Invalidate nest-resolver caches during assignment.

Resolves GH#159.

src/nested_pandas/nestedframe/core.py

@@ -94,13 +94,19 @@ def __getitem__(self, item):
             super().__setitem__(top_nest, _NestedFieldResolver(top_nest, self._outer))
         return super().__getitem__(top_nest)
 
-    def __setitem__(self, key, value):
+    def __setitem__(self, item, _):
         # Called to update the resolver with intermediate values.
         # The important point is to intercept the call so that the evaluator
-        # does not create any new resolvers on the fly.  Storing the value
-        # is not important, since that will have been done already in
-        # the NestedFrame.
-        pass
+        # does not create any new resolvers on the fly.  We do NOT want to
+        # store the given value, since the resolver does lazy-loading.
+        # What we DO want to do, however, is to invalidate the cache for
+        # any field resolver for a given nest that is receiving an assignment.
+        # Since the resolvers are created as-needed in __getitem__, all we need
+        # to do is delete them from the local cache when this pattern is detected.
+        if "." in item:
+            top_nest = item.split(".")[0].strip()
+            if top_nest in self._outer.nested_columns and super().__contains__(top_nest):
+                del self[top_nest]  # force re-creation in __setitem__
 
 
 class _NestedFieldResolver:

tests/nested_pandas/nestedframe/test_nestedframe.py

@@ -884,14 +884,20 @@ def test_eval_assignment():
     assert (nf_n3["p2.d"] == nf_n2["p2.c2"] + nf["packed.d"] * 2 + nf["b"]).all()
 
     # Now test multiline and inplace=True
+    # Verify the resolution of GH#159, where a nested column created in
+    # an existing nest during a multi-line eval was not being recognized
+    # in a subsequent line.
     nf.eval(
         """
         c = a + b
-        p2.e = packed.d * 2 + c
+        packed.e = packed.d * 2
+        p2.e = packed.e + c
         p2.f = p2.e + b
         """,
         inplace=True,
     )
-    assert len(nf.p2.nest.fields) == 2
+    assert set(nf.nested_columns) == {"packed", "p2"}
+    assert set(nf.packed.nest.fields) == {"c", "d", "e", "time"}
+    assert set(nf.p2.nest.fields) == {"e", "f"}
     assert (nf["p2.e"] == nf["packed.d"] * 2 + nf.c).all()
     assert (nf["p2.f"] == nf["p2.e"] + nf.b).all()