Python: Add suggested comments and test case

Author: yoff

python/ql/src/semmle/python/functions/ModificationOfParameterWithDefaultCustomizations.qll

@@ -17,6 +17,7 @@ module ModificationOfParameterWithDefault {
    * A data flow source for detecting modifications of a parameters default value.
    */
   abstract class Source extends DataFlow::Node {
+    /** Result is true if the default value is non-empty for this source and false if not. */
     abstract boolean isNonEmpty();
   }
 
@@ -34,6 +35,7 @@ module ModificationOfParameterWithDefault {
    * A sanitizer guard for detecting modifications of a parameters default value.
    */
   abstract class BarrierGuard extends DataFlow::BarrierGuard {
+    /** Result is true if this guard blocks non-empty values and false if it blocks empty values. */
     abstract boolean blocksNonEmpty();
   }
 
@@ -120,7 +122,7 @@ module ModificationOfParameterWithDefault {
     }
   }
 
-  /** 
+  /**
    * An expression that is checked directly in an `if`, possibly with `not`, such as `if x:` or `if not x:`.
    */
   private class IdentityGuarded extends Expr {
@@ -139,7 +141,7 @@ module ModificationOfParameterWithDefault {
       )
     }
 
-    /** 
+    /**
      * Whether this guard has been inverted. For `if x:` the result is `false`, and for `if not x:` the result is `true`.
      */
     boolean isInverted() { result = inverted }

python/ql/test/query-tests/Functions/ModificationOfParameterWithDefault/test.py

@@ -124,5 +124,11 @@ def dict_update_op_nochange(d = {}):
 # OK
 def sanitizer(l = []):
     if l:
+        l.append(1)  #$ modification=l
+    return l
+
+# OK
+def sanitizer_negated(l = [1]):
+    if not l:
         l.append(1)  #$ SPURIOUS: modification=l
     return l