class zfit.constraint.PoissonConstraint(params, observation)[source]#

Bases: TFProbabilityConstraint

Poisson constraints on a list of parameters to some observed values.

Constraints parameters that can be counts (i.e. from a histogram) or, more generally, are Poisson distributed. This is often used in the case of histogram templates which are obtained from simulation and have a poisson uncertainty due to limited statistics.

\[\text{constraint} = \text{Poisson}(\text{observation}; \text{params})\]
  • params (TypeVar(ParamTypeInput, zfit.core.interfaces.ZfitParameter, Union[int, float, complex, Tensor, zfit.core.interfaces.ZfitParameter])) – The parameters to constraint; corresponds to the mu in the Poisson distribution.

  • observation (Union[int, float, complex, Tensor, zfit.core.interfaces.ZfitParameter]) – observed values of the parameter; corresponds to lambda in the Poisson distribution.


ShapeIncompatibleError – If params and observation have incompatible shapes.

add_cache_deps(cache_deps, allow_non_cachable=True)#

Add dependencies that render the cache invalid if they change.

  • cache_deps (Union[zfit.core.interfaces.ZfitGraphCachable, Iterable[zfit.core.interfaces.ZfitGraphCachable]]) –

  • allow_non_cachable (bool) – If True, allow cache_dependents to be non-cachables. If False, any cache_dependents that is not a ZfitGraphCachable will raise an error.


TypeError – if one of the cache_dependents is not a ZfitGraphCachable _and_ allow_non_cachable if False.

property dtype: DType#

The dtype of the object.

Return type



Return a set of all independent Parameter that this object depends on.


only_floating (bool) – If True, only return floating Parameter

Return type




Deprecated: THIS FUNCTION IS DEPRECATED. It will be removed in a future version. Instructions for updating: Use get_params instead if you want to retrieve the independent parameters or get_cache_deps in case you need the numerical cache dependents (advanced).

Return type


get_params(floating=True, is_yield=None, extract_independent=True, only_floating=<class 'zfit.util.checks.NotSpecified'>)#

Recursively collect parameters that this object depends on according to the filter criteria.

Which parameters should be included can be steered using the arguments as a filter.
  • None: do not filter on this. E.g. floating=None will return parameters that are floating as well as

    parameters that are fixed.

  • True: only return parameters that fulfil this criterion

  • False: only return parameters that do not fulfil this criterion. E.g. floating=False will return

    only parameters that are not floating.

  • floating (bool | None) – if a parameter is floating, e.g. if floating() returns True

  • is_yield (bool | None) – if a parameter is a yield of the _current_ model. This won’t be applied recursively, but may include yields if they do also represent a parameter parametrizing the shape. So if the yield of the current model depends on other yields (or also non-yields), this will be included. If, however, just submodels depend on a yield (as their yield) and it is not correlated to the output of our model, they won’t be included.

  • extract_independent (bool | None) – If the parameter is an independent parameter, i.e. if it is a ZfitIndependentParameter.

Return type


property name: str#

The name of the object.

Return type


property observation#

Return the observed values of the parameters constrained.


Register a cacher that caches values produces by this instance; a dependent.


cacher (Union[zfit.core.interfaces.ZfitGraphCachable, Iterable[zfit.core.interfaces.ZfitGraphCachable]]) –


Clear the cache of self and all dependent cachers.


Sample n points from the probability density function for the observed value of the parameters.


n – The number of samples to be generated.