Cosmology I/O & Convert#

Introduction#

The astropy.cosmology.io package provides a unified interface for reading, writing, and converting Cosmology objects. Most of the features of this package are accessible through the Cosmology class, which provides the methods read(), write(), to_format(), and from_format() for reading, writing, and converting Cosmology objects, respectively.

Getting Started: Reading and Writing#

The Cosmology class includes two methods, read() and write(), that make it possible to read from and write to files.

The registered read / write formats include “ascii.ecsv” and “ascii.html”, like for Table. Also, custom read / write formats may be registered into the Astropy Cosmology I/O framework. For more information on the built-in formats, see Built-in Cosmology To/From Formats, or Custom Cosmology I/O formats for information on registering custom formats.

Writing a cosmology instance requires only the file location and optionally, or if the file format cannot be inferred, a keyword argument “format”. Additional positional arguments and keyword arguments are passed to the relevant writer methods.

>>> from astropy.cosmology import Planck18
>>> Planck18.write("example_cosmology.ecsv", format="ascii.ecsv")

Reading back the cosmology is done from Cosmology.

>>> from astropy.cosmology import Cosmology
>>> cosmo = Cosmology.read("example_cosmology.ecsv", format="ascii.ecsv")
>>> cosmo == Planck18
True

More specific cosmology classes can be used, providing default values for missing information (see Cosmology I/O Details). However using the base class is safest as it provides no default information and therefore requires the file to have all necessary information to describe a cosmology.

To see a list of the available read/write file formats:

>>> from astropy.cosmology import Cosmology
>>> Cosmology.read.list_formats()
  Format   Read Write Auto-identify
---------- ---- ----- -------------
ascii.ecsv  Yes   Yes           Yes
ascii.html  Yes   Yes           Yes

>>> Cosmology.write.list_formats()
   Format   Read Write Auto-identify
----------- ---- ----- -------------
 ascii.ecsv  Yes   Yes           Yes
 ascii.html  Yes   Yes           Yes
ascii.latex   No   Yes           Yes

This list will include both built-in and registered 3rd-party formats.

Getting Started: Converting Formats#

Reading and writing Cosmology objects go through intermediate representations, often a dict or QTable instance. These intermediate representations are accessible through the methods to_format() / from_format().

to_format() / from_format() parse a Cosmology to/from another python object. This can be useful for e.g., iterating through an MCMC of cosmological parameters or printing out a cosmological model to a journal format, like latex or HTML. When 3rd party cosmology packages register with Astropy’s Cosmology I/O, to/from_format can be used to convert cosmology instances between packages!

Another pre-registered format is “table”, for converting a Cosmology to and from a QTable.

>>> from astropy.cosmology import Planck18
>>> ct = Planck18.to_format("astropy.table")
>>> ct
<QTable length=1>
  name        H0        Om0    Tcmb0    Neff      m_nu      Ob0
         km / (Mpc s)            K                 eV
  str8     float64    float64 float64 float64  float64[3] float64
-------- ------------ ------- ------- ------- ----------- -------
Planck18        67.66 0.30966  2.7255   3.046 0.0 .. 0.06 0.04897

Now this QTable can be used to load a new cosmological instance identical to the Planck 2018 cosmology from which it was created.

>>> cosmo = Cosmology.from_format(ct, format="astropy.table")
>>> print(cosmo)
FlatLambdaCDM(name="Planck18", H0=67.66 km / (Mpc s), Om0=0.30966,
              Tcmb0=2.7255 K, Neff=3.046, m_nu=[0. 0. 0.06] eV, Ob0=0.04897)

Perhaps most usefully, QTable itself has read/write methods with numerous options, e.g. FITS, that now work with Cosmology.

To see the a list of the available conversion formats:

>>> from astropy.cosmology import Cosmology
>>> Cosmology.to_format.list_formats()
      Format      Read Write Auto-identify
----------------- ---- ----- -------------
astropy.cosmology  Yes   Yes           Yes
    astropy.model  Yes   Yes           Yes
      astropy.row  Yes   Yes           Yes
    astropy.table  Yes   Yes           Yes
          mapping  Yes   Yes           Yes
             yaml  Yes   Yes            No

This list will include both built-in and registered 3rd-party formats.

Using Cosmology I/O#

More details are provided in the following pages:

Reference/API#

astropy.cosmology.connect Module#

Classes#

`CosmologyRead`(instance, cosmo_cls)	Read and parse data to a `Cosmology`.
`CosmologyWrite`(instance, cls)	Write this Cosmology object out in the specified format.
`CosmologyFromFormat`(instance, cosmo_cls)	Transform object to a `Cosmology`.
`CosmologyToFormat`(instance, cls)	Transform this Cosmology to another format.