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NDData Plan

authors: Thomas Robitaille, Perry Greenfield, Matt Craig

date-created: 2014 October 2

date-last-revised: 2014 December 17

date-accepted: 2014 December 17

type: Standard Track

status: Accepted


This APE is intended to provide a long-term plan for the astropy.nddata sub-package. The package has been the subject of continuous debate since the start of the astropy project, and has changed in scope several times, so this APE is aimed at agreeing on the scope and future of the sub-package and providing a well-structured foundation on which future development of astropy.nddata can occur.

Detailed description


At the first Astropy coordination meeting in 2011, it was decided that as well as having a generic table container, it would be useful to have a generic container for gridded data. The astropy.table package has since then seen a large amount of development, and the API has now stabilized. The added value of the table package compared to using simple Numpy structured arrays is clear - the Table class makes it very easy to do common operations on tables such as adding or removing columns or rows, and reading/writing tables to common file formats.

On the other hand, NDData development has stagnated and we have not been able to converge on a stable API. Part of this is due to the fact that there is in fact a huge variety of 'n-dimensional datasets' and that there is very little in common for example between a spectrum and an image, in terms of what can be done with them. This has prevented the NDData class from including much functionality, and in fact we have been adding functionality then removing it after we have realized that it is not general enough.

An example to illustrate this issue is that of arithmetic - what happens when we add two n-dimensional datasets? Of course, the data values can be added, but what happens to the mask, to the flags, or to the uncertainties? The truth is that there is no general recipe for dealing with this, and that it may be a mistake to try and define it in such a general way.

This APE takes the approach of re-thinking the purpose of NDData and trying to define a scope for the future.

The 'why' of NDData

First, why do we actually need a generic data container that can be sub-classed? What is the benefit of this versus simply defining separate base classes for spectra, images, and other types of data? There are several possible answers:

  1. We want to provide users with a consistent experience across data objects - that is, the user should know that meta-data is always consistently named meta, that a mask can always be accessed with mask, and that the data can be accessed with data.
  2. By providing a common base class which can define a unified I/O interface which taps into the astropy I/O registry, we can seamlessly make it that all data objects have read and write methods that behave consistently.
  3. We want functions and methods in Astropy to know if an object passed to them is an n-dimensional data object that can be expected to have specific attributes (such as wcs, mask, and so on).

In principle, none of these require a base class. We could simply agree on a standard for data objects that defines what certain attributes should be called. This is a valid solution, but at the same time, having a base class can enforce this and factor out some boilerplate code, and as described in Alternatives, the questions raised here would apply to a base Image class that was sub-classed as XRayImage, CCDImage, and so on.

Proposal for an NDDataBase abstract base class

The first proposal in this APE is to separate a definition of the NDData interface from concrete realizations of NDData-like objects by creating an abstract base class called NDDataBase.

The proposed NDDataBase simplifies the current NDData class to the extreme, such that it only defines which properties are needed for NDData.

The NDDataBase class should not define any arithmetic operations, which are impossible to generalize.

The following properties should be included in the base class:

  • data - the data itself. No restrictions are placed on the type of this data in the ABC. Subclasses could, for example, make it a plain Numpy array, masked Numpy array, an Astropy Quantity, or an h5py data buffer. Subclasses could also require, for example, that data provides a shape attribute and/or be sliceable in order to 'prove' that it is an n-dimensional data object.

  • mask - the mask of the data, following the Numpy convention of True meaning masked, and False meaning unmasked. Sub-classes could choose to connect this to data.mask. Masks do not need to be Numpy arrays, they could be for example 'lazy' masks based on functions that will be evaluated on-the-fly.

  • unit - the unit of the data values, which will be internally represented as an Astropy Unit. If present, subclasses should try to ensure numerical operations properly take into account and propagate units. Sub-classes could choose to connect this to data.unit, in which case data should be a Quantity or behave like it.

  • wcs - an object that can be used to describe the relationship between input and world coordinates. This can (but does not have to) be an Astropy WCS object. Once the generalized WCS system is in place in Astropy, we will probably require this to be such an object. Subclasses are free to be more restrictive in what they permit for the wcs object.

  • meta - a dict-like object that can be used to contain arbitrary metadata. This could be a plain Python dict, an ordered dict, a FITS Header object, and so on, provided that it offers dict-like item access and iteration.

  • uncertainty -- an object that represents the uncertainty in the data for each element on the array. This APE places no restriction on what type of uncertainty this is (e.g. variance, standard deviation, or posisson count rate), nor does it require the attribute to be set at all (other than defaulting to None). It places only one restriction on uncertainty: it must have an attribute uncertainty_type, which should be a human-readable string.

