diff --git a/python/ql/src/Functions/ModificationOfParameterWithDefault.ql b/python/ql/src/Functions/ModificationOfParameterWithDefault.ql
index 206174e02b6a..ab94a9735763 100644
--- a/python/ql/src/Functions/ModificationOfParameterWithDefault.ql
+++ b/python/ql/src/Functions/ModificationOfParameterWithDefault.ql
@@ -19,32 +19,47 @@ predicate safe_method(string name) {
     name = "items" or name = "keys" or name = "values" or name = "iteritems" or name = "iterkeys" or name = "itervalues"
 }
 
-predicate maybe_parameter(SsaVariable var, Function f, Parameter p) {
-    p = var.getAnUltimateDefinition().getDefinition().getNode() and
-    f.getAnArg() = p
+/** Gets the truthiness (non emptyness) of the default of `p` if that value is mutable */
+private boolean mutableDefaultValue(Parameter p) {
+    exists(Dict d |
+        p.getDefault() = d |
+        exists(d.getAKey()) and result = true
+        or
+        not exists(d.getAKey()) and result = false
+    )
+    or
+    exists(List l |
+        p.getDefault() = l |
+        exists(l.getAnElt()) and result = true
+        or
+        not exists(l.getAnElt()) and result = false
+    )
 }
 
-predicate has_mutable_default(Parameter p) {
-    exists(SsaVariable v, FunctionExpr f | maybe_parameter(v, f.getInnerScope(), p) and
-        exists(int i, int def_cnt, int arg_cnt |
-            def_cnt = count(f.getArgs().getADefault()) and
-            arg_cnt = count(f.getInnerScope().getAnArg()) and
-            i in [1 .. arg_cnt] and
-           (f.getArgs().getDefault(def_cnt - i) instanceof Dict or f.getArgs().getDefault(def_cnt - i) instanceof List) and
-            f.getInnerScope().getArgName(arg_cnt - i) = v.getId()
-        )
-    )
+
+class NonEmptyMutableValue extends TaintKind {
+    NonEmptyMutableValue() {
+        this = "non-empty mutable value"
+    }
 }
 
-class MutableValue extends TaintKind {
-    MutableValue() {
-        this = "mutable value"
+class EmptyMutableValue extends TaintKind {
+    EmptyMutableValue() {
+        this = "empty mutable value"
     }
+
+    override boolean booleanValue() {
+        result = false
+    }
+
 }
 
 class MutableDefaultValue extends TaintSource {
+
+    boolean nonEmpty;
+
     MutableDefaultValue() {
-        has_mutable_default(this.(NameNode).getNode())
+        nonEmpty = mutableDefaultValue(this.(NameNode).getNode())
     }
 
     override string toString() {
@@ -52,7 +67,9 @@ class MutableDefaultValue extends TaintSource {
     }
 
     override predicate isSourceOf(TaintKind kind) {
-        kind instanceof MutableValue
+        nonEmpty = false and kind instanceof EmptyMutableValue
+        or
+        nonEmpty = true and kind instanceof NonEmptyMutableValue
     }
 }
 
@@ -68,7 +85,9 @@ class Mutation extends TaintSink {
     }
 
     override predicate sinks(TaintKind kind) {
-        kind instanceof MutableValue
+        kind instanceof EmptyMutableValue
+        or
+        kind instanceof NonEmptyMutableValue
     }
 }
 
diff --git a/python/ql/src/semmle/python/security/TaintTracking.qll b/python/ql/src/semmle/python/security/TaintTracking.qll
index bd0037b52caf..16cd1952a90d 100755
--- a/python/ql/src/semmle/python/security/TaintTracking.qll
+++ b/python/ql/src/semmle/python/security/TaintTracking.qll
@@ -148,6 +148,16 @@ abstract class TaintKind extends string {
         none()
     }
 
+    /** Gets the boolean values (may be one, neither, or both) that
+     * may result from the Python expression `bool(this)`
+     */
+    boolean booleanValue() {
+        /* Default to true as the vast majority of taint is strings and 
+         * the empty string is almost always benign.
+         */
+        result = true
+    }
+
     string repr() { result = this }
 
