westonpace commented on a change in pull request #12560:
URL: https://github.com/apache/arrow/pull/12560#discussion_r839137322
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File path: python/pyarrow/_dataset.pyx
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@@ -240,14 +240,22 @@ cdef class Dataset(_Weakrefable):
columns : list of str, default None
The columns to project. This can be a list of column names to
include (order and duplicates will be preserved), or a dictionary
- with {new_column_name: expression} values for more advanced
+ with {{new_column_name: expression}} values for more advanced
projections.
+
+ The list of columns or expressions may use the special fields
+ like `__batch_index` (the index of the batch within the
fragment),
+ `__fragment_index` (the index of the fragment within the dataset),
+ `__last_in_fragment` (whether the batch is last in fragment) and
+ `__filename` (the name of the source file or a description of the
+ source fragment).
+
The columns will be passed down to Datasets and corresponding data
fragments to avoid loading, copying, and deserializing columns
that will not be required further down the compute chain.
- By default all of the available columns are projected. Raises
- an exception if any of the referenced column names does not exist
- in the dataset's Schema.
+ By default all of the available columns are projected.
+ An error will be returned if any of the referenced column name
+ does not exist in the dataset's schema.
Review comment:
Yikes, yes this was totally my fault. My original suggestion was bad.
Thank you for switching it back.
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