    While not a requirement, the following uncertainty_type strings are strongly recommended for common ways of specifying normal distributions:

    • "std": if uncertainty stores the standard deviation/sigma (either a single value or on a per-pixel basis).
    • "var": if uncertainty stores the variance (either a single value or on a per-pixel basis).
    • "ivar": if uncertainty stores the inverse variance (either a single value or on a per-pixel basis).

The only required attribute is data; all others default to None if not initialized or overridden in a subclass.

Proposal for the NDData class

The NDData class would be a concrete class that is far less restrictive than the current NDData while still providing some basic functionality as a container. It will place loose restrictions on what data can be set to and is described further in Implementation.

The NDData class would not include methods such as __array__, __array_prepare__, and so on which allow a class to be treated as a Numpy array. This behavior has been identified as being potentially ambiguous in the general case because it will depend on the details of e.g. how masks are handled and also does not make it explicit in what units the data is required.

Proposal for mixin classes to provide additional functionality

Specific functionality such as uncertainty handling and arithmetic can be developed as mix-in classes that can be used by NDData sub-classes.

Generic slicing capabilities, further described in Implementation, will be provided as a mixin class called NDSlicing

Handling of NDData in Astropy and affiliated packages

If a user has a data object such as an image, it would be nice if they can use functions directly on this image and have them return an image object. At the same time, we do not want to force people to use special data containers if they have for example a Numpy array and a WCS object. This raises the question of whether we should duplicate the API for all functions, to provide one interface for NDData subclasses, and one for separate attributes. The proposal in this APE is that functions should only define a single API that takes separate keyword arguments for e.g. data, mask, and so on, but that we then provide a way for users to be able to call these functions with NDData sub-classes (see Implementation).


NDDataBase class

NDDataBase will be implemented as an abstract base class. The only input validation it will provide is enforcing the existence of an uncertainty_type attribute if uncertainty is not None, as described above.

We will not include setters for properties except mask and uncertainty because it is ambiguous what the meaning of setting e.g. the unit or WCS after initialization means: it could either mean to change the unit or WCS, or it could mean that the user wants to convert the data to this new unit or WCS. Given this ambiguity, it is safer to not have setters for the core attributes and this is consistent with e.g. Quantity.

NDData class

NDData will be a concrete subclass of NDDataBase that provides some logic for handling the setting of data:

  • If the object passed in as data has a shape attribute, is sliceable, and has an __array__ method, so that it can be easily used as a numpy array, then will be set to that object.
  • Otherwise, NDData will attempt to create a numpy.ndarray from the input data and use that as the internal representation of the data.

I/O mixin

The read and write methods will be developed via a mixin class.

Slicing mixin

This APE suggests adding a mixin class, NDSlicing, to handle basic slicing. This could be done by simply having code similar to the following inside __getitem__:

def __getitem__(self, slice):

    new = self.__class__()

    if is not None:
        new._data =[slice]

    if self.mask is not None:
        new._mask = self.mask[slice]

    if self.wcs is not None:
        new._wcs = self.wcs[slice]

Note that this is only meant as an illustration of the idea suggested here, and the final implementation will likely differ from this - but the basic idea is that the slicing would be delegated to the member attributes. For example, the WCS class would need to define itself how it should be sliced. Some attributes (such as meta) would not necessarily need to be sliceable.

Note the effect of slicing on attributes presumably returns a similar object, e.g., for wcs, it returns a new WCS appropriate to the sliced data attribute.

Arithmetic mixin

The arithmetic methods currently in NDData will be implemented in a mixin called NDArithmetic.

Facilitating the use of NDData sub-classes

One question that has come up as part of several affiliated packages is how to deal with NDData objects in functions. For example, if we consider a downsample function that can downsample an image, should the function accept only NDData (or sub-class) objects? Should it also accept plain Numpy arrays? If so, how do we pass any additional meta-data such as WCS? Should we return a downsampled Numpy array and downsampled WCS, or a single downsampled NDData (or sub-class) instance? In this example, one option would be to provide two APIs, one for NDData and/or sub-classes and one for separate Numpy arrays and attributes, but maintaining two parallel APIs is not an ideal solution. An alternative is for each function to encode the logic of checking the input type and deciding on the output type based on the output type. However, this means repeating a lot of similar code such as:

def downsample(data, wcs=None)

    if isinstance(data, NDData):
        if wcs is not None:
            raise ValueError("wcs cannot be specified if NDData instance was passed")
        wcs = data.wcs
        data =

and this will become a lot more complex once more attributes are needed by the function.

