Cosmology I/O Details

Cosmology in a Pickle

For temporary storage an easy means to serialize and deserialize a Cosmology object is using the pickle module. This is good for e.g. passing a Cosmology between threads.

>>> 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 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.

Renaming Fields

Many I/O methods in 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

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.

>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM.read('<file name>')
>>> cosmo == Planck18
True