 }
@@ -1190,7 +1200,8 @@ library module TaintFlowImplementation {
                 sanitizer.sanitizingEdge(kind, test)
             )
             |
-            not Filters::isinstance(test.getTest(), _, var.getSourceVariable().getAUse())
+            not Filters::isinstance(test.getTest(), _, var.getSourceVariable().getAUse()) and
+            not test.getTest() = var.getSourceVariable().getAUse()
             or
             exists(ControlFlowNode c, ClassObject cls |
                 Filters::isinstance(test.getTest(), c, var.getSourceVariable().getAUse())
@@ -1200,6 +1211,8 @@ library module TaintFlowImplementation {
                 or
                 test.getSense() = false and not kind.getClass().getAnImproperSuperType() = cls
             )
+            or
+            test.getTest() = var.getSourceVariable().getAUse() and kind.booleanValue() = test.getSense()
         )
     }
 
diff --git a/python/ql/test/library-tests/taint/general/TestNode.expected b/python/ql/test/library-tests/taint/general/TestNode.expected
index c7ae50289a07..b1f290b4acba 100644
--- a/python/ql/test/library-tests/taint/general/TestNode.expected
+++ b/python/ql/test/library-tests/taint/general/TestNode.expected
@@ -215,6 +215,11 @@
 | Taint simple.test | test.py:169 | SOURCE |  |
 | Taint simple.test | test.py:172 | Subscript |  |
 | Taint simple.test | test.py:173 | Subscript |  |
+| Taint simple.test | test.py:178 | SOURCE |  |
+| Taint simple.test | test.py:179 | t |  |
+| Taint simple.test | test.py:180 | t |  |
+| Taint simple.test | test.py:183 | t |  |
+| Taint simple.test | test.py:186 | t |  |
 | Taint {simple.test} | test.py:169 | Dict |  |
 | Taint {simple.test} | test.py:171 | d |  |
 | Taint {simple.test} | test.py:173 | y |  |
diff --git a/python/ql/test/library-tests/taint/general/TestSink.expected b/python/ql/test/library-tests/taint/general/TestSink.expected
index dd169d78f033..c8f7f22a1fa4 100644
--- a/python/ql/test/library-tests/taint/general/TestSink.expected
+++ b/python/ql/test/library-tests/taint/general/TestSink.expected
@@ -32,3 +32,5 @@
 | simple.test | test.py:159 | 160 | t | simple.test |
 | simple.test | test.py:168 | 172 | Subscript | simple.test |
 | simple.test | test.py:169 | 173 | Subscript | simple.test |
+| simple.test | test.py:178 | 180 | t | simple.test |
+| simple.test | test.py:178 | 186 | t | simple.test |
diff --git a/python/ql/test/library-tests/taint/general/TestSource.expected b/python/ql/test/library-tests/taint/general/TestSource.expected
index ea9012b7735b..d341758b2eae 100644
--- a/python/ql/test/library-tests/taint/general/TestSource.expected
+++ b/python/ql/test/library-tests/taint/general/TestSource.expected
@@ -40,3 +40,4 @@
 | test.py:163 | SOURCE | simple.test |
 | test.py:168 | SOURCE | simple.test |
 | test.py:169 | SOURCE | simple.test |
+| test.py:178 | SOURCE | simple.test |
diff --git a/python/ql/test/library-tests/taint/general/TestStep.expected b/python/ql/test/library-tests/taint/general/TestStep.expected
index a4c72fd6b0ce..dec3e5b03b24 100644
--- a/python/ql/test/library-tests/taint/general/TestStep.expected
+++ b/python/ql/test/library-tests/taint/general/TestStep.expected
@@ -173,6 +173,10 @@
 | Taint simple.test | test.py:163 | SOURCE |  |  -->  | Taint simple.test | test.py:164 | s |  |
 | Taint simple.test | test.py:168 | SOURCE |  |  -->  | Taint [simple.test] | test.py:168 | List |  |
 | Taint simple.test | test.py:169 | SOURCE |  |  -->  | Taint {simple.test} | test.py:169 | Dict |  |
+| Taint simple.test | test.py:178 | SOURCE |  |  -->  | Taint simple.test | test.py:179 | t |  |
+| Taint simple.test | test.py:178 | SOURCE |  |  -->  | Taint simple.test | test.py:180 | t |  |
+| Taint simple.test | test.py:178 | SOURCE |  |  -->  | Taint simple.test | test.py:183 | t |  |
+| Taint simple.test | test.py:178 | SOURCE |  |  -->  | Taint simple.test | test.py:186 | t |  |
