If I change it to

col = [ self._node_cache[argpath] ]

, my test cases ran just fine, but pants failed due to

E   RuntimeError: maximum recursion depth exceeded while calling a Python object
                      ERROR collecting .pants.d/pyprep/sources/d5607ed57d8c38fe7fd622398f7e758f4a7cc2c4/chatlib_test/util/__init__.py