feat(python): add pyarrow to delta compatible schema conversion in wr…

…iter/merge (#1820)

# Description
This ports some functionality that @stinodego and I had worked on in
Polars. Where we converted a pyarrow schema to a compatible delta
schema. It converts the following:

- uint -> int
- timestamp(any timeunit) -> timestamp(us) 

I adjusted the functionality to do schema conversion from large to
normal when necessary, which is still needed in MERGE as workaround
#1753.

Additional things I've added:

- Schema conversion for every input in write_deltalake/merge
- Add Pandas dataframe conversion
- Add Pandas dataframe as input in merge


# Related Issue(s)
- closes #686
- closes #1467

---------

Co-authored-by: Will Jones <willjones127@gmail.com>

python/deltalake/schema.py

@@ -1,9 +1,7 @@
-from typing import TYPE_CHECKING, Tuple, Union
+from typing import Generator, Union
 
 import pyarrow as pa
-
-if TYPE_CHECKING:
-    import pandas as pd
+import pyarrow.dataset as ds
 
 from ._internal import ArrayType as ArrayType
 from ._internal import Field as Field
@@ -17,34 +15,109 @@
 DataType = Union["PrimitiveType", "MapType", "StructType", "ArrayType"]
 
 
-def delta_arrow_schema_from_pandas(
-    data: "pd.DataFrame",
-) -> Tuple[pa.Table, pa.Schema]:
-    """
-    Infers the schema for the delta table from the Pandas DataFrame.
-    Necessary because of issues such as:  https://github.com/delta-io/delta-rs/issues/686
-
-    Args:
-        data: Data to write.
+### Inspired from Pola-rs repo - licensed with MIT License, see license in python/licenses/polars_license.txt.###
+def _convert_pa_schema_to_delta(
+    schema: pa.schema, large_dtypes: bool = False
+) -> pa.schema:
+    """Convert a PyArrow schema to a schema compatible with Delta Lake. Converts unsigned to signed equivalent, and
+    converts all timestamps to `us` timestamps. With the boolean flag large_dtypes you can control if the schema
+    should keep cast normal to large types in the schema, or from large to normal.
 
-    Returns:
-        A PyArrow Table and the inferred schema for the Delta Table
+    Args
+        schema: Source schema
+        large_dtypes: If True, the pyarrow schema is casted to large_dtypes
     """
+    dtype_map = {
+        pa.uint8(): pa.int8(),
+        pa.uint16(): pa.int16(),
+        pa.uint32(): pa.int32(),
+        pa.uint64(): pa.int64(),
+    }
+    if large_dtypes:
+        dtype_map = {
+            **dtype_map,
+            **{pa.string(): pa.large_string(), pa.binary(): pa.large_binary()},
+        }
+    else:
+        dtype_map = {
+            **dtype_map,
+            **{pa.large_string(): pa.string(), pa.large_binary(): pa.binary()},
+        }
 
