PSDPosterior.py

class PSDPosterior(object):
    def __init__(self):
        self.foo = 10

my_lambda = lambda self, x : self.foo * x

psd_posterior = PSDPosterior()

import types
psd_posterior.logprior = types.MethodType(my_lambda, psd_posterior)

print(psd_posterior.logprior(9))