python: add test for captured variables

this illustrates that the function implementing
the comprehension does not capture `mod_local`.

We could handle this case specially, by having
a different implementation for `for`, but the
wider issue would remain.

Author: yoff

python/ql/test/experimental/dataflow/variable-capture/collections.py

@@ -0,0 +1,64 @@
+# Here we test the case where a captured variable is being read.
+
+# All functions starting with "test_" should run and execute `print("OK")` exactly once.
+# This can be checked by running validTest.py.
+
+import sys
+import os
+
+sys.path.append(os.path.dirname(os.path.dirname((__file__))))
+from testlib import expects
+
+# These are defined so that we can evaluate the test code.
+NONSOURCE = "not a source"
+SOURCE = "source"
+
+def is_source(x):
+    return x == "source" or x == b"source" or x == 42 or x == 42.0 or x == 42j
+
+
+def SINK(x):
+    if is_source(x):
+        print("OK")
+    else:
+        print("Unexpected flow", x)
+
+
+def SINK_F(x):
+    if is_source(x):
+        print("Unexpected flow", x)
+    else:
+        print("OK")
+
+l = [NONSOURCE]
+SINK_F(l_mod[0])
+
+l_mod = [SOURCE for x in l]
+SINK(l_mod[0]) #$ captured
+
+l_mod_lambda = [(lambda a : SOURCE)(x) for x in l]
+SINK(l_mod_lambda[0]) #$ captured
+
+def mod(x):
+    return SOURCE
+
+l_mod_function = [mod(x) for x in l]
+SINK(l_mod_function[0]) #$ captured
+
+def mod_list(l):
+    def mod_local(x):
+        return SOURCE
+
+    return [mod_local(x) for x in l]
+
+l_modded = mod_list(l)
+SINK(l_modded[0]) #$ MISSING: captured
+
+def mod_list_first(l):
+    def mod_local(x):
+        return SOURCE
+
+    return [mod_local(l[0])]
+
+l_modded_first = mod_list_first(l)
+SINK(l_modded_first[0]) #$ captured