In order to make it easier for functions to accept NDData sub-classes and return these, we can implement a decorator that will automatically split up an NDData object as needed. Let us consider the following function:

def test(data, wcs=None, unit=None, n_iterations=3):

We can provide a decorator called e.g. support_nddata:

def test(data, wcs=None, unit=None, n_iterations=3):

which makes it so that if the user passes an NDData sub-class called e.g. nd, the function would automatically be called with:

test(, wcs=nd.wcs, unit=nd.unit)

That is, the decorator looks at the signature of the function and checks if any of the arguments are also properties of the NDData object, and passes them as individual arguments.

An error could be raised if an NDData property is set but the function does not accept it - for example, if wcs is set, but the function cannot support WCS objects, an error would be raised. On the other hand, if an argument in the function does not exist in the NDData object or is not set, it is simply left to its default value. This behavior could be customizable but the details are beyond the scope of this APE document.

If the function call succeeds, then the decorator will make a new NDData object (with the correct class) and will populate the properties as needed. In order to figure out what is returned by the function, the decorator will need to accept a list which gives the name of the output values:

@support_nddata(returns=['data', 'wcs'])
def test(data, wcs=None, unit=None, n_iterations=3):

Finally, the decorator could be made to restrict input to specific NDData sub-classes (and sub-classes of those):

@support_nddata(accepts=CCDImage, returns=['data', 'wcs'])
def test(data, wcs=None, unit=None, n_iterations=3):

With this decorator, the functions could be seamlessly used either with separate arguments (e.g. Numpy array and WCS) or with subclasses of NDData such as CCDImage.

Example of subclassing from both NDData and Quantity

The Quantity class would benefit from the ability to share the same interface that NDData provides and to tap into NDData's metadata and WCS handling.

Because subclassing from numpy.ndarray involves subtleties that differ from typical subclassing in python, an example subclass called NDQuantity may be implemented as part of astropy.nddata. Should it turn out to be unreasonably difficult to do, an attempt may be made to implement a class which uses Quantity as the data store, with the ability to link NDData properties like unit to the underlying properties in Quantity. Should that also prove to be unworkable, an explanation of the issues that prevented implementation may be provided in the documentation for NDData.

Branches and pull requests

Initial decorator implementation:

Initial refactoring of NDData:

Backward compatibility

This APE will require packages such as specutils and ccdproc to completely refactor how they use the NDData class. This will also break compatibility with users currently using NDData directly, but this is assumed to be a very small fraction (if any) of users.


Eliminate NDData

One alternative is to remove the NDData class altogether and to start the base classes at the level of Spectrum or Image. In this case many of this ideas of this APE (including the attribute names, decorators, etc.) would still apply to these base classes. The benefits of having a base NDData class instead of starting at the Image and Spectrum level are that:

  • The NDData class enforces the naming of the base properties to ensure consistency across all sub-classes.
  • It allows slicing to be implemented at the core level as a mixin, whereas this would need to be repeated in each base class if we had e.g. Spectrum, Image, SpectralCube as the base classes.
  • It allows the connection to the unified I/O framework to be defined once, whereas this would also need to be repeated in each base class otherwise.

On the other hand, the downsides of having a core NDData class is that it reduces flexibility of the sub-classes - for instance Spectrum has to be implemented taking into consideration the restrictions on e.g. attribute names defined by the sub-classes. In the spectral-cube package, at the moment we do not have a data attribute because we have a custom masking framework and define attributes like unmasked_data. Of course, we should aim to make this more compliant with what is decided here, but this is just to demonstrate that this type of flexibility may be lost. However, this may be a good thing as it enforces consistency for users.

Subclass NDData from astropy.units.Quantity or numpy.ndarray

The original implementation of the NDData class behaved like a numpy ndarray; an alternative to making NDData a more generic container is to make it a full-fledged subclass of ndarray or of Quantity. The advantage of this approach is that it potentially reduces duplication of code by using the infrastructure of Quantity and/or nddata.

It has the disadvantage of reducing the flexibility of NDData and presents the challenge of handling the attributes (especially meta, mask and wcs) in a sensible way for arbitrary operations on an NDData. Even in one of the most straightforward cases, the addition of two NDData objects with metadata, it is unclear what the meta of the result should be.

There is a need for a more generic container with metadata than would be possible if subclassing from ndarray. In addition, it would be straightforward to implement a subclass of the NDData proposed in this APE that ties the unit and (when they are available in `Quantity) mask and uncertainty to those properties of the data attribute. In other words, a subclass which is essentially a Quantity with meta wrapped in the NDData interface is straightforward.

If NDData subclasses from ndarray then it will be difficult or impossible to subclass a more generic container from it, which is likely to lead, down the road, to the need for the type of generic container proposed in this APE.

Decision rationale

This APE led to lengthy discussion both on the mailing list and in the astropy-APEs Pull Request. This produced some major changes to the APE, leading to the current form. A final vote was held, with unanimous support. Hence this APE was accepted 2014 December 17.