# Creating theory classes and dependencies¶

Custom Theory codes can be used to calculate observables needed by a likelihood, or perform any sub-calculation needed by any other likelihood or theory class. By breaking the calculation up in to separate classes the calculation can be modularized, and each class can have its own nuisance parameters and speed, and hence be sampled efficiently using the built-in fast-slow parameter blocking.

Theories should inherit from the base Theory class, either directly or indirectly. Each theory may have its own parameters, and depend on derived parameters or general quantities calculated by other theory classes (or likelihoods as long as this does not lead to a circular dependence). The names of derived parameters or other quantities are for each class to define and document as needed.

To specify requirements you can use the Theory methods

The actual calculation of the quantities requested by the must_provide() method should be done by calculate(), which stores computed quantities into a state dictionary. Derived parameters should be saved into the special state['derived'] dictionary entry. The theory code also needs to tell other theory codes and likelihoods the things that it can calculate using

• get_X methods; any method starting with get_ will automatically indicate that the theory can compute X
• return list of things that can be calculated from get_can_provide().
• return list of derived parameter names from get_can_provide_params()
• specify derived parameters in an associated .yaml file or class params dictionary

Use a get_X method when you need to add optional arguments to provide different outputs from the computed quantity. Quantities returned by get_can_provide() should be stored in the state dictionary by the calculate function or returned by the get_results(X) for each quantity X (which by default just returns the value stored in the current state dictionary). The results stored by calculate for a given set of input parameters are cached, and self._current_state is set to the current state whenever get_X, get_param etc are called.

For example, this is a class that would calculate A = B*b_derived using inputs B and derived parameter b_derived from another theory code, and provide the method to return A with a custom normalization and the derived parameter Aderived:

from cobaya.theory import Theory

class ACalculator(Theory):

def initialize(self):
"""called from __init__ to initialize"""

def initialize_with_provider(self, provider):
"""
Initialization after other components initialized, using Provider class
instance which is used to return any dependencies (see calculate below).
"""
self.provider = provider

def get_requirements(self):
"""
Return dictionary of derived parameters or other quantities that are needed
by this component and should be calculated by another theory class.
"""
return {'b_derived': None}

def must_provide(self, **requirements):
if 'A' in requirements:
# e.g. calculating A requires B computed using same kmax (default 10)
return {'B': {'kmax': requirements['A'].get('kmax', 10)}}

def get_can_provide_params(self):

def calculate(self, state, want_derived=True, **params_values_dict):
state['A'] = self.provider.get_result('B') * self.provider.get_param('b_derived')

def get_A(self, normalization=1):
return self._current_state['A'] * normalization


Likelihood codes (that return A in their get_requirements method) can then use, e.g. self.provider.get_A(normalization=1e-10) to get the result calculated by this component. Some other Theory class would be required to calculate the remaining requirements, e.g. to get b_derived and B:

from cobaya.theory import Theory

class BCalculator(Theory):

def initialize(self):
self.kmax = 0

def get_can_provide_params(self):
return ['b_derived']

def get_can_provide(self):
return ['B']

def must_provide(self, **requirements):
if 'B' in requirements:
self.kmax = max(self.kmax, requirements['B'].get('kmax',10))

def calculate(self, state, want_derived=True, **params_values_dict):
if self.kmax:
state['B'] = ... do calculation using self.kmax

if want_derived:
state['derived'] = {'b_derived': ...xxx...}


So far this example allows the use of ACalculator and BCalculator together with any likelihood that needs the quantity A, but neither theory code yet depends on any parameters. Although theory codes do not need to have their own sampled parameters, often they do, in which case they can be specified in a [ClassName].yaml file as for likelihoods, or as a class params dictionary. For example to specify input parameter Xin and output parameter Xderived the class could be defined like this:

from cobaya.theory import Theory

class X(Theory):
params = {'Xin': None, 'Xderived': {'derived': True}}


Here the user has to specify the input for Xin. Of course you can also provide default sampling settings for ‘Xin’ so that configuring it is transparent to the user, e.g.

class X(Theory):
params = {'Xin': {'prior': {'min': 0, 'max': 1}, 'propose': 0.01, 'ref': 0.9},
'Xderived': {'derived': True}}


If multiple theory codes can provide the same quantity, it may be ambiguous which to use to compute which. When this happens use the provides input .yaml keyword to specify that a specific theory computes a specific quantity.