[tasks] Add way to change interval at run-time

PR: Rapptz#2162 
Fixes: Rapptz#2158

discord/ext/tasks/__init__.py

@@ -18,9 +18,6 @@ class Loop:
     """
     def __init__(self, coro, seconds, hours, minutes, count, reconnect, loop):
         self.coro = coro
-        self.seconds = seconds
-        self.hours = hours
-        self.minutes = minutes
         self.reconnect = reconnect
         self.loop = loop or asyncio.get_event_loop()
         self.count = count
@@ -47,13 +44,7 @@ def __init__(self, coro, seconds, hours, minutes, count, reconnect, loop):
         if self.count is not None and self.count <= 0:
             raise ValueError('count must be greater than 0 or None.')
 
-        self._sleep = sleep = self.seconds + (self.minutes * 60.0) + (self.hours * 3600.0)
-        if sleep >= MAX_ASYNCIO_SECONDS:
-            fmt = 'Total number of seconds exceeds asyncio imposed limit of {0} seconds.'
-            raise ValueError(fmt.format(MAX_ASYNCIO_SECONDS))
-
-        if sleep < 0:
-            raise ValueError('Total number of seconds cannot be less than zero.')
+        self.change_interval(seconds=seconds, minutes=minutes, hours=hours)
 
         if not inspect.iscoroutinefunction(self.coro):
             raise TypeError('Expected coroutine function, not {0.__name__!r}.'.format(type(self.coro)))
@@ -317,6 +308,42 @@ def after_loop(self, coro):
         self._after_loop = coro
         return coro
 
+    def change_interval(self, *, seconds=0, minutes=0, hours=0):
+        """Changes the interval for the sleep time.
+
+        .. note::
+
+            This only applies on the next loop iteration. If it is desirable for the change of interval
+            to be applied right away, cancel the task with :meth:`cancel`.
+
+        Parameters
+        ------------
+        seconds: :class:`float`
+            The number of seconds between every iteration.
+        minutes: :class:`float`
+            The number of minutes between every iteration.
+        hours: :class:`float`
+            The number of hours between every iteration.
+
+        Raises
+        -------
+        ValueError
+            An invalid value was given.
+        """
+
+        sleep = seconds + (minutes * 60.0) + (hours * 3600.0)
+        if sleep >= MAX_ASYNCIO_SECONDS:
+            fmt = 'Total number of seconds exceeds asyncio imposed limit of {0} seconds.'
+            raise ValueError(fmt.format(MAX_ASYNCIO_SECONDS))
+
+        if sleep < 0:
+            raise ValueError('Total number of seconds cannot be less than zero.')
+
+        self._sleep = sleep
+        self.seconds = seconds
+        self.hours = hours
+        self.minutes = minutes
+
 def loop(*, seconds=0, minutes=0, hours=0, count=None, reconnect=True, loop=None):
     """A decorator that schedules a task in the background for you with
     optional reconnect logic.