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# Astropy Glossary

(`n`,)

  A parenthesized number followed by a comma denotes a tuple with one element. The trailing comma distinguishes a one-element tuple from a parenthesized `n`. This is from NumPy; see https://numpy.org/doc/stable/glossary.html#term-n.

-like

  `<Class>-like` is an instance of the `Class` or a valid initializer argument for `Class` as `Class(value)`. E.g. `Quantity`-like includes `"2 * u.km"` because `astropy.units.Quantity("2 * u.km")` works.

[‘physical type’]

  The physical type of a quantity can be annotated in square brackets following a `Quantity` (or similar quantity-like).

  For example, `distance : quantity-like ['length']`

angle-like

  quantity-like and a valid initializer for `Angle`. The `unit` must be an angular. A string input is interpreted as an angle as described in the `Angle` documentation.

buffer-like

  Object that implements Python’s buffer protocol.

coordinate-like

  `BaseCoordinateFrame` subclass instance, or a `SkyCoord` (or subclass) instance, or a valid initializer as described in COORD.

file-like (readable)

  python:file-like object object that supports reading with a method `read`.

  For a formal definition see `ReadableFileLike`.

file-like (writeable)

  python:file-like object object that supports writing with a method `write`.

  For a formal definition see `WriteableFileLike`.

frame-like

  `BaseCoordinateFrame` subclass or subclass instance or a valid Frame name (string).

length-like

  quantity-like and a valid initializer for `Distance`. The `unit` must be a convertible to a unit of length.

number

  Any scalar numeric type. e.g. `float` or `int` or `numpy.number`.

quantity-like

  `Quantity` (or subclass) instance, a number or array-like object, or a string which is a valid initializer for `Quantity`.

  For a formal definition see `QuantityLike`.

table-like

  `Table` (or subclass) instance or valid initializer for `Table` as described in Constructing a Table. Common types include `dict[list]`, `list[dict]`, `list[list]`, and `ndarray` (structured array).

time-like

  `Time` (or subclass) instance or a valid initializer for `Time`, e.g. `str`, array-like[str], `datetime`, or `datetime64`.

trait type

  In short, a trait type is a class with the following properties:

  - It is a class that can be used as a mixin to add functionality to another class.

  - It should never be instantiated directly.

  - It should not be used as a base class for other classes, but only as a mixin.

  - It can define methods, properties, and attributes – any of which can be abstract.

  - It can be generic, i.e. it can have type parameters.

  - It can subclass other traits, but should have a linear MRO.

  These are the same set of properties as orthogonal mixin classes, with the added emphasis that they can serve as compiled types, if so enabled by a compilation system such as mypyc.

unit-like

  `UnitBase` subclass instance or a valid initializer for `Unit`, e.g., `str` or scalar `Quantity`.

## Optional Packages’ Glossary

color

  Any valid Matplotlib color.