-    table = pa.Table.from_pandas(data)
-    schema = table.schema
-    schema_out = []
-    for field in schema:
-        if isinstance(field.type, pa.TimestampType):
-            f = pa.field(
-                name=field.name,
-                type=pa.timestamp("us"),
-                nullable=field.nullable,
-                metadata=field.metadata,
-            )
-            schema_out.append(f)
+    def dtype_to_delta_dtype(dtype: pa.DataType) -> pa.DataType:
+        # Handle nested types
+        if isinstance(dtype, (pa.LargeListType, pa.ListType)):
+            return list_to_delta_dtype(dtype)
+        elif isinstance(dtype, pa.StructType):
+            return struct_to_delta_dtype(dtype)
+        elif isinstance(dtype, pa.TimestampType):
+            return pa.timestamp(
+                "us"
+            )  # TODO(ion): propagate also timezone information during writeonce we can properly read TZ in delta schema
+        try:
+            return dtype_map[dtype]
+        except KeyError:
+            return dtype
+
+    def list_to_delta_dtype(
+        dtype: Union[pa.LargeListType, pa.ListType],
+    ) -> Union[pa.LargeListType, pa.ListType]:
+        nested_dtype = dtype.value_type
+        nested_dtype_cast = dtype_to_delta_dtype(nested_dtype)
+        if large_dtypes:
+            return pa.large_list(nested_dtype_cast)
         else:
-            schema_out.append(field)
-    schema = pa.schema(schema_out, metadata=schema.metadata)
-    table = table.cast(target_schema=schema)
-    return table, schema
+            return pa.list_(nested_dtype_cast)
+
+    def struct_to_delta_dtype(dtype: pa.StructType) -> pa.StructType:
+        fields = [dtype[i] for i in range(dtype.num_fields)]
+        fields_cast = [f.with_type(dtype_to_delta_dtype(f.type)) for f in fields]
+        return pa.struct(fields_cast)
+
+    return pa.schema([f.with_type(dtype_to_delta_dtype(f.type)) for f in schema])
+
+
+def _cast_schema_to_recordbatchreader(
+    reader: pa.RecordBatchReader, schema: pa.schema
+) -> Generator[pa.RecordBatch, None, None]:
+    """Creates recordbatch generator."""
+    for batch in reader:
+        yield pa.Table.from_batches([batch]).cast(schema).to_batches()[0]
+
+
+def convert_pyarrow_recordbatchreader(
+    data: pa.RecordBatchReader, large_dtypes: bool
+) -> pa.RecordBatchReader:
+    """Converts a PyArrow RecordBatchReader to a PyArrow RecordBatchReader with a compatible delta schema"""
+    schema = _convert_pa_schema_to_delta(data.schema, large_dtypes=large_dtypes)
+
+    data = pa.RecordBatchReader.from_batches(
+        schema,
+        _cast_schema_to_recordbatchreader(data, schema),
+    )
+    return data
+
+
+def convert_pyarrow_recordbatch(
+    data: pa.RecordBatch, large_dtypes: bool
+) -> pa.RecordBatchReader:
+    """Converts a PyArrow RecordBatch to a PyArrow RecordBatchReader with a compatible delta schema"""
+    schema = _convert_pa_schema_to_delta(data.schema, large_dtypes=large_dtypes)
+    data = pa.Table.from_batches([data]).cast(schema).to_reader()
+    return data
+
+
+def convert_pyarrow_table(data: pa.Table, large_dtypes: bool) -> pa.RecordBatchReader:
+    """Converts a PyArrow table to a PyArrow RecordBatchReader with a compatible delta schema"""
+    schema = _convert_pa_schema_to_delta(data.schema, large_dtypes=large_dtypes)
+    data = data.cast(schema).to_reader()
+    return data
+
+
+def convert_pyarrow_dataset(
+    data: ds.Dataset, large_dtypes: bool
+) -> pa.RecordBatchReader:
+    """Converts a PyArrow dataset to a PyArrow RecordBatchReader with a compatible delta schema"""
+    data = data.scanner().to_reader()
+    data = convert_pyarrow_recordbatchreader(data, large_dtypes)
+    return data

python/deltalake/table.py

@@ -20,6 +20,7 @@
 )
 
 import pyarrow
+import pyarrow.dataset as ds
 import pyarrow.fs as pa_fs
 from pyarrow.dataset import (
     Expression,
@@ -596,7 +597,13 @@ def optimize(
 
     def merge(
         self,
-        source: Union[pyarrow.Table, pyarrow.RecordBatch, pyarrow.RecordBatchReader],
+        source: Union[
+            pyarrow.Table,
+            pyarrow.RecordBatch,
+            pyarrow.RecordBatchReader,
+            ds.Dataset,
+            "pandas.DataFrame",
+        ],
         predicate: str,
         source_alias: Optional[str] = None,
         target_alias: Optional[str] = None,
@@ -619,25 +626,36 @@ def merge(
         invariants = self.schema().invariants
         checker = _DeltaDataChecker(invariants)
 
