Re: [Pyunit-interest] problem using assertRaises

[Jeremy H. <je...@al...>]

>>>>> "RJ" == robin and jim <rob...@ea...> writes:

  RJ> Hello, I'm using the unittest module shipped with PythonWin
  RJ> 2.2.1, and I am having trouble using assertRaises.  Here is a
  RJ> code fragment example.


  RJ>       def test_01(self):
  RJ>          '''handling the failure to connect with and log onto a
  RJ>          FTP server'''
  RJ>           try:
  RJ>              self.testable.open()
  RJ>           except Exception:
  RJ>              pass
  RJ>           else:
  RJ>              self.fail('exception not raised')

  RJ>       def test_02(self):
  RJ>          '''handling the failure to connect with and log onto a FTP server''' 
  RJ>          self.assertRaises(Exception, self.testable.open())

  RJ> test_01 works as expected (i.e., when the open() operation
  RJ> fails, the test succeeds).

  RJ> However, test_02 does not work as expected; when the open()
  RJ> operation fails, the test also fails (i.e., the exception is not
  RJ> caught by assertRaises).

  RJ> What am I doing wrong?

Do not call open() before passing it to assertRaises().

    self.assertRaises(Exception, self.testable.open)

should work just like test_01.  The code you wrote will call
self.testable.open() before calling assertRaises(), which means
assertRaises() will never get a chance to catch the exception.  You
probably overlooked that assertRaises() takes a callable object -- a
function or method or something like that -- as its second argument.
It also takes a variable number of other arguments that is passes as
arguments to the callable.

Say you were testing this function:

def f(x, y):
    return x / y

You would write:

    self.assertRaises(ZeroDivisionError, f, 12, 0)

Internally, assertRaises() will call f(12, 0) within a try/except
block that catches ZeroDivisionError.

Jeremy