Consider this example, based on two doctests from Twisted:

  from doctest import DocTestSuite
  import twisted.web2.stream
  docTestOne = DocTestSuite(twisted.web2.stream)._tests[0]
  import twisted.web2.test.test_stream
  docTestTwo = DocTestSuite(twisted.web2.test.test_stream)._tests[0]
  print docTestOne.id(), '==', docTestTwo.id(), '?'
  print docTestOne == docTestTwo

One might reasonably expect a false result, since the two DocTestCase
instances represent two different doctests.  One will meet with
surprise, though.

There doesn't seem to be an __eq__ or __cmp__ method in DocTestCase,
which implies that the issue is with unittest.TestCase (from which
DocTestCase inherits).

Or an interaction between TestCase and DocTestCase.  TestCase.__eq__ and
TestCase.__hash__ are both implemented in terms of the _testMethodName
attribute.  It looks like this is *always* "runTest" for a DocTestCase
instance.  It probably doesn't make sense to set a _testMethodName for
DocTestCase instances (maybe it doesn't even make sense for DocTestCase
to subclass TestCase, in fact).  So perhaps overriding comparison and
hashing to use the attributes that do make sense (_dt_test, maybe, I
don't know) would make sense.

Agreed, defining __eq__ and __hash__ on DocTestCase sounds like the way
to go.

Here is a patch that defines __eq__ and __hash__ on DocTestCase.

New patch with test

New patch to add __hash__ and __eq__ to DocTest

The definition of __eq__ is wrong: it shouldn't compare the hashes since the hash() function isn't injective. For example, 0 and "" have equal hashes, yet they are unequal.

Update patch to not use hash in __eq__

Something I forgot: you should also test the "!=" operator in the tests.

Add test for "!="

Add also __eq__ to Example and add __ne__ method.

New changeset c478734ded4b by Antoine Pitrou in branch '3.2':
Issue #7502: Fix equality comparison for DocTestCase instances.
http://hg.python.org/cpython/rev/c478734ded4b

New changeset b8cb6f1e4981 by Antoine Pitrou in branch 'default':
Issue #7502: Fix equality comparison for DocTestCase instances.
http://hg.python.org/cpython/rev/b8cb6f1e4981

New changeset 64d670a8b183 by Antoine Pitrou in branch '2.7':
Issue #7502: Fix equality comparison for DocTestCase instances.
http://hg.python.org/cpython/rev/64d670a8b183

Patch now committed to all 3 branches. Thanks for contributing!

Patch to add __hash__ to prevent warnings in 2.7

Add test for __hash__

New changeset 6a95820b9607 by Antoine Pitrou in branch '2.7':
Followup to #7502: add __hash__ method and tests.
http://hg.python.org/cpython/rev/6a95820b9607