From d34e46184983c8ed25abe98a08d8831c7a8791ec Mon Sep 17 00:00:00 2001
From: Kurt Rose <kurt.rose@simplelegal.com>
Date: Tue, 7 May 2019 14:01:45 -0700
Subject: [PATCH 1/2] added __rsub__ to complement-set so that expressions of
 form set() - complement(set()) eval correctly

---
 boltons/setutils.py    | 16 ++++++++++++++++
 tests/test_setutils.py |  1 +
 2 files changed, 17 insertions(+)

diff --git a/boltons/setutils.py b/boltons/setutils.py
index 5ac478a..03d7b1b 100644
--- a/boltons/setutils.py
+++ b/boltons/setutils.py
@@ -858,6 +858,22 @@ class _ComplementSet(object):
             else:  # + +
                 return _ComplementSet(included=self._included.difference(inc))
 
+    def __rsub__(self, other):
+        inc, exc = _norm_args_notimplemented(other)
+        if inc is NotImplemented:
+            return NotImplemented
+        # rsub, so the expression being evaluated is "other - self"
+        if self._included is None:
+            if exc is None:  # - +
+                return _ComplementSet(included=inc & self._excluded)
+            else:  # - -
+                return _ComplementSet(included=self._excluded - exc)
+        else:
+            if inc is None:  # + -
+                return _ComplementSet(excluded=exc | self._included)
+            else:  # + +
+                return _ComplementSet(included=inc.difference(self._included))
+
     def difference_update(self, other):
         try:
             self -= other
diff --git a/tests/test_setutils.py b/tests/test_setutils.py
index 4b1c5a0..92d5c90 100644
--- a/tests/test_setutils.py
+++ b/tests/test_setutils.py
@@ -113,6 +113,7 @@ def test_complement_set():
     assert (sab ^ cab) == (cbc ^ sbc)
     assert cab - cc == sc
     assert cab - sab == cab
+    assert sab - cab == sab
     assert (cab ^ cbc | set('b')) == (sab | sbc)
     everything = complement(frozenset())
     assert everything in everything  # https://en.wikipedia.org/wiki/Russell%27s_paradox

From d28450f34b9438b654eadb5f6efd32a145c2c63e Mon Sep 17 00:00:00 2001
From: Kurt Rose <kurt.rose@simplelegal.com>
Date: Tue, 7 May 2019 14:34:01 -0700
Subject: [PATCH 2/2] proper __bool__ on complement set and tests

---
 boltons/setutils.py    | 11 +++++++++--
 tests/test_setutils.py |  2 ++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/boltons/setutils.py b/boltons/setutils.py
index 03d7b1b..b426da6 100644
--- a/boltons/setutils.py
+++ b/boltons/setutils.py
@@ -907,11 +907,18 @@ class _ComplementSet(object):
         return hash(self._included) ^ hash(self._excluded)
 
     def __len__(self):
-        if self._included:
+        if self._included is not None:
             return len(self._included)
         raise NotImplementedError('complemented sets have undefined length')
 
     def __iter__(self):
-        if self._included:
+        if self._included is not None:
             return iter(self._included)
         raise NotImplementedError('complemented sets have undefined contents')
+
+    def __bool__(self):
+        if self._included is not None:
+            return bool(self._included)
+        return True
+
+    __nonzero__ = __bool__  # py2 compat
diff --git a/tests/test_setutils.py b/tests/test_setutils.py
index 92d5c90..a71d39a 100644
--- a/tests/test_setutils.py
+++ b/tests/test_setutils.py
@@ -117,6 +117,8 @@ def test_complement_set():
     assert (cab ^ cbc | set('b')) == (sab | sbc)
     everything = complement(frozenset())
     assert everything in everything  # https://en.wikipedia.org/wiki/Russell%27s_paradox
+    assert bool(cab)
+    assert not complement(u)
     # destructive testing
     cab ^= sab
     cab ^= sab