Unittest for context-manager fails with AttributeError: exit

Question

I'm trying to understand the correct way to unit test code with context-manager (with statement).

Here is my sample code:

class resources():
    def __init__(self):
        self.data = 'at-init'

    def __enter__(self):
        self.data = 'at-enter'
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.data = 'at-exit'

Here is my unittest code:

import unittest
import ctxmgr     

class TestResources(unittest.TestCase):
    def setUp(self):
        pass

    def test_ctxmgr(self):
        with ctxmgr.resources as r:
            self.assertEqual(r.data, 'at-enter')

The sample code runs fine but the unittest code above fails with,

======================================================================
ERROR: test_ctxmgr (__main__.TestResources)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_ctxmgr.py", line 12, in test_ctxmgr
    with ctxmgr.resources as r:
AttributeError: __exit__

----------------------------------------------------------------------
Ran 1 test in 0.003s

FAILED (errors=1)

What is causing this error? What am I missing?

user2555451 · Accepted Answer · 2015-02-13 01:47:30Z

12

You need to instantiate the resources class when you use it with the with-statement:

with ctxmgr.resources() as r:
#                    ^^

Demo:

>>> class resources():
...     def __init__(self):
...         self.data = 'at-init'
...     def __enter__(self):
...         self.data = 'at-enter'
...         return self
...     def __exit__(self, exc_type, exc_val, exc_tb):
...         self.data = 'at-exit'
...
>>> with resources as r:
...     r
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __exit__
>>>
>>> with resources() as r:
...     r
...
<__main__.resources object at 0x02112510>
>>>

answered Feb 13, 2015 at 1:47

user2555451

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