import functools
import tango

tasks = {}

# note: f.__name__ is of type is_XXX_allowed
# f.__name__[3:-8] select the command name
# this decorator build a dictionary with the command name as key and
# the handler as value.
task = lambda f:tasks.setdefault(f.__name__[3:-8], f)

@task
def is_standby_allowed(device_instance):
    """
    Allowed method for Standby method.
    Command *Standby* is allowed when the device *State* is ON or
    STANDBY.

    :return: True if the method is allowed, otherwise False.
    """
    if device_instance.get_state() in [tango.DevState.ON, 
                                        tango.DevState.STANDBY]:
        return True
    return False

@task
def is_on_allowed(device_instance):
    """
    Allowed method for On method.
    Command *On* is allowed when the device *State* is ON or
    STANDBY.

    :return: True if the method is allowed, otherwise False.
    """
    if device_instance.get_state() in [tango.DevState.ON, 
                                        tango.DevState.STANDBY]:
        return True
    return False

@task
def is_off_allowed(device_instance):
    """
    Allowed method for Off method.
    Command *Off* is allowed when the device *State* is OFF or
    STANDBY.

    :return: True if the method is allowed, otherwise False.
    """
    if device_instance.get_state() in [tango.DevState.OFF, 
                                       tango.DevState.STANDBY]:
        return True
    return False

def is_command_allowed(device_instance, cmd_name):
    """
    Call the allowed method for the command name specified
    as input argument
    :param device_istance: the TANGO device instance
    :param cmd_name: the name of command to execute

    :return: True/False
    """
    tasks[cmd_name](device_instance)

class IsCommandAllowed(object):
    """
    Class designed to be a decorator for the Master power methods.
    The *decorator function* performs a check on the input argument
    to control if the command is issued on the whole sub-element.
    If this is the case, it checks the State of the sub-element Master
    device and rejects the command accordingly to the State
    machine setting.

    :raise: tango.DevFailed exception if the command can't be executed 
    """
    def __init__(self, *args, **kwargs):
        # store the decorators parameters:
        # args: the list of sub-element attributes to subscribe, to track the
        #       sub-element command progress and detect a command timeout
        # kwargs: a dictionary: key ="cmd_name",
        #                       value = command to execute ('on', 'off'...)
        self._args = args
        self._kwargs = kwargs

    def __call__(self, f):
        @functools.wraps(f)
        def input_args_check(*args, **kwargs):
            # the Master device instance
            dev_instance = args[0]
            # the command name
            cmd_to_exec = f.__name__
            # the command input argument
            input_arg = args[1]
            #device_list = input_arg
            # Note: device list is a reference to args[1]: changing
            # device_list content, args[1] changes accordingly!
            num_of_devices = len(input_arg)
            if num_of_devices == 0:
                # check the device State: if it not the proper value the command is
                # not executed
                if not is_command_allowed(dev_instance, cmd_to_exec.lower()):
                    msg = "Command {} can't be executed when the device is {}".format(cmd_to_exec,
                                                                                       dev_instance.get_state())
                    tango.Except.throw_exception("Command failure",msg,
                                             "IsCommandAllowed decorator",
                                             tango.ErrSeverity.ERR)
            return f(*args, **kwargs)
        return input_args_check