[GitHub] [arrow] Fokko commented on a diff in pull request #35568: GH-33986: [Python] Add a minimal protocol for datasets

["Fokko (via GitHub)" <gi...@apache.org>]

Fokko commented on code in PR #35568:
URL: https://github.com/apache/arrow/pull/35568#discussion_r1230059528


##########
python/pyarrow/dataset/protocol.py:
##########
@@ -0,0 +1,180 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Protocol definitions for pyarrow.dataset
+
+These provide the abstract interface for a dataset. Other libraries may implement
+this interface to expose their data, without having to extend PyArrow's classes.
+
+Applications and libraries that want to consume datasets should accept datasets
+that implement these protocols, rather than requiring the specific
+PyArrow classes.
+
+See Extending PyArrow Datasets for more information:
+
+https://arrow.apache.org/docs/python/integration/dataset.html
+"""
+from abc import abstractmethod, abstractproperty
+from typing import Iterator, List, Optional
+
+# TODO: remove once we drop support for Python 3.7
+if sys.version_info >= (3, 8):
+    from typing import Protocol, runtime_checkable
+else:
+    from typing_extensions import Protocol, runtime_checkable
+
+from pyarrow.dataset import Expression
+from pyarrow import Table, RecordBatchReader, Schema
+
+
+@runtime_checkable
+class Scanner(Protocol):
+    """
+    A scanner implementation for a dataset.
+
+    This may be a scan of a whole dataset, or a scan of a single fragment.
+    """
+    @abstractmethod
+    def count_rows(self) -> int:
+        """
+        Count the number of rows in this dataset.
+
+        Implementors may provide optimized code paths that compute this from metadata.
+
+        Returns
+        -------
+        int
+            The number of rows in the dataset.
+        """
+        ...
+
+    @abstractmethod
+    def head(self, num_rows: int) -> Table:
+        """
+        Get the first ``num_rows`` rows of the dataset.
+
+        Parameters
+        ----------
+        num_rows : int
+            The number of rows to return.
+
+        Returns
+        -------
+        Table
+            A table containing the first ``num_rows`` rows of the dataset.
+        """
+        ...
+
+    @abstractmethod
+    def to_reader(self) -> RecordBatchReader:
+        """
+        Create a Record Batch Reader for this scan.
+
+        This is used to read the data in chunks.
+
+        Returns
+        -------
+        RecordBatchReader
+        """
+        ...
+
+
+@runtime_checkable
+class Scannable(Protocol):
+    @abstractmethod
+    def scanner(self, columns: Optional[List[str]] = None,
+                filter: Optional[Expression] = None, batch_size: Optional[int] = None,
+                use_threads: bool = True,
+                **kwargs) -> Scanner:
+        """Create a scanner for this dataset.
+
+        Parameters
+        ----------
+        columns : List[str], optional
+            Names of columns to include in the scan. If None, all columns are
+            included.
+        filter : Expression, optional
+            Filter expression to apply to the scan. If None, no filter is applied.
+        batch_size : int, optional
+            The number of rows to include in each batch. If None, the default
+            value is used. The default value is implementation specific.
+        use_threads : bool, default True
+            Whether to use multiple threads to read the rows. It is expected
+            that consumers reading a whole dataset in one scanner will keep this
+            as True, while consumers reading a single fragment per worker will
+            typically set this to False.
+
+        Notes
+        -----
+        The filters must be fully satisfied. If the dataset cannot satisfy the
+        filter, it should raise an error.
+
+        Only the following expressions are allowed in the filter:
+        - Equality / inequalities (==, !=, <, >, <=, >=)
+        - Conjunctions (and, or)
+        - Field references (e.g. "a" or "a.b.c")
+        - Literals (e.g. 1, 1.0, "a", True)
+        - cast
+        - is_null / not_null
+        - isin
+        - between
+        - negation (not)
+

Review Comment:
   ```suggestion
   - is_nan / not_nan
   ```



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