class astropy.coordinates.Attribute(default=None, secondary_attribute='')[source]

Bases: astropy.utils.misc.OrderedDescriptor

A non-mutable data descriptor to hold a frame attribute.

This class must be used to define frame attributes (e.g. equinox or obstime) that are included in a frame class definition.

default : object

Default value for the attribute if not provided

secondary_attribute : str

Name of a secondary instance attribute which supplies the value if default is None and no value was supplied during initialization.


The FK4 class uses the following class attributes:

class FK4(BaseCoordinateFrame):
    equinox = TimeAttribute(default=_EQUINOX_B1950)
    obstime = TimeAttribute(default=None,

This means that equinox and obstime are available to be set as keyword arguments when creating an FK4 class instance and are then accessible as instance attributes. The instance value for the attribute must be stored in '_' + <attribute_name> by the frame __init__ method.

Note in this example that equinox and obstime are time attributes and use the TimeAttributeFrame class. This subclass overrides the convert_input method to validate and convert inputs into a Time object.

Attributes Summary


Methods Summary

convert_input(value) Validate the input value and convert to expected attribute class.

Attributes Documentation

name = '<unbound>'

Methods Documentation


Validate the input value and convert to expected attribute class.

The base method here does nothing, but subclasses can implement this as needed. The method should catch any internal exceptions and raise ValueError with an informative message.

The method returns the validated input along with a boolean that indicates whether the input value was actually converted. If the input value was already the correct type then the converted return value should be False.

value : object

Input value to be converted.


The value converted to the correct type (or just value if converted is False)

converted : bool

True if the conversion was actually performed, False otherwise.


If the input is not valid for this attribute.