Redesign asset retrieval in the library.

Most assets now return a new class named `Asset`. This allows for the
assets to be consistently saved via a `save` method instead of special
casing for `Attachment`.

`AppInfo` is no longer a namedtuple it is a fully documented dataclass,
as well as having the state attached to it.

Fixes #1997

discord/emoji.py

@@ -24,11 +24,10 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 """
 
-from collections import namedtuple
-
+from .asset import Asset
 from . import utils
 
-class PartialEmoji(namedtuple('PartialEmoji', 'animated name id')):
+class PartialEmoji:
     """Represents a "partial" emoji.
 
     This model will be given in two scenarios:
@@ -65,7 +64,19 @@ class PartialEmoji(namedtuple('PartialEmoji', 'animated name id')):
         The ID of the custom emoji, if applicable.
     """
 
-    __slots__ = ()
+    __slots__ = ('animated', 'name', 'id', '_state')
+
+    def __init__(self, *, animated, name, id=None):
+        self.animated = animated
+        self.name = name
+        self.id = id
+        self._state = None
+
+    @classmethod
+    def with_state(cls, state, *, animated, name, id=None):
+        self = cls(animated=animated, name=name, id=id)
+        self._state = state
+        return self
 
     def __str__(self):
         if self.id is None:
@@ -81,6 +92,9 @@ class PartialEmoji(namedtuple('PartialEmoji', 'animated name id')):
         if isinstance(other, (PartialEmoji, Emoji)):
             return self.id == other.id
 
+    def __ne__(self, other):
+        return not self == other
+
     def __hash__(self):
         return hash((self.id, self.name))
 
@@ -99,12 +113,13 @@ class PartialEmoji(namedtuple('PartialEmoji', 'animated name id')):
 
     @property
     def url(self):
-        """Returns a URL version of the emoji, if it is custom."""
+        """:class:`Asset`:Returns an asset of the emoji, if it is custom."""
         if self.is_unicode_emoji():
-            return None
+            return Asset(self._state)
 
         _format = 'gif' if self.animated else 'png'
-        return "https://cdn.discordapp.com/emojis/{0.id}.{1}".format(self, _format)
+        url = "https://cdn.discordapp.com/emojis/{0.id}.{1}".format(self, _format)
+        return Asset(self._state, url)
 
 class Emoji:
     """Represents a custom emoji.
@@ -186,6 +201,9 @@ class Emoji:
     def __eq__(self, other):
         return isinstance(other, (PartialEmoji, Emoji)) and self.id == other.id
 
+    def __ne__(self, other):
+        return not self == other
+
     def __hash__(self):
         return self.id >> 22
 
@@ -198,7 +216,8 @@ class Emoji:
     def url(self):
         """Returns a URL version of the emoji."""
         _format = 'gif' if self.animated else 'png'
-        return "https://cdn.discordapp.com/emojis/{0.id}.{1}".format(self, _format)
+        url = "https://cdn.discordapp.com/emojis/{0.id}.{1}".format(self, _format)
+        return Asset(self._state, url)
 
     @property
     def roles(self):