from io import StringIO


class BaseCommand(object):
    """Commands can be used to chain the execution of multiple programs
    together. You can chain multiple commands together using commands

    For example:
    ls = RawCommand(["ls"])
    grep = RawCommand(["grep", "potato"])
    printer = Printer()
    ls.chain(grep).chain(printer).call()
    """

    def __init__(self, args=None):
        self.cmd_args = args
        self.prev_cmd = None

    def call(self, *args, **kwargs):
        """Implicitly calls any chained commands, returning a function
        to make an input generator."""
        raise NotImplementedError("Must implement call")

    def get_input_generator(self):
        """Gets the input generator from the previous command, if it
        exists. If it doesn't exist, we just return an empy list so that
        when you iterate over it, it does nothing."""
        if self.prev_cmd is not None:
            make_input_generator = self.prev_cmd.call()
            input_generator = make_input_generator()
        else:
            input_generator = []
        return input_generator

    def chain(self, cmd):
        """Chains a command to another command, returning the other command"""
        cmd.prev_cmd = self
        return cmd


registered_cmds = []

def register_cmd(cls):
    """Decorator for putting all of the commands in one nice place."""
    registered_cmds.append(cls.__name__)
    return cls