzowNSArMDIwMA== test_square_vlaue gives an eroror because assertRaises wants for 2nd arg a callable not a function call. this should do: self.assertRaises(TypeError, df.square_value, 'A') id author author_email author_url date date_gmt content 873 Julien Lengrand-Lambert julien@lengrand.fr http://www.lengrand.fr MjAxMi0wNC0yNCAxMjo0MToyNCArMDIwMA== MjAxMi0wNC0yNCAxMTo0MToyNCArMDIwMA== Thanks for the info, I'll try this way of doing next time. It is prettier than my current solution :)

Hi all,

Today, a small hint about unit tests in Python I discovered while working on Tippy.

In order to get as reliable code as possible, I am currently experiencing Agile techniques, and especially TDD. I develop Tippy in Python, and test methods with the excellent unittest framework.One of (in my mind at least) the most important tips Agile provides is the "fail fast" rule. And to fit with this rule, all my methods check their inputs before performing anything else.

It implies a lot of assertRaises assertions in my tests. Here is an example of how it could be used :

Let's say I want to create a (dum) function calculating the square value of a number. I will have to test that the input can really be multiplied.

• First, here is the corresponding (still dum :p) function in file dum_function.py :

{% highlight python %} def square_value(a): """ Returns the square value of a. """ try: out = a*a except TypeError: raise TypeError("Input should be a string:")

return out

{% endhighlight %}

• Here is the test to be performed (only this test is inserted):

{% highlight python %} import dum_function as df # import function module import unittest class Test(unittest.TestCase): """ The class inherits from unittest """ def setUp(self): """ This method is called before each test """ self.false_int = "A"

def tearDown(self):
    """
    This method is called after each test
    """
    pass
#---
## TESTS
def test_square_value(self):
    # assertRaises(excClass, callableObj) prototype
    self.assertRaises(TypeError, df.square_value(self.false_int))

if name == "main": unittest.main() {% endhighlight %}

• We are now ready to test our function! Here is what happens when trying to run the test :

{% highlight python %} ====================================================================== ERROR: test_square_value (__main__.Test) ---------------------------------------------------------------------- Traceback (most recent call last): File "test_dum_function.py", line 22, in test_square_value self.assertRaises(TypeError, df.square_value(self.false_int)) File "/home/jlengrand/Desktop/function.py", line 8, in square_value raise TypeError("Input should be a string:") TypeError: Input should be a string:

---

Ran 1 test in 0.000s

FAILED (errors=1) {% endhighlight %}

The TypeError is actullay raised, and generates a test failure. The problem is that this is exactly the behavior we wanted :s.

To avoid this error, simply run the function using lambda in the test call :

{% highlight python %}

self.assertRaises(TypeError, lambda: df.square_value(self.false_int))

{% endhighlight %}

The final output :

{% highlight python %}

Ran 1 test in 0.000s

OK {% endhighlight %}

Perfect !

Note : For the purpose of this article, I did not work "backwards" (created test before the function). I think this would have been counter productive for the effect I wanted to highlight.