(core) Don't swallow TypeErrors in functions like SUM

Summary: Math functions like SUM which call `_chain` were catching `TypeError`s raised by the iterable arguments themselves, e.g. `SUM(r.A / r.B for r in $group)` where `r.A / r.B` raises a `TypeError` would silently return wrong results. This diff narrows the `try/catch` to only check whether the argument is iterable as intended, but not catch errors from the process of iterating.

Test Plan: Added Python unit test.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3679

sandbox/grist/functions/math.py

@@ -21,10 +21,12 @@ import roman
 def _chain(*values_or_iterables):
   for v in values_or_iterables:
     try:
-      for x in v:
-        yield x
+      v = iter(v)
     except TypeError:
       yield v
+    else:
+      for x in v:
+        yield x
 
 
 # Iterates through iterable or other arguments, skipping non-numeric ones.

sandbox/grist/test_functions.py

@@ -80,3 +80,9 @@ class TestUuid(unittest.TestCase):
       self.check_uuids(1)  # because of the `random.seed(0)` in `check_uuids()`
     finally:
       uuid.uuid4 = v4
+
+
+class TestChain(unittest.TestCase):
+  def test_chain_type_error(self):
+    with self.assertRaises(TypeError):
+      functions.SUM(x / "2" for x in [1, 2, 3])