from paddle.v2.framework import framework as framework

__all__ = ['append_backward_ops']


def append_backward_ops(loss, parameter_list=None, no_grad_set=None):
    """
    Create and add gradient Operators in BlockDesc to compute
    gradients of `loss` for parameters in parameter_list

    :param loss: an variable generated by cost function.
    :type loss: Variable
    :param no_grad_set: variable that should not create gradient
    :type no_grad_set: set
    :param parameter_list: parameters that need to compute gradient and 
    update to optimize the lost.
    :type: list
    :return: list of (parameters, gradients) pair.
    :rtype: list[Variable]
    """
    assert isinstance(loss, framework.Variable)
    param_grad_map = loss.block.program.append_backward(loss, no_grad_set or
                                                        set())
    if parameter_list is not None:
        parameters = parameter_list
    else:
        params = loss.block.program.global_block().all_parameters()
        parameters = [param.name for param in params]
    params_and_grads = []
    for param in parameters:
        if param not in param_grad_map:
            raise ValueError("param %s is not in map" % param)
        grad_info = param_grad_map[param]
        grad_block = loss.block.program.block(grad_info[1])
        if not grad_block.has_var(grad_info[0]):
            raise ValueError("grad block[{0}] did not have grad var {1}".format(
                grad_info[1], grad_info[0]))
        # Get the param var from the global block
        param_var = loss.block.program.global_block().var(param)
        grad_var = grad_block.var(grad_info[0])
        if loss.block.has_var(grad_info[0]):
            params_and_grads.append((param_var, grad_var))
        else:
            params_and_grads.append((param_var, None))
    return params_and_grads
