get_covariance is loops twice over arguments

[mikofski]

during the first loop, it is just getting the dimensions, mainly of nobs which seems like this should already be known, but even still, this could be popped from a sequence on the 2nd loop which would eliminate redundant code and try-except blocks.

    def get_covariance(datargs, outargs, vargs, datvar, outvar):
        """
        Get covariance matrix.

        :param datargs: data arguments
        :param outargs: output arguments
        :param vargs: variable arguments
        :param datvar: variance of data arguments
        :param outvar: variance of output arguments
        :return: covariance
        """
        # number of formula arguments that are not constant
        argn = len(vargs)
        # number of observations must be the same for all vargs
        nobs = 1
        c = []
        for m in xrange(argn):
            a = vargs[m]
            try:
                a = datargs[a]
            except (KeyError, TypeError):
                a = outargs[a]
                avar = outvar[a]
            else:
                avar = datvar[a]
            c.append([])  # add a list
            for n in xrange(argn):
                b = vargs[n]
                try:
                    b = datargs[b]
                except (KeyError, TypeError):
                    b = outargs[b]
                c.append(avar.get(b, 0.0))  # add covariance to sequence
                try:
                    nobs = max(nobs, len(c))
                except (TypeError, ValueError):
                    LOGGER.debug('c of %s vs %s = %g', a, b, c)
        # covariance matrix is initially zeros
        cov = np.zeros((nobs, argn, argn))
        # loop over arguments in both directions, fill in covariance
        for m in xrange(argn):
            d = c.pop()
            for n in xrange(argn):
                cov[:, argn-1-m, argn-1-n] = d.pop()
        if nobs == 1:
            cov = cov.squeeze()  # squeeze out any extra dimensions
        LOGGER.debug('covariance:\n%r', cov)
        return cov