<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_covariance(self, chain=0, parameters=None):
 """ Takes a chain and returns the covariance between chain parameters. Parameters chain : int|str, optional The chain index or name. Defaults to first chain. parameters : list[str], optional The list of parameters to compute correlations. Defaults to all parameters for the given chain. Returns ------- tuple The first index giving a list of parameter names, the second index being the 2D covariance matrix. """

        index = self.parent._get_chain(chain)
        assert len(index) == 1, "Please specify only one chain, have %d chains" % len(index)
        chain = self.parent.chains[index[0]]
        if parameters is None:
            parameters = chain.parameters

        data = chain.get_data(parameters)
        cov = np.atleast_2d(np.cov(data, aweights=chain.weights, rowvar=False))

        return parameters, cov