Extending pyarrow

Controlling conversion to pyarrow.Array with the __arrow_array__ protocol

The pyarrow.array() function has built-in support for Python sequences, numpy arrays and pandas 1D objects (Series, Index, Categorical, ..) to convert those to Arrow arrays. This can be extended for other array-like objects by implementing the __arrow_array__ method (similar to numpy’s __array__ protocol).

For example, to support conversion of your duck array class to an Arrow array, define the __arrow_array__ method to return an Arrow array:

class MyDuckArray:

    ...

    def __arrow_array__(self, type=None):
        # convert the underlying array values to a pyarrow Array
        import pyarrow
        return pyarrow.array(..., type=type)

The __arrow_array__ method takes an optional type keyword which is passed through from pyarrow.array(). The method is allowed to return either a Array or a ChunkedArray.

Defining extension types (“user-defined types”)

Arrow has the notion of extension types in the metadata specification as a possibility to extend the built-in types. This is done by annotating any of the built-in Arrow logical types (the “storage type”) with a custom type name and optional serialized representation (“ARROW:extension:name” and “ARROW:extension:metadata” keys in the Field’s custom_metadata of an IPC message). See the Extension Types section of the metadata specification for more details.

Pyarrow allows you to define such extension types from Python.

There are currently two ways:

  • Subclassing PyExtensionType: the (de)serialization is based on pickle. This is a good option for an extension type that is only used from Python.

  • Subclassing ExtensionType: this allows to give a custom Python-independent name and serialized metadata, that can potentially be recognized by other (non-Python) Arrow implementations such as PySpark.

For example, we could define a custom UUID type for 128-bit numbers which can be represented as FixedSizeBinary type with 16 bytes. Using the first approach, we create a UuidType subclass, and implement the __reduce__ method to ensure the class can be properly pickled:

class UuidType(pa.PyExtensionType):

    def __init__(self):
        pa.PyExtensionType.__init__(self, pa.binary(16))

    def __reduce__(self):
        return UuidType, ()

This can now be used to create arrays and tables holding the extension type:

>>> uuid_type = UuidType()
>>> uuid_type.extension_name
'arrow.py_extension_type'
>>> uuid_type.storage_type
FixedSizeBinaryType(fixed_size_binary[16])

>>> import uuid
>>> storage_array = pa.array([uuid.uuid4().bytes for _ in range(4)], pa.binary(16))
>>> arr = pa.ExtensionArray.from_storage(uuid_type, storage_array)
>>> arr
<pyarrow.lib.ExtensionArray object at 0x7f75c2f300a0>
[
  A6861959108644B797664AEEE686B682,
  718747F48E5F4058A7261E2B6B228BE8,
  7FE201227D624D96A5CD8639DEF2A68B,
  C6CA8C7F95744BFD9462A40B3F57A86C
]

This array can be included in RecordBatches, sent over IPC and received in another Python process. The custom UUID type will be preserved there, as long as the definition of the class is available (the type can be unpickled).

For example, creating a RecordBatch and writing it to a stream using the IPC protocol:

>>> batch = pa.RecordBatch.from_arrays([arr], ["ext"])
>>> sink = pa.BufferOutputStream()
>>> with pa.RecordBatchStreamWriter(sink, batch.schema) as writer:
...    writer.write_batch(batch)
>>> buf = sink.getvalue()

and then reading it back yields the proper type:

>>> with pa.ipc.open_stream(buf) as reader:
...    result = reader.read_all()
>>> result.column('ext').type
UuidType(extension<arrow.py_extension_type>)

We can define the same type using the other option:

class UuidType(pa.ExtensionType):

    def __init__(self):
        pa.ExtensionType.__init__(self, pa.binary(16), "my_package.uuid")

    def __arrow_ext_serialize__(self):
        # since we don't have a parameterized type, we don't need extra
        # metadata to be deserialized
        return b''

    @classmethod
    def __arrow_ext_deserialize__(self, storage_type, serialized):
        # return an instance of this subclass given the serialized
        # metadata.
        return UuidType()

This is a slightly longer implementation (you need to implement the special methods __arrow_ext_serialize__ and __arrow_ext_deserialize__), and the extension type needs to be registered to be received through IPC (using register_extension_type()), but it has now a unique name:

>>> uuid_type = UuidType()
>>> uuid_type.extension_name
'my_package.uuid'

>>> pa.register_extension_type(uuid_type)

The receiving application doesn’t need to be Python but can still recognize the extension type as a “uuid” type, if it has implemented its own extension type to receive it. If the type is not registered in the receiving application, it will fall back to the storage type.

Parameterized extension type

The above example used a fixed storage type with no further metadata. But more flexible, parameterized extension types are also possible.

