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 % TODO: ask if anyone needs to acknowledge funding for development % TODO: update list of contributors \documentclass[traditabstract]{aa} \usepackage{graphicx} \usepackage{xspace} \usepackage{natbib} \usepackage{url} \usepackage[xindy, toc, hyperfirst=false, nolist, nostyles, sanitize={name=false,description=false,symbol=false}]{glossaries} \bibpunct{(}{)}{;}{a}{}{,} \newcommand{\astropy}{\texttt{astropy}\xspace} \begin{document} % Reading the glossary \input{glossary.tex} \titlerunning{Astropy} \authorrunning{The Astropy Collaboration} \title{Astropy: A Community Python Package for Astronomy} \author{ The Astropy Collaboration \and % Coordination committee Thomas P. Robitaille\inst{\ref{inst:mpia}} % confirmed \and Erik J. Tollerud\inst{\ref{inst:yale}, \ref{inst:hubble}} % confirmed \and Perry Greenfield\inst{\ref{inst:stsci}} % confirmed \and % Developers (other than coordination committee). Michael Droettboom\inst{\ref{inst:stsci}} % confirmed \and Erik Bray\inst{\ref{inst:stsci}} % confirmed \and Tom Aldcroft\inst{\ref{inst:cfa}} % confirmed \and Matt Davis\inst{\ref{inst:stsci}} % confirmed \and Adam Ginsburg\inst{\ref{inst:colorado}} % confirmed \and Adrian M. Price-Whelan\inst{\ref{inst:columbia}} % confirmed \and Wolfgang E. Kerzendorf\inst{\ref{inst:toronto}} % confirmed \and Alexander Conley\inst{\ref{inst:colorado}} % confirmed \and Neil Crighton\inst{\ref{inst:mpia}} % confirmed \and Kyle Barbary\inst{\ref{inst:argonne}} % confirmed \and Demitri Muna\inst{\ref{inst:osu}} % confirmed \and Henry Ferguson\inst{\ref{inst:stsci}} % confirmed \and Fr\'ed\'eric Grollier\inst{\ref{inst:freelance}} % confirmed \and Madhura M. Parikh\inst{\ref{inst:surat}} % confirmed \and Prasanth H. Nair\inst{\ref{inst:freelance}} % confirmed \and Hans M. G\"unther\inst{\ref{inst:cfa}} % confirmed \and Christoph Deil\inst{\ref{inst:mpik}} % confirmed \and Julien Woillez\inst{\ref{inst:eso_garching}} % confirmed \and Simon Conseil\inst{\ref{inst:oamp}} % confirmed \and Roban Kramer\inst{\ref{inst:eth}} % confirmed \and James E. H. Turner\inst{\ref{inst:gemini_s}} % confirmed \and Leo Singer\inst{\ref{inst:ligo}} % confirmed \and Ryan Fox\inst{\ref{inst:freelance}} % confirmed \and Benjamin A. Weaver\inst{\ref{inst:nyu}} % confirmed \and Victor Zabalza\inst{\ref{inst:mpik}} % confirmed \and Zachary I. Edwards\inst{\ref{inst:louisiana}} % confirmed \and % Other contributors, alphabetical, including coordination meeting % participants K. Azalee Bostroem\inst{\ref{inst:stsci}} % confirmed \and D. J. Burke\inst{\ref{inst:cfa}} % confirmed \and Andrew R. Casey\inst{\ref{inst:stromlo}} % confirmed \and Steven M. Crawford\inst{\ref{inst:saao}} % confirmed \and Nadia Dencheva\inst{\ref{inst:stsci}} % confirmed \and Justin Ely\inst{\ref{inst:stsci}} % confirmed \and Tim Jenness\inst{\ref{inst:jac},\ref{inst:cornell}} % confirmed \and Kathleen Labrie\inst{\ref{inst:gemini_n}} % confirmed \and Pey Lian Lim\inst{\ref{inst:stsci}} % confirmed \and Francesco Pierfederici\inst{\ref{inst:stsci}} % confirmed \and Andrew Pontzen\inst{\ref{inst:oxford},\ref{inst:ucl}} % confirmed \and Andy Ptak\inst{\ref{inst:gsfc}} % confirmed \and Brian Refsdal\inst{\ref{inst:cfa}} % confirmed \and Mathieu Servillat\inst{\ref{inst:saclay},\ref{inst:cfa}} % confirmed \and Ole Streicher\inst{\ref{inst:leibniz}} % confirmed } \institute{ Max-Planck-Institut f\"{u}r Astronomie, K\"onigstuhl 17, Heidelberg 69117, Germany \label{inst:mpia} \and Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06510, USA \label{inst:yale} \and Hubble Fellow \label{inst:hubble} \and Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA \label{inst:stsci} \and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA \label{inst:cfa} \and Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309, USA \label{inst:colorado} \and Department of Astronomy, Columbia University, Pupin Hall, 550W 120th St., New York, NY 10027, USA \label{inst:columbia} \and Department of Astronomy and Astrophysics, University of Toronto, 50 Saint George Street, Toronto, ON M5S3H4, Canada \label{inst:toronto} \and Argonne National Laboratory, High Energy Physics Division, 9700 South Cass Avenue, Argonne, IL 60439, USA \label{inst:argonne} \and Department of Astronomy, Ohio State University, Columbus, OH 43210, USA \label{inst:osu} \and S.V.National Institute of Technology, Surat., India \label{inst:surat} \and Independent developer \label{inst:freelance} \and Max-Planck-Institut f\"{u}r Kernphysik, P.O. Box 103980, 69029 Heidelberg, Germany \label{inst:mpik} \and European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching bei M\"{u}nchen, Germany \label{inst:eso_garching} \and Laboratoire d'Astrophysique de Marseille, OAMP, Universit\'e Aix-Marseille et CNRS, Marseille, France \label{inst:oamp} \and ETH Z\"{u}rich, Institute for Astronomy, Wolfgang-Pauli-Strasse 27, Building HIT, Floor J, CH-8093 Zurich, Switzerland \label{inst:eth} \and Gemini Observatory, Casilla 603, La Serena, Chile \label{inst:gemini_s} \and LIGO Laboratory, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125, USA \label{inst:ligo} \and Center for Cosmology and Particle Physics, New York University, New York, NY 10003, USA \label{inst:nyu} \and Department of Physics and Astronomy, Louisiana State University, Nicholson Hall, Baton Rouge, LA 70803, USA \label{inst:louisiana} \and Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, via Cotter Road, Weston Creek ACT 2611, Australia \label{inst:stromlo} \and SAAO, P.O. Box 9, Observatory 7935, Cape Town, South Africa \label{inst:saao} \and Joint Astronomy Centre, 660 N.\ Aoh\=ok\=u Place, Hilo, HI 96720, USA \label{inst:jac} \and Department of Astronomy, Cornell University, Ithaca, NY 14853, USA \label{inst:cornell} \and Gemini Observatory, 670 N.