Mocking open(file_name) in unit tests

Question

I have a source code that opens a csv file and sets up a header to value association. The source code is given below:

def ParseCsvFile(source): 
  \"\"\"Pa         


        

Answer 1

Hi I was having a similar problem, and was tearing my hair out flipping between different mocking libraries. I finally found a solution that I am happy with, and maybe it might help you? In the end I went with the Mocker library http://labix.org/mocker and here is the code for mocking open:

from mocker import Mocker
from StringIO import StringIO
import __builtin__
mocker = Mocker()
sourceFile = 'myTestFile.txt'
__builtin__.open = mocker.mock()
__builtin__.open(sourceFile)
mocker.result(StringIO('this,is,a,test,file'))

<the rest of your test setup goes here>

mocker.replay()

ParseCsvFile(sourceFile)

mocker.restore()
mocker.verify()

Incidentaly the reason I went with Mocker is because I was testing a function which used open to read a file, and then used open again to overwrite the same file with new data. What I needed to be able to do was test the case where the initial file didn't exist, so set up a mock, that threw an IOError the first time, and then worked the second time. The setup for which looked like this:

from mocker import Mocker
import __builtin__

mocker = Mocker()

mockFileObject = mocker.mock()
__builtin__.open = mocker.mock()

__builtin__.open('previousState.pkl', 'r') 
mocker.throw(IOError('Boom'))

__builtin__.open('previousState.pkl','w') 
mocker.result(mockFileObject)

<rest of test setup >

mocker.replay()

<test>

mocker.restore() #required to restore the open method
mocker.verify()

Hope this helps!

Answer 2

>>> class A(object):
...     def __init__(self):
...         self.x = open('test.py')
... 
>>> old_open = open
>>> def open(s):
...     return "test\n"
... 
>>> a = A()
>>> a.x
'test\n'
>>> open = old_open
>>> a = A()
>>> a.x
<open file 'test.py', mode 'r' at 0xb7736230>