refactor: clean up bigframes/core/__init__.py (#1380)

* refactor: move ArrayValue out of init.py

* fix mypy

* Remove unused import

* make identifier import consistent

Author: sycai

bigframes/core/__init__.py

@@ -48,6 +48,7 @@
 import pandas as pd
 import pyarrow as pa
 
+from bigframes import session
 import bigframes._config.sampling_options as sampling_options
 import bigframes.constants
 import bigframes.core as core
@@ -257,7 +258,7 @@ def dtypes(
         return [self.expr.get_column_type(col) for col in self.value_columns]
 
     @property
-    def session(self) -> core.Session:
+    def session(self) -> session.Session:
         return self._expr.session
 
     @functools.cached_property
@@ -2653,7 +2654,7 @@ def dtypes(
         ]
 
     @property
-    def session(self) -> core.Session:
+    def session(self) -> session.Session:
         return self._expr.session
 
     @property
@@ -3171,7 +3172,7 @@ def unpivot(
 
 
 def _pd_index_to_array_value(
-    session: core.Session,
+    session: session.Session,
     index: pd.Index,
 ) -> core.ArrayValue:
     """

bigframes/core/array_value.py

@@ -22,8 +22,8 @@
 import jellyfish
 import pandas as pd
 
+from bigframes import session
 from bigframes.core import log_adapter
-import bigframes.core as core
 import bigframes.core.block_transforms as block_ops
 import bigframes.core.blocks as blocks
 import bigframes.core.expression
@@ -76,7 +76,7 @@ def __init__(
             ]
 
     @property
-    def _session(self) -> core.Session:
+    def _session(self) -> session.Session:
         return self._block.session
 
     def __getitem__(
@@ -492,7 +492,7 @@ def _aggregate_all(
     def _apply_window_op(
         self,
         op: agg_ops.WindowOp,
-        window: typing.Optional[core.WindowSpec] = None,
+        window: typing.Optional[window_specs.WindowSpec] = None,
         numeric_only: bool = False,
     ):
         """Apply window op to groupby. Defaults to grouped cumulative window."""
@@ -536,7 +536,7 @@ def __init__(
         self._dropna = dropna  # Applies to aggregations but not windowing
 
     @property
-    def _session(self) -> core.Session:
+    def _session(self) -> session.Session:
         return self._block.session
 
     @validations.requires_ordering()
@@ -759,7 +759,7 @@ def _apply_window_op(
         self,
         op: agg_ops.WindowOp,
         discard_name=False,
-        window: typing.Optional[core.WindowSpec] = None,
+        window: typing.Optional[window_specs.WindowSpec] = None,
         never_skip_nulls: bool = False,
     ):
         """Apply window op to groupby. Defaults to grouped cumulative window."""