The example given here implements an extension type for the pandas “period” data type, representing time spans (e.g., a frequency of a day, a month, a quarter, etc). It is stored as an int64 array which is interpreted as the number of time spans of the given frequency since 1970.

class PeriodType(pa.ExtensionType):

    def __init__(self, freq):
        # attributes need to be set first before calling
        # super init (as that calls serialize)
        self._freq = freq
        pa.ExtensionType.__init__(self, pa.int64(), 'my_package.period')

    @property
    def freq(self):
        return self._freq

    def __arrow_ext_serialize__(self):
        return "freq={}".format(self.freq).encode()

    @classmethod
    def __arrow_ext_deserialize__(cls, storage_type, serialized):
        # return an instance of this subclass given the serialized
        # metadata.
        serialized = serialized.decode()
        assert serialized.startswith("freq=")
        freq = serialized.split('=')[1]
        return PeriodType(freq)

Here, we ensure to store all information in the serialized metadata that is needed to reconstruct the instance (in the __arrow_ext_deserialize__ class method), in this case the frequency string.

Note that, once created, the data type instance is considered immutable. If, in the example above, the freq parameter would change after instantiation, the reconstruction of the type instance after IPC will be incorrect. In the example above, the freq parameter is therefore stored in a private attribute with a public read-only property to access it.

Parameterized extension types are also possible using the pickle-based type subclassing PyExtensionType. The equivalent example for the period data type from above would look like:

class PeriodType(pa.PyExtensionType):

    def __init__(self, freq):
        self._freq = freq
        pa.PyExtensionType.__init__(self, pa.int64())

    @property
    def freq(self):
        return self._freq

    def __reduce__(self):
        return PeriodType, (self.freq,)

Also the storage type does not need to be fixed but can be parameterized.

Custom extension array class

By default, all arrays with an extension type are constructed or deserialized into a built-in ExtensionArray object. Nevertheless, one could want to subclass ExtensionArray in order to add some custom logic specific to the extension type. Arrow allows to do so by adding a special method __arrow_ext_class__ to the definition of the extension type.

For instance, let us consider the example from the Numpy Quickstart of points in 3D space. We can store these as a fixed-size list, where we wish to be able to extract the data as a 2-D Numpy array (N, 3) without any copy:

class Point3DArray(pa.ExtensionArray):
    def to_numpy_array(self):
        return self.storage.flatten().to_numpy().reshape((-1, 3))


class Point3DType(pa.PyExtensionType):
    def __init__(self):
        pa.PyExtensionType.__init__(self, pa.list_(pa.float32(), 3))

    def __reduce__(self):
        return Point3DType, ()

    def __arrow_ext_class__(self):
        return Point3DArray

Arrays built using this extension type now have the expected custom array class:

>>> storage = pa.array([[1, 2, 3], [4, 5, 6]], pa.list_(pa.float32(), 3))
>>> arr = pa.ExtensionArray.from_storage(Point3DType(), storage)
>>> arr
<__main__.Point3DArray object at 0x7f40dea80670>
[
    [
        1,
        2,
        3
    ],
    [
        4,
        5,
        6
    ]
]

The additional methods in the extension class are then available to the user:

>>> arr.to_numpy_array()
array([[1., 2., 3.],
   [4., 5., 6.]], dtype=float32)

This array can be sent over IPC, received in another Python process, and the custom extension array class will be preserved (as long as the definitions of the classes above are available).

The same __arrow_ext_class__ specialization can be used with custom types defined by subclassing ExtensionType.

Conversion to pandas

The conversion to pandas (in Table.to_pandas()) of columns with an extension type can controlled in case there is a corresponding pandas extension array for your extension type.

For this, the ExtensionType.to_pandas_dtype() method needs to be implemented, and should return a pandas.api.extensions.ExtensionDtype subclass instance.

Using the pandas period type from above as example, this would look like:

class PeriodType(pa.ExtensionType):
    ...

    def to_pandas_dtype(self):
        import pandas as pd
        return pd.PeriodDtype(freq=self.freq)

Secondly, the pandas ExtensionDtype on its turn needs to have the __from_arrow__ method implemented: a method that given a pyarrow Array or ChunkedArray of the extension type can construct the corresponding pandas ExtensionArray. This method should have the following signature:

class MyExtensionDtype(pd.api.extensions.ExtensionDtype):
    ...

    def __from_arrow__(self, array: pyarrow.Array/ChunkedArray) -> pandas.ExtensionArray:
        ...

This way, you can control the conversion of a pyarrow Array of your pyarrow extension type to a pandas ExtensionArray that can be stored in a DataFrame.

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