\ Aoh\=ok\=u Place, Hilo, Hawaii 96720, USA \label{inst:gemini_n} \and Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK \label{inst:oxford} \and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK \label{inst:ucl} \and NASA Goddard Space Flight Center, X-ray Astrophysics Lab Code 662, Greenbelt, MD 20771, USA \label{inst:gsfc} \and Laboratoire AIM, CEA Saclay, Bat. 709, 91191 Gif-sur-Yvette, France \label{inst:saclay} \and Leibniz-Institut f\"{u}r Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany \label{inst:leibniz} } \abstract{ We present the first public version (v0.2) of the open-source and community-developed Python package, Astropy. This package provides core astronomy-related functionality to the community, including support for domain-specific file formats such as Flexible Image Transport System (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity conversions, physical constants specific to astronomy, celestial coordinate and time transformations, world coordinate system (WCS) support, generalized containers for representing gridded as well as tabular data, and a framework for cosmological transformations and conversions. Significant functionality is under active development, such as a model fitting framework, VO client and server tools, and aperture and point spread function (PSF) photometry tools. The core development team is actively making additions and enhancements to the current code base, and we encourage anyone interested to participate in the development of future Astropy versions. } \keywords{Methods: data analysis -- Methods: miscellaneous -- Virtual Observatory Tools} \maketitle \section{Introduction} The Python programming language\footnote{\url{http://www.python.org}} has become one of the fastest-growing programming languages in the astronomy community in the last decade (see e.g. \citealt{greenfield11} for a recent review). While there have been a number of efforts to develop Python packages for astronomy-specific functionality, these efforts have been fragmented, and several dozens of packages have been developed across the community with little or no coordination. This has led to duplication and a lack of homogeneity across packages, making it difficult for users to install all the required packages needed in an astronomer's toolkit. Because a number of these packages depend on individual or small groups of developers, packages are sometimes no longer maintained, or simply become unavailable, which is detrimental to long-term research and reproducibility. Motivated by these issues, the Astropy project was started in 2011 out of a desire to bring together developers across the field of astronomy in order to coordinate the development of a common set of Python tools for astronomers and simplify the landscape of available packages. The project has grown rapidly, and to date, over 200 individuals are signed up to the development mailing list for the Astropy project.\footnote{ \url{https://groups.google.com/forum/?fromgroups#!forum/astropy-dev}} One of the primary aims of the Astropy \textit{project} is to develop a core \astropy \textit{package} that covers much of the astronomy-specific functionality needed by researchers, complementing more general scientific packages such as \gls{numpy} and \gls{scipy}, which are invaluable for numerical array-based calculations and more general scientific algorithms (e.g. interpolation, integration, clustering). In addition, the Astropy project includes work on more specialized Python packages (which we call \textit{affiliated packages}) that are not included in the core package for various reasons: for some the functionality is in early stages of development and is not robust; the license is not compatible with Astropy; the package includes large files; or the functionality is mature, but too domain-specific to be included in a core package. The driving interface design philosophy behind the core package is that code using \astropy should result in concise and easily readable code, even by those new to Python. Typical operations should appear in code similar to how they would appear if expressed in spoken or written language. Such an interface results in code that is less likely to contain errors and is easily understood, enabling astronomers to focus more of their effort on their science objectives rather than interpreting obscure function or variable names or otherwise spending time trying to understand the interface. In this paper, we present the first public release (v0.2) of the \astropy package. We provide an overview of the current capabilities (\S\ref{sec:capabilities}), our development workflow (\S\ref{sec:workflow}), and planned functionality (\S\ref{sec:future}). This paper is not intended to provide a detailed documentation for the package (which is available online\footnote{\url{http://docs.astropy.org}}), but is rather intended to give an overview of the functionality and design. \section{Capabilities} This section provides a broad overview of the capabilities of the different \astropy sub-packages, which covers units and unit conversions (\S\ref{sec:units_main}), absolute dates and times (\S\ref{sec:time}), celestial coordinates (\S\ref{sec:coordinates}), tabular and gridded data (\S\ref{sec:table}), common astronomical file formats (\S\ref{sec:io}), world coordinate system transformations (\S\ref{sec:wcs}), and cosmological utilities (\S\ref{sec:cosmology}). We have illustrated each section with simple and concise code examples, but for more details and examples, we refer the reader to the online documentation.\footnotemark[3] \label{sec:capabilities} \subsection{Units, Quantities, and Physical Constants} \label{sec:units_main} The \texttt{astropy.units} sub-package provides support for physical units. It originates from code in the \texttt{pynbody} package \citep{pynbody}, but has been significantly enhanced in behavior and implementation (with the intent that the \texttt{pynbody} will eventually become interoperable with \texttt{astropy.units}). This sub-package can be used to attach units to scalars and arrays, convert from one set of units to another, define custom units, define equivalencies for units that are not strictly the same (such as wavelength and frequency), and decompose units into base units. Unit definitions are included in both the International System of Units (SI) and the Centimeter-Gram-Second (CGS) systems, as well as a number of astronomy- and astrophysics-specific units. \subsubsection{Units} \begin{figure} \center \caption{Quantity conversion using the \texttt{astropy.units} sub-package.