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# Mixin Columns

`astropy` tables support the concept of “mixin columns”, which allows integration of appropriate non-|Column| based class objects within a |Table| object. These mixin column objects are not converted in any way but are used natively.

The available built-in mixin column classes are:

- |Quantity| and subclasses

- |SkyCoord| and coordinate frame classes

- |Time| and `TimeDelta`

- `EarthLocation`

- `NdarrayMixin`

## Basic Example

As an example we can create a table and add a time column:

    >>> from astropy.table import Table
    >>> from astropy.time import Time
    >>> t = Table()
    >>> t['index'] = [1, 2]
    >>> t['time'] = Time(['2001-01-02T12:34:56', '2001-02-03T00:01:02'])
    >>> print(t)
    index           time
    ----- -----------------------
        1 2001-01-02T12:34:56.000
        2 2001-02-03T00:01:02.000

The important point here is that the `time` column is a bona fide |Time| object:

    >>> t['time']
    <Time object: scale='utc' format='isot' value=['2001-01-02T12:34:56.000' '2001-02-03T00:01:02.000']>
    >>> t['time'].mjd
    array([51911.52425926, 51943.00071759])

## Quantity and QTable

The ability to natively handle |Quantity| objects within a table makes it more convenient to manipulate tabular data with units in a natural and robust way. However, this feature introduces an ambiguity because data with a unit (e.g., from a FITS binary table) can be represented as either a |Column| with a `unit` attribute or as a |Quantity| object. In order to cleanly resolve this ambiguity, `astropy` defines a minor variant of the |Table| class called |QTable|. The |QTable| class is exactly the same as |Table| except that |Quantity| is the default for any data column with a defined unit.

If you take advantage of the |Quantity| infrastructure in your analysis, then |QTable| is the preferred way to create tables with units. If instead you use table column units more as a descriptive label, then the plain |Table| class is probably the best class to use.

### Example

To illustrate these concepts we first create a standard |Table| where we supply as input a |Time| object and a |Quantity| object with units of `m / s`. In this case the quantity is converted to a |Column| (which has a `unit` attribute but does not have all of the features of a |Quantity|):

    >>> import astropy.units as u
    >>> t = Table()
    >>> t['index'] = [1, 2]
    >>> t['time'] = Time(['2001-01-02T12:34:56', '2001-02-03T00:01:02'])
    >>> t['velocity'] = [3, 4] * u.m / u.s

    >>> print(t)
    index           time          velocity
                                   m / s
    ----- ----------------------- --------
        1 2001-01-02T12:34:56.000      3.0
        2 2001-02-03T00:01:02.000      4.0

    >>> type(t['velocity'])
    <class 'astropy.table.column.Column'>

    >>> t['velocity'].unit
    Unit("m / s")

    >>> (t['velocity'] ** 2).unit  # WRONG because Column is not smart about unit
    Unit("m / s")

So instead let’s do the same thing using a |QTable|:

    >>> from astropy.table import QTable

    >>> qt = QTable()
    >>> qt['index'] = [1, 2]
    >>> qt['time'] = Time(['2001-01-02T12:34:56', '2001-02-03T00:01:02'])
    >>> qt['velocity'] = [3, 4] * u.m / u.s

The `velocity` column is now a |Quantity| and behaves accordingly:

    >>> type(qt['velocity'])
    <class 'astropy.units.quantity.Quantity'>

    >>> qt['velocity'].unit
    Unit("m / s")

    >>> (qt['velocity'] ** 2).unit  # GOOD!
    Unit("m2 / s2")

You can conveniently convert |Table| to |QTable| and vice-versa:

    >>> qt2 = QTable(t)
    >>> type(qt2['velocity'])
    <class 'astropy.units.quantity.Quantity'>

    >>> t2 = Table(qt2)
    >>> type(t2['velocity'])
    <class 'astropy.table.column.Column'>

Note

To summarize: the only difference between `QTable` and `Table` is the behavior when adding a column that has a specified unit. With `QTable` such a column is always converted to a `Quantity` object before being added to the table. Likewise if a unit is specified for an existing unit-less `Column` in a `QTable`, then the column is converted to `Quantity`.

The converse is that if you add a `Quantity` column to an ordinary `Table` then it gets converted to an ordinary `Column` with the corresponding `unit` attribute.

Attention

When a column of `int` `dtype` is converted to `Quantity`, its `dtype` is converted to `float`.

For example, for a quality flag column of `int`, if it is assigned with the dimensionless unit, it will still be converted to `float`. Therefore such columns typically should not be assigned with any unit.

## Mixin Attributes

The usual column attributes `name`, `dtype`, `unit`, `format`, `meta`, and `description` are available in any mixin column via the `info` property:

    >>> qt['velocity'].info.name
    'velocity'

This `info` property is a key bit of glue that allows a non-|Column| object to behave much like a |Column|.

The same `info` property is also available in standard `Column` objects. These `info` attributes like `t['a'].info.name` refer to the direct `Column` attribute (e.g., `t['a'].name`) and can be used interchangeably. Likewise in a `Quantity` object, `info.dtype` attribute refers to the native `dtype` attribute of the object.

Note

When writing generalized code that handles column objects which might be mixin columns, you must always use the `info` property to access column attributes.

## Details and Caveats

Most common table operations behave as expected when mixin columns are part of the table. However, there are limitations in the current implementation.

Adding or inserting a row

Adding or inserting a row works as expected only for mixin classes that are mutable (data can be changed internally) and that have an `insert()` method. Adding rows to a |Table| with |Quantity|, |Time| or |SkyCoord| columns does work.

Masking

Masking of mixin columns is enabled by the |Masked| class. See Masked Values (astropy.utils.masked) for details.