 | Taint {simple.test} | test.py:169 | Dict |  |  -->  | Taint {simple.test} | test.py:171 | d |  |
 | Taint {simple.test} | test.py:169 | Dict |  |  -->  | Taint {simple.test} | test.py:175 | d |  |
 | Taint {simple.test} | test.py:171 | d |  |  -->  | Taint {simple.test} | test.py:173 | y |  |
diff --git a/python/ql/test/library-tests/taint/general/TestVar.expected b/python/ql/test/library-tests/taint/general/TestVar.expected
index 5aad11fb5476..9939820a0a4d 100644
--- a/python/ql/test/library-tests/taint/general/TestVar.expected
+++ b/python/ql/test/library-tests/taint/general/TestVar.expected
@@ -177,3 +177,8 @@
 | test.py:174 | l_2 | test.py:168 | Taint [simple.test] | List |
 | test.py:175 | d2_0 | test.py:175 | Taint {simple.test} | dict() |
 | test.py:175 | d_2 | test.py:169 | Taint {simple.test} | Dict |
+| test.py:178 | t_0 | test.py:178 | Taint simple.test | SOURCE |
+| test.py:180 | t_1 | test.py:178 | Taint simple.test | SOURCE |
+| test.py:180 | t_2 | test.py:178 | Taint simple.test | SOURCE |
+| test.py:183 | t_3 | test.py:178 | Taint simple.test | SOURCE |
+| test.py:186 | t_4 | test.py:178 | Taint simple.test | SOURCE |
diff --git a/python/ql/test/library-tests/taint/general/test.py b/python/ql/test/library-tests/taint/general/test.py
index 5a6fc1e2900d..6c752b2c1d71 100644
--- a/python/ql/test/library-tests/taint/general/test.py
+++ b/python/ql/test/library-tests/taint/general/test.py
@@ -173,3 +173,14 @@ def test_update_extend(x, y):
     SINK(y["key"])
     l2 = list(l)
     d2 = dict(d)
+
+def test_truth():
+    t = SOURCE
+    if t:
+        SINK(t)
+    else:
+        SINK(t)
+    if not t:
+        SINK(t)
+    else:
+        SINK(t)
diff --git a/python/ql/test/query-tests/Functions/general/ModificationOfParameterWithDefault.expected b/python/ql/test/query-tests/Functions/general/ModificationOfParameterWithDefault.expected
index fa8feaf186ba..a65d2ca8dc15 100644
--- a/python/ql/test/query-tests/Functions/general/ModificationOfParameterWithDefault.expected
+++ b/python/ql/test/query-tests/Functions/general/ModificationOfParameterWithDefault.expected
@@ -1,20 +1,22 @@
 edges
-| functions_test.py:36:9:36:9 | mutable value | functions_test.py:37:16:37:16 | mutable value |
-| functions_test.py:39:9:39:9 | mutable value | functions_test.py:40:5:40:5 | mutable value |
-| functions_test.py:238:15:238:15 | mutable value | functions_test.py:239:5:239:5 | mutable value |
-| functions_test.py:290:25:290:25 | mutable value | functions_test.py:291:5:291:5 | mutable value |
-| functions_test.py:293:21:293:21 | mutable value | functions_test.py:294:5:294:5 | mutable value |
-| functions_test.py:296:27:296:27 | mutable value | functions_test.py:297:25:297:25 | mutable value |
-| functions_test.py:296:27:296:27 | mutable value | functions_test.py:298:21:298:21 | mutable value |
-| functions_test.py:297:25:297:25 | mutable value | functions_test.py:290:25:290:25 | mutable value |
-| functions_test.py:298:21:298:21 | mutable value | functions_test.py:293:21:293:21 | mutable value |
+| functions_test.py:36:9:36:9 | empty mutable value | functions_test.py:37:16:37:16 | empty mutable value |
+| functions_test.py:39:9:39:9 | empty mutable value | functions_test.py:40:5:40:5 | empty mutable value |
+| functions_test.py:238:15:238:15 | empty mutable value | functions_test.py:239:5:239:5 | empty mutable value |
+| functions_test.py:290:25:290:25 | empty mutable value | functions_test.py:291:5:291:5 | empty mutable value |
+| functions_test.py:293:21:293:21 | empty mutable value | functions_test.py:294:5:294:5 | empty mutable value |
+| functions_test.py:296:27:296:27 | empty mutable value | functions_test.py:297:25:297:25 | empty mutable value |
+| functions_test.py:296:27:296:27 | empty mutable value | functions_test.py:298:21:298:21 | empty mutable value |