\label{code:quantities}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_quantities.pdf}} \end{figure} \label{sec:units} The \texttt{astropy.units} sub-package defines a \texttt{Unit} class to represent base units, which can be manipulated without attaching them to values, for example to determine the conversion factor from one set of units to another. Users can also define their own units, either as standalone base units or by composing other units together. It is also possible to decompose units into their base units, or alternatively search for higher-level units that are identical. This sub-package includes the concept of equivalencies'' in units, which is intended to be used where there exists an equation that provides a relationship between two different physical quantities. A standard astronomical example is the relationships between the frequency, wavelength and energy of a photon - it is common practice to treat such units as equivalent even though they are not strictly comparable. Such a conversion can be carried out in \texttt{astropy.units} by supplying an equivalency list (see Figure~\ref{code:quantities}). The inclusion of these equivalencies is an important improvement over existing unit-handling software, which typically does not have this functionality. Equivalencies are also included for monochromatic flux densities, which allows users to convert between $F_\nu$ and $F_\lambda$, and users can easily implement their own equivalencies. There are multiple string representations for units used in the astronomy community. The FITS Standard \citep{fits3} defines a unit standard, as well as both the Centre de Donn\'ees astronomiques de Strasbourg (CDS) \citep{ochsenbein2000cds} and NASA/Goddard's Office of Guest Investigator Programs (OGIP) \citep{george1995ogip}. In addition, the International Virtual Observatory Alliance (IVOA) has a forthcoming VOUnit standard \citep{derriere2012vounit} in an attempt to resolve some of these differences. Rather than choose one of these, \texttt{astropy.units} supports most of these standards (OGIP support is planned for the next major release of \astropy), and allows the user to select the appropriate one when reading and writing unit string definitions to and from external file formats. \subsubsection{Quantities and Physical Constants} \label{sec:quantities} While the previous section described the use of the \texttt{astropy.units} sub-package to manipulate the units themselves, a more common use-case is to attach the units to quantities, and use them together in expressions. The \texttt{astropy.units} package allows units to be attached to Python scalars, or \gls{numpy} arrays, producing \texttt{Quantity} objects. These objects support arithmetic with other numbers and \texttt{Quantity} objects while preserving their units. For multiplication and division, the resulting object will retain all units used in the expression. The final object can then be converted to a specified set of units or decomposed, effectively canceling and combining any equivalent units and returning a \texttt{Quantity} object in some set of base units. This is demonstrated in Figure~\ref{code:quantities}. Using the \texttt{.to()} method, \texttt{Quantity} objects can easily be converted to different units. The units must either be dimensionally equivalent, or users should pass equivalencies through the \texttt{equivalencies} argument (c.f. \S\ref{sec:units} or Figure~\ref{code:quantities}). Since \texttt{Quantity} objects can operate with \gls{numpy} arrays, it is very simple and efficient to convert the units on large datasets. \begin{figure} \center \caption{Using the \texttt{astropy.constants} sub-package.\label{code:constants}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_constants.pdf}} \end{figure} The \texttt{Quantity} objects are used to define a number of useful astronomical constants included in \texttt{astropy.constants}, each with an associated unit (where applicable) and additional metadata describing their provenance and uncertainties. These can be used along with \texttt{Quantity} objects to provide a convenient framework for computing any quantity in astronomy. Figure~\ref{code:constants} includes a simple example that shows how the gravitational force between two bodies can be calculated in Newtons using physical constants and user-specified quantities. \subsection{Time} \label{sec:time} The \texttt{astropy.time} package provides functionality for manipulating times and dates. Specific emphasis is placed on supporting time scales (e.g. UTC, TAI, UT1) and time formats or representations (e.g. JD, MJD, ISO 8601) that are used in astronomy \citep{guinot88,kovalevsky01,sofa_wallace}. Examples of using this sub-package are provided in Figure \ref{code:time}. The most common way to use \texttt{astropy.time} is to create a \texttt{Time} object by supplying one or more input time values as well as the time format or representation and time scale of those values. The input time(s) can either be a single scalar such as \verb|"2010-01-01 00:00:00"| or \verb|2455348.5| or a sequence of such values; the format or representation specifies how to interpret the input values, such as ISO, JD, or Unix time; and the scale specifies the time standard used for the values, such as Coordinated Universal Time (UTC), Terrestial Time (TT), or International Atomic Time (TAI). The full list of available time scales is given in Table~\ref{tab:time_systems}. Many of these formats and scales are used within astronomy, and it is especially important to treat the different time scales properly when converting between celestial coordinate systems. To facilitate