Text table writing

Tables with mixin columns can be written out to file using the `astropy.io.ascii` module, but the fast C-based writers are not available. Instead, the pure-Python writers will be used. For writing tables with mixin columns it is recommended to use the ECSV Format. This will fully serialize the table data and metadata, allowing full “round-trip” of the table when it is read back.

Binary table writing

Tables with mixin columns can be written to binary files using FITS, HDF5 and Parquet formats. These can be read back to recover exactly the original |Table| including mixin columns and metadata. See High-level Unified File I/O for details.

## Mixin Protocol

A key idea behind mixin columns is that any class which satisfies a specified protocol can be used. That means many user-defined class objects which handle array-like data can be used natively within a |Table|. The protocol is relatively concise and requires that a class behave like a minimal `numpy` array with the following properties:

- Contains array-like data.

- Implements `__getitem__()` to support getting data as a single item, slicing, or index array access.

- Has a `shape` attribute.

- Has a `__len__()` method for length.

- Has an `info` class descriptor which is an instance of a subclass of the `astropy.utils.data_info.MixinInfo` class.

The Example: ArrayWrapper section shows a minimal working example of a class which can be used as a mixin column. A `pandas.Series` object can function as a mixin column as well.

Other interesting possibilities for mixin columns include:

- Columns which are dynamically computed as a function of other columns (AKA spreadsheet).

- Columns which are themselves a |Table| (i.e., nested tables). A proof of concept is available.

### new_like() method

In order to support high-level operations like `join()` and `vstack()`, a mixin class must provide a `new_like()` method in the `info` class descriptor. A key part of the functionality is to ensure that the input column metadata are merged appropriately and that the columns have consistent properties such as the shape.

A mixin class that provides `new_like()` must also implement `__setitem__()` to support setting via a single item, slicing, or index array.

The `new_like()` method has the following signature:

    def new_like(self, cols, length, metadata_conflicts='warn', name=None):
        """
        Return a new instance of this class which is consistent with the
        input ``cols`` and has ``length`` rows.

        This is intended for creating an empty column object whose elements can
        be set in-place for table operations like join or vstack.

        Parameters
        ----------
        cols : list
            List of input columns
        length : int
            Length of the output column object
        metadata_conflicts : {'warn', 'error', 'silent'}
            How to handle metadata conflicts
        name : str
            Output column name

        Returns
        -------
        col : object
            New instance of this class consistent with ``cols``
        """

Examples of this are found in the `ColumnInfo` and `QuantityInfo` classes.

## Example: ArrayWrapper

The code listing below shows an example of a data container class which acts as a mixin column class. This class is a wrapper around a |ndarray|. It is used in the `astropy` mixin test suite and is fully compliant as a mixin column.

    from astropy.utils.data_info import ParentDtypeInfo

    class ArrayWrapper:
        """
        Minimal mixin using a simple wrapper around a numpy array
        """
        info = ParentDtypeInfo()

        def __init__(self, data):
            self.data = np.array(data)
            if 'info' in getattr(data, '__dict__', ()):
                self.info = data.info

        def __getitem__(self, item):
            if isinstance(item, (int, np.integer)):
                out = self.data[item]
            else:
                out = self.__class__(self.data[item])
                if 'info' in self.__dict__:
                    out.info = self.info
            return out

        def __setitem__(self, item, value):
            self.data[item] = value

        def __len__(self):
            return len(self.data)

        @property
        def dtype(self):
            return self.data.dtype

        @property
        def shape(self):
            return self.data.shape

        def __repr__(self):
            return f"<{self.__class__.__name__} name='{self.info.name}' data={self.data}>"

## Registering array-like objects as mixin columns

In some cases, you may want to directly add an array-like object as a table column while maintaining the original object properties (instead of the default conversion of the object to a `Column`). This is done by registering the object class as a mixin column and defining a handler which allows `Table` to treat that object class as a mixin similar to the built-in mixin columns such as `Time` or `Quantity`.

This can be done for data classes that are defined in third-party packages and which you have no control over. As an example, we define a class that is not numpy-like and stores the data in a private attribute:

    >>> import numpy as np
    >>> class ExampleDataClass:
    ...     def __init__(self):
    ...         self._data = np.array([0, 1, 3, 4], dtype=float)

By default, the outcome of setting a column using this class is not helpful:

    >>> t = Table([np.arange(4)], names=["index"])
    >>> t['data'] = ExampleDataClass()
    >>> t
    <Table length=4>
    index                         data
    int64                        object
    ----- -------------------------------------...-
        0 <__main__.ExampleDataClass object at ...>
        1 <__main__.ExampleDataClass object at ...>
        2 <__main__.ExampleDataClass object at ...>
        3 <__main__.ExampleDataClass object at ...>

What happened is that the instance is seen as a scalar object, and a |Column| with `dtype=object` is created, which has the same entry for each row. The same would happen if, e.g., you set `t['data'] = None`.

However, you can create a function (or ‘handler’) which takes an instance of the data class you want to have automatically handled and turns it into a mixin column:

    >>> from astropy.table.table_helpers import ArrayWrapper
    >>> def handle_example_data_class(obj):
    ...     return ArrayWrapper(obj._data)

You can then register this by providing the fully qualified name of the class and the handler function:

    >>> from astropy.table.mixins.registry import register_mixin_handler
    >>> register_mixin_handler('__main__.ExampleDataClass', handle_example_data_class)
    >>> t['data'] = ExampleDataClass()
    >>> t
    <Table length=4>
    index   data
    int64 float64
    ----- -------
        0     0.0
        1     1.0
        2     3.0
        3     4.0

Because we defined the data class as part of the example above, the fully qualified name starts with `__main__`, but for a class in a third-party package, this might look like `package.Class` for example.