chore: revise imports and import order to account for circular imports

Author: chalmerlowe

packages/bigframes/bigframes/__init__.py

@@ -24,12 +24,24 @@
     message=".*Google will stop supporting.*Python.*",
 )
 
+# import configuration and types.
+# This ensures that when the deeper 'core' modules ask for 'dtypes','options', et. al.,
+# they are already defined and available.
+import bigframes.dtypes   # noqa: E402 # isort: skip
+import bigframes._config  # noqa: E402 # isort: skip
+from bigframes._config import option_context, options  # noqa: E402 # isort: skip
+
 import bigframes.enums as enums  # noqa: E402
 import bigframes.exceptions as exceptions  # noqa: E402
 
+# We import operations early to resolve a circular dependency between
+# bigframes.core.expression and bigframes.operations.
+# This ensures the 'Expression' base class is defined before 'Aggregation'
+# subclasses attempt to inherit from it.
+import bigframes.operations  # noqa: E402 # isort: skip
+
 # Register pandas extensions
 import bigframes.extensions.pandas.dataframe_accessor  # noqa: F401, E402
-from bigframes._config import option_context, options  # noqa: E402
 from bigframes._config.bigquery_options import BigQueryOptions  # noqa: E402
 from bigframes.core.global_session import (  # noqa: E402
     close_session,

packages/bigframes/bigframes/core/expression.py

@@ -28,6 +28,9 @@
 from bigframes import dtypes
 from bigframes.core import field
 
+if typing.TYPE_CHECKING:
+    import bigframes.operations
+
 
 def const(
     value: typing.Hashable, dtype: dtypes.ExpressionType = None

packages/bigframes/bigframes/dtypes.py

@@ -31,7 +31,6 @@
 import pyarrow as pa
 import shapely.geometry  # type: ignore
 
-import bigframes.core.backports
 import bigframes.exceptions
 
 # Type hints for Pandas dtypes supported by BigQuery DataFrame
@@ -397,6 +396,9 @@ def get_struct_fields(type_: ExpressionType) -> dict[str, Dtype]:
     assert isinstance(type_.pyarrow_dtype, pa.StructType)
     struct_type = type_.pyarrow_dtype
     result: dict[str, Dtype] = {}
+
+    # Local import to break circular dependency with core.backports
+    import bigframes.core.backports
     for field in bigframes.core.backports.pyarrow_struct_type_fields(struct_type):
         result[field.name] = arrow_dtype_to_bigframes_dtype(field.type)
     return result
@@ -582,6 +584,9 @@ def to_storage_type(
         return pa.list_(to_storage_type(arrow_type.value_type))
     if pa.types.is_struct(arrow_type):
         assert isinstance(arrow_type, pa.StructType)
+
+        # Local import to break circular dependency with core.backports
+        import bigframes.core.backports
         return pa.struct(
             field.with_type(to_storage_type(field.type))
             for field in bigframes.core.backports.pyarrow_struct_type_fields(arrow_type)
@@ -595,6 +600,9 @@ def arrow_type_to_literal(
     """Create a representative literal value for an arrow type."""
     if pa.types.is_list(arrow_type):
         return [arrow_type_to_literal(arrow_type.value_type)]
+
+    # Local import to break circular dependency with core.backports
+    import bigframes.core.backports
     if pa.types.is_struct(arrow_type):
         return {
             field.name: arrow_type_to_literal(field.type)
@@ -1001,6 +1009,8 @@ def contains_db_dtypes_json_arrow_type(type_):
         return contains_db_dtypes_json_arrow_type(type_.value_type)
 
     if isinstance(type_, pa.StructType):
+        # Local import to break circular dependency with core.backports
+        import bigframes.core.backports
         return any(
             contains_db_dtypes_json_arrow_type(field.type)
             for field in bigframes.core.backports.pyarrow_struct_type_fields(type_)