this, the \texttt{Time} class makes the conversion to a different format such as Julian Date straightforward, as well as the conversion to a different time scale, for instance from UTC to TT. We note that the \texttt{Time} class includes support for leap seconds in the UTC time scale. This package is based on a derived version of the Standards of Fundamental Astronomy (SOFA) time and calendar library\footnote{\url{http://www.iausofa.org}} \citep{sofa_wallace}. Leveraging the robust and well-tested SOFA routines ensures that the fundamental time scale conversions are being computed correctly. An important feature of the SOFA time library which is supported by \texttt{astropy.time} is that each time is represented as a pair of double-precision (64-bit) floating-point values, which enables extremely high precision time computations. Using two 64-bit floating-point values allows users to represent times with a dynamic range of 30 orders of magnitude, providing for example times accurate to better than a nanosecond over timescales of tens of Gyr. All time scale conversions are done by vectorized versions of the SOFA routines using Cython \citep{cython}, a Python package that makes it easy to use C code in Python. \begin{table} \caption{Supported time scales for \texttt{astropy.time}\label{tab:time_systems}} \center \begin{tabular}{ll} \hline Scale & Description \\ \hline TAI & International Atomic Time \\ TCB & Barycentric Coordinate Time \\ TCG & Geocentric Coordinate Time \\ TDB & Barycentric Dynamical Time \\ TT & Terrestrial Time \\ UT1 & Universal Time \\ UTC & Coordinated Universal Time \\ \hline \end{tabular} \end{table} \begin{figure} \center \caption{Time representation and conversion using the \texttt{astropy.time} sub-package.\label{code:time}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_time.pdf}} \end{figure} \subsection{Celestial Coordinates} \label{sec:coordinates} An essential element of any astronomy workflow is the manipulation, parsing, and conversion of astronomical coordinates. This functionality is provided in Astropy by the \texttt{astropy.coordinates} sub-package. The aim of this package is to provide a common application programming interface (API) for Python astronomy packages that use coordinates, and to relieve users from having to (re)implement extremely common utilities. To achieve this, it combines API and implementation ideas from existing Python coordinates packages. Some aspects, such as coordinate transformation approaches from \texttt{kapteyn} \citep{kapteyn} and class structures resembling \texttt{astropysics} \citep{astropysics}, have already been implemented. Others, such as the frames of \texttt{palpy} \citep{palpy} and \texttt{pyast} \citep{pyast} or the ephemeris system of \texttt{pyephem} \citep{pyephem}, are still under design for \texttt{astropy}. By combining the best aspects of these other packages, as well as testing against them, \texttt{astropy.coordinates} seeks to provide a high-quality, flexible Python coordinates library. The sub-package has been designed to present a natural Python interface for representing coordinates in computations, simplify input and output formatting, and allow straightforward transformation between coordinate systems. It also supports implementation of new or custom coordinate systems that work consistently with the built-in systems. A future design goal is also to seamlessly support arbitrarily large data sets. To that end, Figure \ref{fig:code_coordinates} shows some typical usage examples for \texttt{astropy.coordinates}. Coordinate objects are created using standard Python object instantiation via a Python class named after the coordinate system (e.g., \texttt{ICRSCoordinates}). Astronomical coordinates may be expressed in a myriad of ways: the classes support string, numeric, and tuple value specification through a sophisticated input parser. A design goal of the input parser is to be able to determine the angle value and unit from the input alone if a person can unambiguously determine them. For example, an astronomer seeing the input string 12h53m11.5123s'' would understand the units to be in hours, minutes, and seconds, so this value is alone sufficient to pass to the angle initializer. This functionality is built around the \texttt{Angle} object, which can be instantiated and used on its own. It provides additional functionality such as string formatting and mechanisms to specify the valid bounds of an angle. As a convenience, it is also possible to query the online SIMBAD\footnote{\url{http://simbad.u-strasbg.fr}} database to resolve the name of a source (see Figure~\ref{fig:code_coordinates} for an example showing how to find the ICRS coordinates of M32). \begin{figure} \center \caption{Celestial coordinate representation and conversion.\label{code:coords}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_coordinates.pdf}} \label{fig:code_coordinates} \end{figure} The coordinate classes represent different coordinate systems, and provide most of the user-facing functionality for \texttt{astropy.coordinates}. The systems provide customized initializers and appropriate formatting and representation defaults. For some classes, they also contain added functionality specific to a subset of systems, such as code to precess a coordinate to a new equinox. The implemented systems include a variety of equatorial coordinate systems (ICRS, FK4, and FK5), Galactic coordinates, and horizontal (Alt/Az) coordinates, and modern (IAU 2006/200A) precession/nutation models for the relevant systems. Coordinate objects can easily be transformed from one coordinate system to another: Figure \ref{fig:code_coordinates} illustrates the most basic use of this functionality to convert a position on the sky from ICRS to Galactic coordinates. Transformations are provided between all coordinate systems built into version v0.2 of Astropy, with the exception of conversions from celestial to horizontal coordinates. Future versions of Astropy will include additional common