+| functions_test.py:297:25:297:25 | empty mutable value | functions_test.py:290:25:290:25 | empty mutable value |
+| functions_test.py:298:21:298:21 | empty mutable value | functions_test.py:293:21:293:21 | empty mutable value |
+| functions_test.py:300:26:300:26 | empty mutable value | functions_test.py:301:8:301:8 | empty mutable value |
+| functions_test.py:300:26:300:26 | empty mutable value | functions_test.py:303:12:303:12 | empty mutable value |
 parents
-| functions_test.py:290:25:290:25 | mutable value | functions_test.py:297:25:297:25 | mutable value |
-| functions_test.py:291:5:291:5 | mutable value | functions_test.py:297:25:297:25 | mutable value |
-| functions_test.py:293:21:293:21 | mutable value | functions_test.py:298:21:298:21 | mutable value |
-| functions_test.py:294:5:294:5 | mutable value | functions_test.py:298:21:298:21 | mutable value |
+| functions_test.py:290:25:290:25 | empty mutable value | functions_test.py:297:25:297:25 | empty mutable value |
+| functions_test.py:291:5:291:5 | empty mutable value | functions_test.py:297:25:297:25 | empty mutable value |
+| functions_test.py:293:21:293:21 | empty mutable value | functions_test.py:298:21:298:21 | empty mutable value |
+| functions_test.py:294:5:294:5 | empty mutable value | functions_test.py:298:21:298:21 | empty mutable value |
 #select
-| functions_test.py:40:5:40:5 | Taint sink | functions_test.py:39:9:39:9 | mutable value | functions_test.py:40:5:40:5 | mutable value | $@ flows to here and is mutated. | functions_test.py:39:9:39:9 | mutable default value | Default value |
-| functions_test.py:239:5:239:5 | Taint sink | functions_test.py:238:15:238:15 | mutable value | functions_test.py:239:5:239:5 | mutable value | $@ flows to here and is mutated. | functions_test.py:238:15:238:15 | mutable default value | Default value |
-| functions_test.py:291:5:291:5 | Taint sink | functions_test.py:296:27:296:27 | mutable value | functions_test.py:291:5:291:5 | mutable value | $@ flows to here and is mutated. | functions_test.py:296:27:296:27 | mutable default value | Default value |
-| functions_test.py:294:5:294:5 | Taint sink | functions_test.py:296:27:296:27 | mutable value | functions_test.py:294:5:294:5 | mutable value | $@ flows to here and is mutated. | functions_test.py:296:27:296:27 | mutable default value | Default value |
+| functions_test.py:40:5:40:5 | Taint sink | functions_test.py:39:9:39:9 | empty mutable value | functions_test.py:40:5:40:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:39:9:39:9 | mutable default value | Default value |
+| functions_test.py:239:5:239:5 | Taint sink | functions_test.py:238:15:238:15 | empty mutable value | functions_test.py:239:5:239:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:238:15:238:15 | mutable default value | Default value |
+| functions_test.py:291:5:291:5 | Taint sink | functions_test.py:296:27:296:27 | empty mutable value | functions_test.py:291:5:291:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:296:27:296:27 | mutable default value | Default value |
+| functions_test.py:294:5:294:5 | Taint sink | functions_test.py:296:27:296:27 | empty mutable value | functions_test.py:294:5:294:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:296:27:296:27 | mutable default value | Default value |
diff --git a/python/ql/test/query-tests/Functions/general/functions_test.py b/python/ql/test/query-tests/Functions/general/functions_test.py
index 280af2bc2279..91613675aec1 100644
--- a/python/ql/test/query-tests/Functions/general/functions_test.py
+++ b/python/ql/test/query-tests/Functions/general/functions_test.py
@@ -296,3 +296,9 @@ def mutate_argument(x):
 def indirect_modification(y = []):
     aug_assign_argument(y)
     mutate_argument(y)
+
+def guarded_modification(z=[]):
+    if z:
+        z.append(0)
+    return z
+