class_docstr: Poisson Distribution.
  
  The probability mass function of the distribution is expressed as
  
  .. math::
      P(x; \lambda) = \frac{\lambda ^x e^{-\lambda}}{x!}
  
  Args:
      lam(:class:`~chainer.Variable` or :ref:`ndarray`): Parameter of
          distribution. :math:`\lambda`
code: def _log_lam(self):
      return exponential.log(self.lam)
  
imports: import chainer
  from chainer.backends import cuda
  from chainer import distribution
  from chainer.functions.math import exponential
  from chainer.functions.math import lgamma
  from chainer import utils
  from chainer.utils import cache
label_desc: Use image node_id 3 for calling the Poisson obj's underlying member method code with example usage: obj._log_lam() and returns: exponential