Python unittest - opposite of assertRaises

Question

I want to write a test to establish that an Exception is not raised in a given circumstance    It s straightforward to test if an Exception is raised      sInvalidPath AlwaysSuppliesAnInvalidPath   self assertRaises PathIsNotAValidOne  MyObject  sInvalidPath         but how can you do the opposite    Something like this i what I m after      sValidPath AlwaysSuppliesAValidPath   self assertNotRaises PathIsNotAValidOne  MyObject  sValidPath

User · Answer

Hi - I want to write a test to establish that an Exception is not raised in a given circumstance.

That's the default assumption -- exceptions are not raised.

If you say nothing else, that's assumed in every single test.

You don't have to actually write an any assertion for that.

User · Answer

One straight forward way to ensure the object is initialized without any error is to test the object s type instance   Here is an example    p   SomeClass param1  param1 value  self assertTrue isinstance p  SomeClass

User · Answer

I am the original poster and I accepted the above answer by DGH without having first used it in the code    Once I did use I realised that it needed a little tweaking to actually do what I needed it to do  to be fair to DGH he she did say  or something similar        I thought it was worth posting the tweak here for the benefit of others       try          a   Application  abcdef           except pySourceAidExceptions PathIsNotAValidOne          pass     except          self assertTrue False    What I was attempting to do here was to ensure that if an attempt was made to instantiate an Application object with a second argument of spaces the pySourceAidExceptions PathIsNotAValidOne would be raised    I believe that using the above code  based heavily on DGH s answer  will do that

User · Answer

def  assertNotRaises self  exception  obj  attr                                                                                                                                      try                                                                                                                                                                                  result   getattr obj  attr                                                                                                                                                       if hasattr result     call                                                                                                                                                           result                                                                                                                                                                   except Exception as e                                                                                                                                                                if isinstance e  exception                                                                                                                                                          raise AssertionError        raises      format obj  attr  exception      could be modified if you need to accept parameters   call like  self  assertNotRaises IndexError  array   sort

User · Answer

I ve found it useful to monkey-patch unittest as follows   def assertMayRaise self  exception  expr     if exception is None      try        expr       except        info   sys exc info         self fail   s raised    repr info 0      else      self assertRaises exception  expr   unittest TestCase assertMayRaise   assertMayRaise   This clarifies intent when testing for the absence of an exception   self assertMayRaise None  does not raise    This also simplifies testing in a loop  which I often find myself doing     ValueError is raised only for op x x   op y y  and op z z   for i  a b  in enumerate itertools product  x y z    x y z       self assertMayRaise None if i 4 else ValueError  lambda  op a  b

User · Answer

def run test self       try          myFunc       except ExceptionType          self fail  myFunc   raised ExceptionType unexpectedly

User · Answer

If you pass an Exception class to assertRaises    a context manager is provided  This can improve the readability of your tests     raise exception if Application created with bad data with self assertRaises pySourceAidExceptions PathIsNotAValidOne       application   Application  abcdef         This allows you to test error cases in your code    In this case  you are testing the PathIsNotAValidOne is raised when you pass invalid parameters to the Application constructor

User · Answer

You can define assertNotRaises by reusing about 90  of the original implementation of assertRaises in the unittest module  With this approach  you end up with an assertNotRaises method that  aside from its reversed failure condition  behaves identically to assertRaises   TLDR and live demo  It turns out to be surprisingly easy to add an assertNotRaises method to unittest TestCase  it took me about 4 times as long to write this answer as it did the code   Here s a live demo of the assertNotRaises method in action  Just like assertRaises  you can either pass a callable and args to assertNotRaises  or you can use it in a with statement  The live demo includes a test cases that demonstrates that assertNotRaises works as intended   Details  The implementation of assertRaises in unittest is fairly complicated  but with a little bit of clever subclassing you can override and reverse its failure condition   assertRaises is a short method that basically just creates an instance of the unittest case  AssertRaisesContext class and returns it  see its definition in the unittest case module   You can define your own  AssertNotRaisesContext class by subclassing  AssertRaisesContext and overriding its   exit   method   import traceback from unittest case import  AssertRaisesContext  class  AssertNotRaisesContext  AssertRaisesContext       def   exit   self  exc type  exc value  tb           if exc type is not None              self exception   exc value with traceback None               try                  exc name   self expected   name               except AttributeError                  exc name   str self expected               if self obj name                  self  raiseFailure     raised by     format exc name                      self obj name               else                  self  raiseFailure     raised  format exc name            else              traceback clear frames tb           return True   Normally you define test case classes by having them inherit from TestCase  If you instead inherit from a subclass MyTestCase   class MyTestCase unittest TestCase       def assertNotRaises self  expected exception   args    kwargs           context    AssertNotRaisesContext expected exception  self          try              return context handle  assertNotRaises   args  kwargs          finally              context   None   all of your test cases will now have the assertNotRaises method available to them

User · Answer

Just call the function   If it raises an exception  the unit test framework will flag this as an error   You might like to add a comment  e g    sValidPath AlwaysSuppliesAValidPath     Check PathIsNotAValidOne not thrown MyObject sValidPath

User · Answer

you can try like that           try              self assertRaises None function arg1  arg2          except              pass if you don t put code inside try block it will through exception  AssertionError  None not raised   and test case will be failed  Test case will be pass if put inside try block which is expected behaviour

[python] Python unittest - opposite of assertRaises?

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