+        from .schema import (
+            convert_pyarrow_dataset,
+            convert_pyarrow_recordbatch,
+            convert_pyarrow_recordbatchreader,
+            convert_pyarrow_table,
+        )
+
         if isinstance(source, pyarrow.RecordBatchReader):
-            schema = source.schema
+            source = convert_pyarrow_recordbatchreader(source, large_dtypes=True)
         elif isinstance(source, pyarrow.RecordBatch):
-            schema = source.schema
-            source = [source]
+            source = convert_pyarrow_recordbatch(source, large_dtypes=True)
         elif isinstance(source, pyarrow.Table):
-            schema = source.schema
-            source = source.to_reader()
+            source = convert_pyarrow_table(source, large_dtypes=True)
+        elif isinstance(source, ds.Dataset):
+            source = convert_pyarrow_dataset(source, large_dtypes=True)
+        elif isinstance(source, pandas.DataFrame):
+            source = convert_pyarrow_table(
+                pyarrow.Table.from_pandas(source), large_dtypes=True
+            )
         else:
             raise TypeError(
-                f"{type(source).__name__} is not a valid input. Only PyArrow RecordBatchReader, RecordBatch or Table are valid inputs for source."
+                f"{type(source).__name__} is not a valid input. Only PyArrow RecordBatchReader, RecordBatch, Table or Pandas DataFrame are valid inputs for source."
             )
 
         def validate_batch(batch: pyarrow.RecordBatch) -> pyarrow.RecordBatch:
             checker.check_batch(batch)
             return batch
 
         source = pyarrow.RecordBatchReader.from_batches(
-            schema, (validate_batch(batch) for batch in source)
+            source.schema, (validate_batch(batch) for batch in source)
         )
 
         return TableMerger(

python/deltalake/writer.py

@@ -34,13 +34,17 @@
 import pyarrow.fs as pa_fs
 from pyarrow.lib import RecordBatchReader
 
-from deltalake.schema import delta_arrow_schema_from_pandas
-
 from ._internal import DeltaDataChecker as _DeltaDataChecker
 from ._internal import batch_distinct
 from ._internal import convert_to_deltalake as _convert_to_deltalake
 from ._internal import write_new_deltalake as _write_new_deltalake
 from .exceptions import DeltaProtocolError, TableNotFoundError
+from .schema import (
+    convert_pyarrow_dataset,
+    convert_pyarrow_recordbatch,
+    convert_pyarrow_recordbatchreader,
+    convert_pyarrow_table,
+)
 from .table import MAX_SUPPORTED_WRITER_VERSION, DeltaTable
 
 try:
@@ -159,27 +163,38 @@ def write_deltalake(
         overwrite_schema: If True, allows updating the schema of the table.
         storage_options: options passed to the native delta filesystem. Unused if 'filesystem' is defined.
         partition_filters: the partition filters that will be used for partition overwrite.
-        large_dtypes: If True, the table schema is checked against large_dtypes
+        large_dtypes: If True, the data schema is kept in large_dtypes, has no effect on pandas dataframe input
     """
-    if _has_pandas and isinstance(data, pd.DataFrame):
-        if schema is not None:
-            data = pa.Table.from_pandas(data, schema=schema)
-        else:
-            data, schema = delta_arrow_schema_from_pandas(data)
 
     table, table_uri = try_get_table_and_table_uri(table_or_uri, storage_options)
 
