Add test for FP for py/modification-of-locals

Author: owen-mc

python/ql/test/query-tests/Statements/general/ModificationOfLocals.expected

@@ -3,3 +3,10 @@
 | test.py:101:5:101:14 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |
 | test.py:102:9:102:14 | Subscript | Modification of the locals() dictionary will have no effect on the local variables. |
 | test.py:103:5:103:13 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:206:5:206:11 | Subscript | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:207:5:207:23 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:208:5:208:15 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:209:9:209:15 | Subscript | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:210:5:210:14 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:228:9:228:24 | Subscript | Modification of the locals() dictionary will have no effect on the local variables. |
+| test.py:229:9:229:35 | Attribute() | Modification of the locals() dictionary will have no effect on the local variables. |

python/ql/test/query-tests/Statements/general/test.py

@@ -196,3 +196,41 @@ class false_positive:
 class MyClass:
     locals()['x'] = 43  # OK
     y = x
+
+
+# Once a `locals()` dictionary is passed out of the scope that created it, it is
+# just an ordinary mapping. Modifying it in a different scope is meaningful and
+# effective, so these modifications must NOT be flagged: the "no effect on local
+# variables" gotcha only applies within the scope that called `locals()`.
+def modify_passed_dict(ns):
+    ns['k'] = 1  # OK: `ns` is a parameter here, not this scope's locals()
+    ns.update({'j': 2})  # OK
+    ns.pop('k')  # OK
+    del ns['j']  # OK
+    ns.clear()  # OK
+
+
+def pass_locals_to_function():
+    y = 1
+    modify_passed_dict(locals())
+    return y
+
+
+# The same situation, but where the `locals()` dictionary is laundered through an
+# instance attribute (as instance-attribute type tracking now models). These must
+# also not be flagged.
+class NamespaceHolder(object):
+
+    def __init__(self, ns):
+        self.ns = ns
+
+    def populate(self):
+        self.ns['extra'] = 1  # OK: different scope from the `locals()` call
+        self.ns.update({'more': 2})  # OK
+
+
+def launder_locals_through_instance():
+    x = 1
+    holder = NamespaceHolder(locals())
+    holder.populate()
+    return x