mypy fixups

Author: rhshadrach

pandas/core/arrays/arrow/array.py

@@ -1704,7 +1704,8 @@ def _str_accumulate(
                 tail = pa.nulls(len(pa_array) - idx, type=pa_array.type)
                 pa_array = pa_array[:idx].combine_chunks()
 
-        pa_result = pa.array(np_func(pa_array), type=pa_array.type)
+        # error: Cannot call function of unknown type
+        pa_result = pa.array(np_func(pa_array), type=pa_array.type)  # type: ignore[operator]
 
         assert head is None or tail is None
         if head is not None:

pandas/tests/extension/test_string.py

@@ -24,6 +24,8 @@
 
 from pandas.compat import HAS_PYARROW
 
+from pandas.core.dtypes.base import StorageExtensionDtype
+
 import pandas as pd
 import pandas._testing as tm
 from pandas.api.types import is_string_dtype
@@ -193,6 +195,7 @@ def _supports_reduction(self, ser: pd.Series, op_name: str) -> bool:
         )
 
     def _supports_accumulation(self, ser: pd.Series, op_name: str) -> bool:
+        assert isinstance(ser.dtype, StorageExtensionDtype)
         return ser.dtype.storage == "pyarrow" and op_name in [
             "cummin",
             "cummax",