systems, including ecliptic systems, supergalactic coordinates, and all necessary intermediate coordinate systems for the IAU 2000/2006 equatorial-to-horizontal mapping \citep[e.g.,][]{soffel03, usnocircular179}. A final significant feature of \texttt{astropy.coordinates} is support for line-of-sight distances. While the term celestial coordinates'' can be taken to refer to only on-sky angles, in \texttt{astropy.coordinates} a coordinate object is conceptually treated as a point in three dimensional space. Users have the option of specifying a line of sight distance to the object from the origin of the coordinate system (typically the origin is the Earth or solar system barycenter). These distances can be given in physical units or as redshifts. The \texttt{astropy.coordinates} sub-package will in the latter case transparently make use of the cosmological calculations in \texttt{astropy.cosmology} (c.f. \S\ref{sec:cosmology}) for conversion to physical distances. Figure \ref{fig:code_coordinates} illustrates an application of this information in the form of computing three-dimensional distances between two objects. The \texttt{astropy.coordinates} sub-package was designed such that it should be easy for a user to add new coordinate systems. This flexibility is achieved in \texttt{astropy.coordinates} through the internal use of a transformation graph, which keeps track of a network of coordinate systems and the transformations between them. When a coordinate object is to be transformed from one system into another, the package determines the shortest path on the transformation graph to the new system and applies the necessary sequence of transformations. Thus, implementing a new coordinate system simply requires implementing one pair of transformations to and from a system that is already connected to the transformation graph. Once this pair is specified, \texttt{astropy.coordinates} can transform from that coordinate system to any other in the graph. An example of a user-defined system is provided in the documentation,\footnote{\url{http://docs.astropy.org/en/v0.2/coordinates/sgr-example.html}} illustrating the definition of a coordinate system useful for a specific scientific task (Price-Whelan \& Johnston 2013, in prep). \subsection{Tables and Gridded data} \label{sec:table} \begin{figure} \center \caption{Table input/output and manipulation using the \texttt{astropy.table} sub-package.\label{code:tables}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_tables.pdf}} \end{figure} Tables and $n$-dimensional data arrays are the most common forms of data encountered in astronomy. The \gls{python} community has various solutions for tables, such as \gls{numpy} structured arrays or \texttt{DataFrame} objects in Pandas \citep{pandas} to name only a couple. For n-dimensional data the \gls{numpy} \texttt{ndarray} is the most popular. However, for use in astronomy all of these implementations lack some key features. The data that is stored in arrays and tables often contains vital metadata: the data is associated with units, and might also contain additional arrays that either mask or provide additional attributes to each cell. Furthermore, the data often includes a set of keyword-value pairs and comments (such as FITS headers). Finally, the data comes in a plethora of astronomy specific formats (FITS, specially formatted ASCII tables, etc.), which are not recognized by the pre-existing packages. The \texttt{astropy.table} and \texttt{astropy.nddata} sub-packages contain classes (\texttt{Table} and \texttt{NDData}) that try to alleviate these problems. They allow users to represent astronomical data in the form of tables or n-dimensional gridded datasets, including all metadata. Examples of usage of \texttt{astropy.table} are shown in Figure~\ref{code:tables}. The \texttt{Table} class provides a high-level wrapper to \gls{numpy} structured arrays, which are essentially arrays that have fields (or columns) with heterogeneous data types, and any number of rows. \gls{numpy} structured arrays are however difficult to modify, so the \texttt{Table} class makes it easy for users to create a table from columns, add and remove columns or rows, and mask values from the table. Furthermore, tables can be easily read from and written to common file formats using the \texttt{Table.read} and \texttt{Table.write} methods. These methods are connected to sub-packages in \texttt{astropy.io} such as \texttt{astropy.io.ascii} (\S\ref{sec:ascii}) and \texttt{astropy.io.votable} (\S\ref{sec:votable}), which allow ASCII and VO tables to be seamlessly read or written respectively. In addition to providing easy manipulation and input or output of table objects, the \texttt{Table} class allows units to be specified for each column using the \texttt{astropy.units} framework (\S\ref{sec:units_main}), and also allows the \texttt{Table} object to contain arbitrary metadata (stored in \texttt{Table.meta}). Similarly, the \texttt{NDData} class provides a way to store $n$-dimensional array data easily and builds upon the \gls{numpy} \texttt{ndarray} class. The actual data is stored in an \texttt{ndarray}, which allows for easy compatibility with other scientific packages. In addition to keyword-value metadata, the \texttt{NDData} class can store a boolean mask with the same dimensions as the data, several sets of flags ($n$-dimensional arrays that store attributes for each cell of the data array), uncertainties, units, and a transformation between array-index coordinate system and other coordinate systems (c.f. \S\ref{sec:wcs}). In addition, the \texttt{NDData} class intends to provide methods to arithmetically combine the data in a meaningful way. \texttt{NDData} is not meant for direct user interaction but more for providing a framework for higher-level subclasses that can represent for example spectra or astronomical images. \subsection{File Formats} \label{sec:io} \subsubsection{FITS} \label{sec:fits} \begin{figure} \center \caption{Accessing data in FITS format.