     # We need to write against the latest table version
     if table:
         table.update_incremental()
 
-    if schema is None:
-        if isinstance(data, RecordBatchReader):
-            schema = data.schema
-        elif isinstance(data, Iterable):
-            raise ValueError("You must provide schema if data is Iterable")
+    if isinstance(data, RecordBatchReader):
+        data = convert_pyarrow_recordbatchreader(data, large_dtypes)
+    elif isinstance(data, pa.RecordBatch):
+        data = convert_pyarrow_recordbatch(data, large_dtypes)
+    elif isinstance(data, pa.Table):
+        data = convert_pyarrow_table(data, large_dtypes)
+    elif isinstance(data, ds.Dataset):
+        data = convert_pyarrow_dataset(data, large_dtypes)
+    elif _has_pandas and isinstance(data, pd.DataFrame):
+        if schema is not None:
+            data = pa.Table.from_pandas(data, schema=schema)
         else:
-            schema = data.schema
+            data = convert_pyarrow_table(pa.Table.from_pandas(data), False)
+    elif isinstance(data, Iterable):
+        if schema is None:
+            raise ValueError("You must provide schema if data is Iterable")
+    else:
+        raise TypeError(
+            f"{type(data).__name__} is not a valid input. Only PyArrow RecordBatchReader, RecordBatch, Iterable[RecordBatch], Table, Dataset or Pandas DataFrame are valid inputs for source."
+        )
+
+    if schema is None:
+        schema = data.schema
 
     if filesystem is not None:
         raise NotImplementedError("Filesystem support is not yet implemented.  #570")
@@ -226,7 +241,7 @@ def write_deltalake(
         current_version = -1
 
     dtype_map = {
-        pa.large_string(): pa.string(),  # type: ignore
+        pa.large_string(): pa.string(),
     }
 
     def _large_to_normal_dtype(dtype: pa.DataType) -> pa.DataType:
@@ -328,19 +343,8 @@ def validate_batch(batch: pa.RecordBatch) -> pa.RecordBatch:
 
             return batch
 
-        if isinstance(data, RecordBatchReader):
-            batch_iter = data
-        elif isinstance(data, pa.RecordBatch):
-            batch_iter = [data]
-        elif isinstance(data, pa.Table):
-            batch_iter = data.to_batches()
-        elif isinstance(data, ds.Dataset):
-            batch_iter = data.to_batches()
-        else:
-            batch_iter = data
-
         data = RecordBatchReader.from_batches(
-            schema, (validate_batch(batch) for batch in batch_iter)
+            schema, (validate_batch(batch) for batch in data)
         )
 
     if file_options is not None:

python/licenses/README.md

@@ -0,0 +1,8 @@
+# Licenses
+Below are described which licenses apply to the deltalake package and to which areas of the source code.
+
+### deltalake_license.txt (APACHE 2.0 License)
+Applies to the full deltalake package source code.
+
+### polars_license.txt (MIT License)
+Applies solely to the `_convert_pa_schema_to_delta` function in `deltalake/schema.py`.

python/licenses/polars_license.txt

@@ -0,0 +1,19 @@
+Copyright (c) 2020 Ritchie Vink
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

python/pyproject.toml

@@ -6,7 +6,7 @@ build-backend = "maturin"
 name = "deltalake"
 description = "Native Delta Lake Python binding based on delta-rs with Pandas integration"
 readme = "README.md"
-license = {file = "LICENSE.txt"}
+license = {file = "licenses/deltalake_license.txt"}
 requires-python = ">=3.8"
 keywords = ["deltalake", "delta", "datalake", "pandas", "arrow"]
 classifiers = [

python/stubs/pyarrow/__init__.pyi

@@ -19,10 +19,23 @@ type_for_alias: Any
 date32: Any
 date64: Any
 decimal128: Any
+int8: Any
+int16: Any
 int32: Any
+int64: Any
+uint8: Any
+uint16: Any
+uint32: Any
+uint64: Any
 float16: Any
 float32: Any
 float64: Any
+large_string: Any
+string: Any
+large_binary: Any
+binary: Any
+large_list: Any
+LargeListType: Any
 dictionary: Any
 timestamp: Any
 TimestampType: Any