CLASS (classy)

Synopsis:Managing the CLASS cosmological code
Author:Jesus Torrado (import and get_Cl based on MontePython’s CLASS wrapper Benjamin Audren)

This module imports and manages the CLASS cosmological code.


If you use this cosmological code, please cite it as:
D. Blas, J. Lesgourgues, T. Tram, The Cosmic Linear Anisotropy Solving System (CLASS). Part II: Approximation schemes (arXiv:1104.2933)


CLASS is renamed classy for most purposes within cobaya, due to CLASS’s name being a python keyword.


If you are using a likelihood that requires some observable from CLASS, simply add classy to the theory block.

You can specify any parameter that CLASS understands in the params block:

      [any param that CLASS understands]

    [any param that CLASS understands, fixed, sampled or derived]

If you want to use your own version of CLASS, you need to specify its location with a path option inside the classy block. If you do not specify a path, CLASS will be loaded from the automatic-install modules folder, if specified, or otherwise imported as a globally-installed Python package. Cobaya will print at initialisation where it is getting CLASS from.

Modifying CLASS

If you modify CLASS and add new variables, make sure that the variables you create are exposed in the Python interface (instructions here). If you follow those instructions you do not need to make any additional modification in Cobaya.

You can use the model wrapper to test your modification by evaluating observables or getting derived quantities at known points in the parameter space (set debug: True to get more detailed information of what exactly is passed to CLASS).

In your CLASS modification, remember that you can raise a CosmoComputationError whenever the computation of any observable would fail, but you do not expect that observable to be compatible with the data (e.g. at the fringes of the parameter space). Whenever such an error is raised during sampling, the likelihood is assumed to be zero, and the run is not interrupted.



If the installation folder of CLASS is moved, due to CLASS hard-coding some folders, CLASS needs to be recompiled, either manually or by deleting the CLASS installation and repeating the cobaya-install command in the renamed modules folder.

If you do not recompile CLASS, it causes a memory leak (thanks to Stefan Heimersheim).

Automatic installation

If you do not plan to modify CLASS, the easiest way to install it is using the automatic installation script. Just make sure that theory: classy: appears in one of the files passed as arguments to the installation script.

Manual installation (or using your own version)

If you are planning to modify CLASS or use an already modified version, you should not use the automatic installation script. Use the method below instead.

CLASS’s python interface utilizes the cython compiler. If typing cython in the shell produces an error, install it with pip install cython --user.


The fast way, assuming you are installing all your cosmological codes under /path/to/cosmo/:

$ cd /path/to/cosmo/
$ git clone
$ mv class_public CLASS
$ cd CLASS
$ make

If the second line produces an error (because you don’t have git installed), try

$ cd /path/to/cosmo/
$ wget
$ unzip
$ rm
$ mv class_public-master CLASS
$ cd CLASS
$ make

If the instructions above failed, follow those in the official CLASS web page to get CLASS compiled with the Python interface ready.

classy class

class theories.classy.classy(info_theory, modules=None, timing=None)

Importing CLASS from the correct path, if given, and if not, globally.


Specifies the quantities that each likelihood needs from the Cosmology code.

Typical requisites in Cosmology (as keywords, case insensitive):

  • Cl={...}: CMB lensed power spectra, as a dictionary {spectrum:l_max}, where the possible spectra are combinations of “t”, “e”, “b” and “p” (lensing potential). Get with get_Cl().
  • [BETA: CAMB only; notation may change!] source_Cl={...}: \(C_\ell\) of given sources with given windows, e.g.: source_name: {"function": "spline"|"gaussian", [source_args]; for now, [source_args] follow the notation of CAMBSources. If can also take lmax: [int], limber: True if Limber approximation desired, and non_linear: True if non-linear contributions requested. Get with get_source_Cl().
  • Pk_interpolator={...}: Matter power spectrum interpolator in \((z, k)\). Takes "z": [list_of_evaluated_redshifts], "k_max": [k_max], "extrap_kmax": [max_k_max_extrapolated], "nonlinear": [True|False], "vars_pairs": [["delta_tot", "delta_tot"], ["Weyl", "Weyl"], [...]]}.
  • H={'z': [z_1, ...], 'units': '1/Mpc' or 'km/s/Mpc'}: Hubble rate at the redshifts requested, in the given units. Get it with get_H().
  • angular_diameter_distance={'z': [z_1, ...]}: Physical angular diameter distance to the redshifts requested. Get it with get_angular_diameter_distance().
  • comoving_radial_distance={'z': [z_1, ...]}: Comoving radial distance from us to the redshifts requested. Get it with get_comoving_radial_distance().
  • fsigma8={'z': [z_1, ...]}: Structure growth rate \(f\sigma_8\) at the redshifts requested. Get it with get_fsigma8().
  • k_max=[...]: Fixes the maximum comoving wavenumber considered.
  • Other derived parameters that are not included in the input but whose value the likelihood may need.
compute(_derived=None, cached=True, **params_values_dict)

Takes a dictionary of parameter values and computes the products needed by the likelihood. If passed a keyword derived with an empty dictionary, it populates it with the value of the derived parameters for the present set of sampled and fixed parameter values.


Interface function for likelihoods to get sampled and derived parameters.

Always use this one; don’t try to access theory code attributes directly!

get_Cl(ell_factor=False, units='muK2')

Returns a dictionary of lensed CMB power spectra and the lensing potential pp power spectrum.

Set the units with the keyword units='1'|'muK2'|'K2' (default: ‘muK2’, except for the lensing potential power spectrum, which is always unitless).

If ell_factor=True (default: False), multiplies the spectra by \(\ell(\ell+1)/(2\pi)\) (or by \(\ell^2(\ell+1)^2/(2\pi)\) in the case of the lensing potential pp spectrum).

get_H(z, units='km/s/Mpc')

Returns the Hubble rate at the given redshifts.

The redshifts must be a subset of those requested when needs() was called.

The available units are km/s/Mpc (i.e. c*H(Mpc^-1)) and 1/Mpc.


Returns the physical angular diameter distance to the given redshifts.

The redshifts must be a subset of those requested when needs() was called.


Returns a (dict of) power spectrum interpolator(s) PowerSpectrumInterpolator.


Finalizes the theory code, if something needs to be done (releasing memory, etc.)