\label{code:fits}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_fits.pdf}} \end{figure} Support for reading and writing FITS files is provided by the \texttt{astropy.io.fits} sub-package, which at the time of writing is a direct port of the PyFITS\footnote{\url{http://www.stsci.edu/institute/software_hardware/}} project \citep{barrett1999pyfits}. Users already familiar with PyFITS will therefore feel at home with this package. The \texttt{astropy.io.fits} sub-package implements all features from the FITS standard \citep{fits3} such as images, binary tables, and ASCII tables, and includes common compression algorithms. Header-data units (HDUs) are represented by Python classes, with the data itself stored using NumPy arrays, and with the headers stored using a \texttt{Header} class. Files can easily be read and written, and once in memory can be easily modified. This includes support for transparently reading from and writing to gzip-compressed FITS files. Figure~\ref{code:fits} shows a simple example of how to open an existing FITS file, access and modify the header and data, and write a new file back to disk. Creating new FITS files is also made simple. Since the code in this sub-package has been developed over more than a decade, it has been made to work with an extensive variety of FITS files, including ones that deviate from the FITS standard. This includes support for deprecated formats such as \texttt{GROUPS} HDUs as well as more obscure non-standard HDU types such as \texttt{FITS} HDUs which allow encapsulating multiple FITS files within FITS files. Support is also included for common but non-standard header conventions such as \texttt{CONTINUE} cards and ESO \texttt{HIERARCH} cards. Two command-line utilities for working with FITS files are packaged with Astropy: \texttt{fitscheck} can be used to validate FITS files against the standard. \texttt{fitsdiff} can be used to compare two FITS files on a number of criteria, and also includes a powerful API for programmatically comparing FITS files. Because the interface is exactly the same as that of PyFITS, users may directly replace PyFITS with Astropy in existing code by changing import statements such as \texttt{import pyfits} to \texttt{from astropy.io import fits as pyfits} without any additional code changes. Although PyFITS will continue to be released as a separate package in the near term, the long term plan is to discontinue PyFITS releases in favor of Astropy. It is expected that direct support of PyFITS will end mid-2014, so users of PyFITS should plan to make suitable changes to support the eventual transition to Astropy. Becoming integrated with Astropy as the \texttt{astropy.io.fits} sub-package will greatly enhance future development on the existing PyFITS code base in several areas. First and perhaps foremost is integration with Astropy's \texttt{Table} interface (\S\ref{sec:table}) which is much more flexible and powerful than PyFITS' current table interface. We will also be able to integrate Astropy's unit support (\S\ref{sec:units_main}) in order to attach units to FITS table columns as well as header values that specify units in their comments in accordance with the FITS standard. Finally, as the PyWCS package has also been integrated into Astropy as \texttt{astropy.wcs} (\S\ref{sec:wcs}) tighter association between data from FITS files and their world coordinate system (WCS) will be possible. \subsubsection{ASCII table formats} \label{sec:ascii} The \texttt{astropy.io.ascii} sub-package (formerly the standalone project \texttt{asciitable}\footnote{\url{https://asciitable.readthedocs.org}}) provides the ability to read and write tabular data for a wide variety of ASCII-based formats. In addition to generic formats such as space-delimited, tab-delimited or comma-separated values, \texttt{astropy.io.ascii} provides classes for specialized table formats such as CDS,\footnote{\url{http://vizier.u-strasbg.fr/doc/catstd.htx}} IPAC,\footnote{\url{http://irsa.ipac.caltech.edu/applications/DDGEN/}} IRAF DAOphot \citep{daophot}, and LaTeX. Also included is a flexible class for handling a wide variety of fixed-width table formats. Finally, this sub-package is designed to be extensible, making it easy for users to define their own readers and writers for any other ASCII formats. \subsubsection{Virtual Observatory tables} \label{sec:votable} The \texttt{astropy.io.votable} sub-package (formerly the standalone project \texttt{vo.table}) provides full support for reading and writing VOTable format files versions 1.1, 1.2, and the proposed 1.3 \citep{ochsenbein2004votable,ochsenbein2009votable}. It efficiently stores the tables in memory as \gls{numpy} structured arrays. The file is read using streaming to avoid reading in the entire file at once and greatly reducing the memory footprint. VOTable files compressed using the gzip and bzip2 algorithms are supported transparently, as are VOTable files where the table data is stored in an external FITS file. It is possible to convert any one of the tables in a VOTable file to a \texttt{Table} object (\S\ref{sec:table}), where it can be edited and then written back to a VOTable file without any loss of data. The VOTable standard is not strictly adhered to by all VOTable file writers in the wild. Therefore, \texttt{astropy.io.votable} provides a number of tricks and workarounds to support as many VOTable sources as possible, whenever the result would not be ambiguous. A validation tool (\texttt{volint}) is also provided that outputs recommendations to improve the standard compliance of a given file, as well as validate it against the official VOTable schema. \subsection{World Coordinate Systems} \label{sec:wcs} The \texttt{astropy.wcs} sub-package contains utilities for managing World Coordinate System (WCS) transformations in FITS files. These transformations map the pixel locations in an image to their real-world units, such as their position on the celestial sphere. This library is specific to WCS as it relates to FITS as described in the FITS WCS papers \citep{greisen2002wcs,calabretta2002wcs,greisen2006wcs} and is distinct from a planned Astropy package that will handle WCS transformations in general, regardless of their representation. This sub-package is a wrapper around the \texttt{wcslib} library.