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.. _cosmology_io_details:
*********************
Cosmology I/O Details
*********************
.. _cosmology_io_details_pickle:
Cosmology in a Pickle
=====================
For *temporary* storage an easy means to serialize and deserialize a Cosmology
object is using the :mod:`pickle` module. This is good for e.g. passing a
|Cosmology| between threads.
.. doctest-skip::
>>> import pickle
>>> from astropy.cosmology import Planck18
>>> with open("planck18.pkl", mode="wb") as file:
... pickle.dump(Planck18, file)
>>> # and to read back
>>> with open("planck18.pkl", mode="rb") as file:
... cosmo = pickle.load(file)
>>> cosmo
FlatLambdaCDM(name="Planck18", ...
However this method has all the attendant drawbacks of :mod:`pickle` — security
vulnerabilities and non-human-readable files. Pickle files just generally don't
make for good persistent storage.
Solving both these issues, ``astropy`` provides a unified interface for reading
and writing data in different formats.
.. _cosmology_io_renaming_fields:
Renaming Fields
===============
Many I/O methods in :mod:`~astropy.cosmology` support renaming fields of the
|Cosmology| class when converting to a different format. This is done by
passing a ``rename`` dictionary to the ``to_format`` method.
Similarly, when converting from a different format, a ``rename`` dictionary
can be passed to the ``from_format`` method, mapping the fields of the input
to the fields of the |Cosmology| class.
For example, to rename the ``H0`` field to ``Hubble`` when converting to a table
format::
>>> from astropy.cosmology import Cosmology, Planck18
>>> renamed_table = Planck18.to_format("astropy.table", rename={"H0": "Hubble"})
>>> renamed_table
<QTable length=1>
name Hubble 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
>>> cosmo = Cosmology.from_format(renamed_table, format="astropy.table",
... rename={"Hubble": "H0"})
>>> cosmo == Planck18
True
.. _cosmology_io_subclasses:
I/O from Subclasses
===================
When a subclass of |Cosmology| is used to read a file, the subclass will provide
a keyword argument ``cosmology=<class>`` to the registered read method. The
method uses this cosmology class, regardless of the class indicated in the
file, and sets parameters' default values from the class' signature.
.. doctest-skip::
>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM.read('<file name>')
>>> cosmo == Planck18
True
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