Clarify that exceptions are memoized but not checkpointed

Author: benclifford

docs/userguide/workflows/checkpoints.rst

@@ -10,10 +10,11 @@ This can save time and computational resources.
 
 This is done in two ways:
 
-* Firstly, *app caching* will allow reuse of results within the same run.
+* Firstly, *app caching* will allow reuse of results and exceptions within
+  the same run.
 
-* Building on top of that, *checkpointing* will store results on the filesystem
-  and reuse those results in later runs.
+* Building on top of that, *checkpointing* will store results (but not
+  exceptions) on the filesystem and reuse those results in later runs.
 
 .. _label-appcaching:
 

parsl/tests/test_python_apps/test_memoize_exception.py

@@ -0,0 +1,22 @@
+import argparse
+
+import parsl
+from parsl.app.app import python_app
+
+
+@python_app(cache=True)
+def raise_exception(x, cache=True):
+    raise RuntimeError("exception from raise_exception")
+
+
+def test_python_memoization(n=2):
+    """Testing python memoization with exceptions."""
+    x = raise_exception(0)
+
+    # wait for x to be done
+    x.exception()
+
+    for i in range(0, n):
+        foo = raise_exception(0)
+        print(foo.exception())
+        assert foo.exception() == x.exception(), "Memoized exceptions were not used"