\footnote{\url{http://www.atnf.csiro.au/people/mcalabre/WCS/}} Since all of the FITS header parsing is done using \texttt{wcslib}, it is assured the same behavior as the many other tools that use \texttt{wcslib}. On top of the basic FITS WCS support, it adds support for the Simple Imaging Polynomial (SIP) convention and table lookup distortions \citep{calabretta_sip,sip}. Each of these transformations can be used independently or together in a fixed pipeline. The \texttt{astropy.wcs} sub-package also serves as a useful FITS WCS validation tool, as it is able to report on many common mistakes or deviations from the standard in a given FITS file. As mentioned above, the long-term plan is to build a generalized'' WCS for mapping world coordinates to image coordinates (and vice versa). While only in early planning stages, such a package would aim to not be tied to the FITS representation used for the current \texttt{astropy.wcs}. Such a package would also include closer connection to other parts of Astropy, for example \texttt{astropy.coordinates} (\S\ref{sec:coordinates}). \subsection{Cosmology} \label{sec:cosmology} The \texttt{astropy.cosmology} sub-package contains classes for representing widely used cosmologies, and functions for calculating quantities that depend on a cosmological model. It also contains a framework for working with less frequently employed cosmologies that may not be flat, or have a time-varying pressure to density ratio, $w$, for dark energy. The quantities that can be calculated are generally taken from those described by \citet{Hogg99}. Some examples are the angular diameter distance, comoving distance, critical density, distance modulus, lookback time, luminosity distance, and Hubble parameter as a function of redshift. \begin{figure} \center \caption{Cosmology utilities.\label{code:cosmology}} \vspace{0.1in} \fbox{\includegraphics[scale=0.82,clip]{examples/code_cosmology.pdf}} \end{figure} The fundamental model for this sub-package is that any given cosmology is represented by a class. An instance of this class has attributes giving all the parameters required to specify the cosmology uniquely, such as the Hubble parameter, CMB temperature and the baryonic, cold dark matter, and dark energy densities at $z=0$. One can then use methods of this class to perform calculations using these parameters. Figure~\ref{code:cosmology} shows how the \texttt{FlatLambdaCDM} class can be used to create an object representing a flat $\Lambda$CDM cosmology, and how the methods of this object can be called to calculate the comoving volume, age and transverse separation at a given redshift. Further calculations can be performed using the many methods of the cosmology object as described in the Astropy documentation. For users who are more comfortable using a procedural coding style, these methods are also available as functions that take a cosmology class instance as a keyword argument. The sub-package provides several pre-defined cosmology instances corresponding to commonly used cosmological parameter sets. Currently parameters from the WMAP 5-year \citep{Komatsu09}, 7-year \citep{Komatsu11} and 9-year results \citep{Hinshaw13} are included (the \texttt{WMAP5}, \texttt{WMAP7}, and \texttt{WMAP9} classes). The parameters from the \textit{Planck} results \citep{Planck13} will be included in the next release of Astropy. There are several classes corresponding to non-flat cosmologies, and the most common dark energy models are supported: a cosmological constant, constant $w$, and $w(a) = w_0 + w_a (1-a)$ (e.g. \citealt{Linder03}, here $a$ is the scale factor). Figure~\ref{code:cosmology} gives examples showing how to use the pre-defined cosmologies, and how to define a new cosmology with a time-varying dark energy $w(a)$. Any other arbitrary cosmology can be represented by sub-classing one of the basic cosmology classes. All of the code in the sub-package is tested against the web-based cosmology calculator by \citet{Wright06} and two other widely-used calculators.\footnote{\url{http://www.kempner.net/cosmic.php}}$^,$\footnote{\url {http://www.icosmos.co.uk}} In cases when these calculators are not precise enough to enable a meaningful comparison, the code is tested against calculations performed with \textsc{Mathematica}. \section{Development Approach} \label{sec:workflow} A primary guiding philosophy of Astropy is that it is developed for and (at least in part) \emph{by} the astronomy user community. This ensures the interface is designed with the workflow of working astronomers in mind. At the same time, it aims to make use of the expertise of software developers to design code that encourages good software practices such as a consistent and clean API, thorough documentation, and integrated testing. It is also dedicated to remaining open source to enable wide adoption and render input from all users easier, and is thus released with a 3-clause BSD-style license (A license of this sort is permissive'' in that it allows usage of the code for any purposes as long as notice of the Astropy copyright and disclaimers of warranty are given). Achieving these aims requires code collaboration between over 30 geographically-distributed developers, and here we describe our development workflow with the hope that it may be replicated by other astronomy software projects that are likely to have similar needs. To enable this collaboration, we have made use of the GitHub\footnote{\url{http://www.github.com}} open source code hosting and development platform. The main repository for \astropy is stored in a git\footnote{\url{http://git-scm.com}} repository on GitHub, and any non-trivial changes are made via \textit{pull requests}, which are a mechanism for submitting code for review