How to properly use unit-testing s assertRaises with NoneType objects

Question

I did a simple test case   def setUp self      self testListNone   None  def testListSlicing self      self assertRaises TypeError  self testListNone  1     and I am expecting test to pass  but I am getting exception   Traceback  most recent call last        self assertRaises TypeError  self testListNone  1    TypeError   NoneType  object is unsubscriptable   I thought that assertRaises will pass since TypeError exception will be raised

User · Answer

The usual way to use assertRaises is to call a function   self assertRaises TypeError  test function  args    to test that the function call test function args  raises a TypeError   The problem with self testListNone  1  is that Python evaluates the expression immediately  before the assertRaises method is called  The whole reason why test function and args is passed as separate arguments to self assertRaises is to allow assertRaises to call test function args  from within a try   except block  allowing assertRaises to catch the exception   Since you ve defined self testListNone   None  and you need a function to call  you might use operator itemgetter like this   import operator self assertRaises TypeError  operator itemgetter   self testListNone slice None 1      since   operator itemgetter self testListNone slice None 1     is a long-winded way of saying self testListNone  1   but which separates the function  operator itemgetter  from the arguments

User · Answer

If you are using python2 7 or above you can use the ability of assertRaises to be use as a context manager and do   with self assertRaises TypeError       self testListNone  1    If you are using python2 6 another way beside the one given until now is to use unittest2 which is a back port of unittest new feature to python2 6  and you can make it work using the code above   N B  I m a big fan of the new feature  SkipTest  test discovery      of unittest so I intend to use unittest2 as much as I can  I advise to do the same because there is a lot more than what unittest come with in python2 6  lt

User · Answer

The problem is the TypeError gets raised  before  assertRaises gets called since the arguments to assertRaises need to be evaluated before the method can be called  You need to pass a lambda expression like   self assertRaises TypeError  lambda  self testListNone  1

User · Answer

Complete snippet would look like the following  It expands  mouad s answer to asserting on error s message  or generally str representation of its args   which may be useful   from unittest import TestCase   class TestNoneTypeError TestCase      def setUp self        self testListNone   None    def testListSlicing self       with self assertRaises TypeError  as ctx          self testListNone  1      self assertEqual   NoneType  object is not subscriptable   str ctx exception

[python] How to properly use unit-testing's assertRaises() with NoneType objects?

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