question about an exciting gotcha for unittests (and elsewhere) ...

Cameron Simpson cs at
Fri Apr 23 01:37:15 EDT 2010

I've run into a problem unittesting something I'm writing.

I have a little node tracking class I'm using to track items and
attributes.  An item is a "Node" object, and the collection is a

So I'm writing some unittests, thus:

  class Node(dict): [...]
  class NodeDB(dic): [...]
  class Backend(object):
    def serialise(self, value):
      ''' Convert a value for external string storage.
      if isinstance(value, Node): [...]
        return ":%s:%s" % (value.type,
      t = type(value)
      assert t in (str,int), repr(t)+" "+repr(value)+" "+repr(Node)

  class TestAll(unittest.TestCase):
    def setUp(self):
      from cs.nodedb.sqla import Backend_SQLAlchemy
    def test01serialise(self):
      H = self.db.newNode('HOST', 'foo')
      for value in 1, 'str1', ':str2', '::', H:
        s = self.backend.serialise(value)
        assert type(s) is str
        self.assert_(value == self.backend.deserialise(s))

  if __name__ == '__main__':
    import sqlalchemy
    print 'SQLAlchemy version =', sqlalchemy.__version__

and it's failing. I've traced the failure cause, ending up with this
assertion message from the end of serialise() above:

  AssertionError: <class '__main__.Node'> HOST:foo:{} <class 'cs.nodedb.node.Node'>

Experienced users will see at once what's happened: I've made a Node
myself in the test using the local class, and the Node class is thus
__main__.Node. However, my sql Backend class has independently imported
the "Node" and "Backend" classes from "cs.nodedb.node". So when _it_
calls serialise(), "Node" is "cs.nodedb.node.Node".

And lo, the:

  if isinstance(value, Node):

test at the top of serialise() fails.

What's a sensible way of doing this correctly?

I _don't_ want to duck-type the Node and merely test for "type" and "name"
values, because I want to be rather picky about what gets into the backend
database - the wrong types indicate bad app code, and should not be allowed
to introduce corrupt database values.

Cameron Simpson <cs at> DoD#743

That particular mistake will not be repeated.  There are plenty of mistakes
left that have not yet been used. - Andy Tanenbaum <ast at>

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