TST: assert that CUDD statistics interfaces issue UserWarning

specifically the methods `dd.cudd.BDD.statistics` and
`dd.cudd_zdd.BDD.statistics`. This `UserWarning` was
introduced to inform the users about a recent API change.

Moving the function `test_str` from the module `tests/common.py`
to the modules `tests/autoref_test.py`, `tests/cudd_test.py`,
and `tests/cudd_zdd_test.py`, with appropriate modification in
each case, turned out to be the simplest way of implementing
this check.

The function `nose.tools.assert_warns` is used.
In the upcoming commit that switches to using `pytest` for
testing, the function [`pytest.warns`][1] will be used
(read also ["Asserting warnings with the warns function"][2]).

[1]: https://docs.pytest.org/en/latest/reference/reference.html#pytest-warns
[2]: https://docs.pytest.org/en/latest/how-to/capture-warnings.html#warns

tests/autoref_test.py

@@ -16,6 +16,12 @@
 BDDTests.DD = _bdd.BDD
 
 
+def test_str():
+    bdd = _bdd.BDD()
+    s = str(bdd)
+    s + 'must be a string'
+
+
 def test_find_or_add():
     bdd = _bdd.BDD()
     bdd.declare('x', 'y')

tests/common.py

@@ -83,11 +83,6 @@ def test_contains(self):
         with assert_raises(AssertionError):
             other_true in bdd
 
-    def test_str(self):
-        bdd = self.DD()
-        s = str(bdd)
-        s + 'must be a string'
-
     def test_var_levels(self):
         bdd = self.DD()
         # single variable

tests/cudd_test.py

@@ -2,7 +2,7 @@
 import logging
 
 from dd import cudd
-from nose.tools import assert_raises
+from nose.tools import assert_raises, assert_warns
 
 from common import Tests
 from common_bdd import Tests as BDDTests
@@ -18,6 +18,13 @@
 CuddTests.MODULE = cudd
 
 
+def test_str():
+    bdd = cudd.BDD()
+    with assert_warns(UserWarning):
+        s = str(bdd)
+    s + 'must be a string'
+
+
 def test_insert_var():
     bdd = cudd.BDD()
     level = 0

tests/cudd_zdd_test.py

@@ -4,6 +4,7 @@
 from dd import cudd
 from dd import cudd_zdd
 from dd import _copy
+from nose.tools import assert_warns
 
 from common import Tests as Tests
 from common_cudd import Tests as CuddTests
@@ -14,6 +15,13 @@
 CuddTests.MODULE = cudd_zdd
 
 
+def test_str():
+    bdd =  cudd_zdd.ZDD()
+    with assert_warns(UserWarning):
+        s = str(bdd)
+    s + 'must be a string'
+
+
 def test_false():
     zdd = cudd_zdd.ZDD()
     u = zdd.false