# Source code for bayespy.inference.vmp.nodes.exponential

```################################################################################
# Copyright (C) 2014 Jaakko Luttinen
#
# This file is licensed under the MIT License.
################################################################################

"""
Module for the exponential distribution node.
"""

from .gamma import (GammaMoments,
Gamma)
from .expfamily import ExponentialFamily

ExponentialMoments = GammaMoments

[docs]class Exponential(Gamma):
r"""
Node for exponential random variables.

.. warning::

Use :class:`Gamma` instead of this. `Exponential(l)` is equivalent to
`Gamma(1, l)`.

Parameters
----------

l : gamma-like node or scalar or array

Rate parameter

See also
--------

Gamma, Poisson

Notes
-----

For simplicity, this is just a gamma node with the first parent fixed to
one.  Note that this is a bit inconsistent with the BayesPy philosophy which
states that the node does not only define the form of the prior distribution
but more importantly the form of the posterior approximation.  Thus, one
might expect that this node would have exponential posterior distribution
approximation.  However, it has a gamma distribution.  Also, the moments are
gamma moments although only E[x] would be the moment of a exponential random
variable.  All this was done because: a) gamma was already implemented, so
there was no need to implement anything, and b) people might easily use
Exponential node as a prior definition and expect to get gamma posterior
(which is what happens now).  Maybe some day a pure Exponential node is
implemented and the users are advised to use Gamma(1,b) if they want to use
an exponential prior distribution but gamma posterior approximation.
"""

[docs]    def __init__(self, l, **kwargs):
raise NotImplementedError("Not yet implemented. Use Gamma(1, lambda)")
super().__init__(1, l, **kwargs)

@classmethod
def _constructor(cls, l, **kwargs):
raise NotImplementedError("Not yet implemented. Use Gamma(1, lambda)")
```