unittest of file-reading function

[André Malo]

* Helge Stenstroem wrote:

> Say I have a function
> 
> def f(filename):
>     result = openFileAndProcessContents(filename)
>     return result
> 
> Can that function be unit tested without having a real file as input?
> Something along the lines of
> 
> import unittest
> class tests(unittest.TestCase):
>     def test1(self):
>         fileContents = "bla bla bla\nmore bla bla bla"
>         ???   # make f read this string instead of opening a file
>         expected = expectedResult
>         result = f(filename)
>         self.assertEqual(result, expected)
> 
> One possibility would be to make the unit test write to a temporary
> file. Are there better alternatives?

You can also just replace or shadow the file/open builtin (that's what I'm
doing). Something like:

     def test1(self):
         import StringIO

         fileContents = "bla bla bla\nmore bla bla bla"

         def myfile(name, mode, buffering):
             return StringIO.StringIO(fileContents)
         testedmodule.file = myfile
         try:
             expected = expectedResult
             result = f(filename)
         finally:
             del testedmodule.file

         self.assertEqual(result, expected)

(if you're using open, replace .file with .open)
(testedmodule is of course the module where the file call happens)

nd
-- 
die (eval q-qq[Just Another Perl Hacker
]
;-)
# André Malo, <http://www.perlig.de/> #