by other developers prior to merging into the main code base. This workflow aids in increasing the quality, documentation and testing of the code to be included in \astropy. Not all contributions are necessarily accepted - community consensus is needed for incorporating major new functionality in \astropy, and any new feature has to be justified to avoid implementing features that are only useful to a minority of users, but may cause issues in the future. At the time of writing, \astropy includes several thousand tests, which are small units of code that check that functions, methods, and classes in \astropy are behaving as expected, both in terms of scientific correctness and from a programming interface perspective. We make use of \textit{continuous integration}, which is the process of running all the tests under various configurations (such as different versions of Python or \gls{numpy}, and on different platforms) in order to ensure that the package is held to the highest standard of stability. In particular, any change made via a pull request is subject to extensive testing before being merged into the core repository. For the latter, we make use of Travis,\footnote{\url{https://travis-ci.org}} while for running more extensive tests across Linux, MacOS X, and Windows, we make use of Jenkins\footnote{\url{http://jenkins-ci.org}} (both are examples of continuous integration systems). This development workflow has worked very well so far, allowing contributions by many developers, and blurring the line between developers and users. Indeed, users who encounter bugs and who know how to fix them can submit suggested changes. We have also implemented a feature that means that anyone reading the documentation at \url{http://docs.astropy.org} can suggest improvements to the documentation with just a few clicks in a web browser without any prior knowledge of the git version control system. \section{Planned functionality} \label{sec:future} Development on the Astropy package is very active, and in addition to some of the incremental improvements to existing sub-packages described in the text, we are focusing on implementing major new functionality in several areas for the next (v0.3) release (some of which have already been implemented in the publicly-available developer version): \begin{itemize} \item Improving interoperability between packages, which includes for example seamlessly integrating the \texttt{astropy.units} framework across all sub-packages \item Adding support for NumPy arrays in the coordinates sub-package, which will allow the efficient representation and conversions of coordinates in large datasets \item Supporting more file formats for reading and writing \texttt{Table} and \texttt{NDData} objects \item Implementing a Virtual Observatory cone search tool \citep{vo-cone-search} \item Implementing a generalized model-fitting framework \item Implementing statistical functions commonly used in Astronomy \end{itemize} In the longer term, we are already planning the following major functionality: \begin{itemize} \item Image analysis tools, including aperture and point spread function (PSF) photometry \item Spectroscopic analysis tools \item Generalized WCS transformations beyond the FITS WCS standard \item A SAMP server/client (ported from the SAMPy\footnote{\url{http://pythonhosted.org/sampy}} package) \item Support for the Simple Image Access Protocol (SIAP) \citep{vo-siap} \item Support for the Table Access Protocol (TAP) \citep{vo-tap} is under consideration \end{itemize} \noindent and undoubtedly the core functionality will grow beyond this. In fact, the \astropy package will likely remain a continuously-evolving package, and will thus never be considered `complete' in the traditional sense. \section{Summary} \label{sec:summary} We have presented the first public release of the Astropy package (v0.2), a core Python package for astronomers. In this paper we have described the main functionality in this release, which includes: \begin{itemize} \item Units and unit conversions (\S\ref{sec:units_main}) \item Absolute dates and times (\S\ref{sec:time}) \item Celestial coordinate systems (\S\ref{sec:coordinates}) \item Tabular and gridded data (\S\ref{sec:table}) \item Support for common astronomical file formats (\S\ref{sec:io}) \item World Coordinate System transformations (\S\ref{sec:wcs}) \item Cosmological calculations (\S\ref{sec:cosmology}). \end{itemize} We also briefly described our development approach (\S\ref{sec:workflow}), which has enabled an international collaboration of scientists and software developers to create and contribute to the package. We outlined our plans for the future (\S\ref{sec:future}) which includes more interoperability of sub-packages, as well as new functionality. We invite members of the community to join the effort by adopting the Astropy package for their own projects, reporting any issues, and whenever possible, developing new functionality. \section*{Acknowledgements} \label{sec:acknowledgements} We thank the referee, Igor Chiligarian, for suggestions that helped improve this paper. We would like to thank the \gls{numpy}, \gls{scipy}, \gls{ipython} and \gls{matplotlib} communities for providing their packages which are invaluable to the development of Astropy. We thank the GitHub team for providing us with an excellent free development platform. We also are grateful to Read the Docs (\url{https://readthedocs.org/}), Shining Panda (\url{https://www.shiningpanda-ci.com/}), and Travis (\url{https://www.travis-ci.org/}) for providing free documentation hosting and testing respectively. Finally, we would like to thank all the \astropy users that have provided feedback and submitted bug reports. The contribution by T. Aldcroft was funded by NASA contract NAS8-39073. The name resolution functionality shown in Figure~\ref{fig:code_coordinates} makes use of the SIMBAD database, operated at CDS, Strasbourg, France. \bibliographystyle{apj_custom} \bibliography{apj-jour,references} \end{document}