XEP-0428: add fallback body and subject elements

+ tests
+ helpers to strip the fallback content

slixmpp/plugins/xep_0428/stanza.py

@@ -1,8 +1,13 @@
-
 # Slixmpp: The Slick XMPP Library
 # Copyright (C) 2020 Mathieu Pasquet <mathieui@mathieui.net>
 # This file is part of Slixmpp.
 # See the file LICENSE for copying permissio
+from abc import ABC
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
+
 from slixmpp.stanza import Message
 from slixmpp.xmlstream import (
     ElementBase,
@@ -10,14 +15,83 @@ from slixmpp.xmlstream import (
 )
 
 
-NS = 'urn:xmpp:fallback:0'
+NS = "urn:xmpp:fallback:0"
 
 
 class Fallback(ElementBase):
     namespace = NS
-    name = 'fallback'
-    plugin_attrib = 'fallback'
+    name = "fallback"
+    plugin_attrib = "fallback"
+    plugin_multi_attrib = "fallbacks"
+    interfaces = {"for"}
+
+    def _find_fallback(self, fallback_for: str) -> "Fallback":
+        if self["for"] == fallback_for:
+            return self
+        for fallback in self.parent()["fallbacks"]:
+            if fallback["for"] == fallback_for:
+                return fallback
+        raise AttributeError("No fallback for this namespace", fallback_for)
+
+    def get_stripped_body(
+        self, fallback_for: str, element: Literal["body", "subject"] = "body"
+    ) -> str:
+        """
+        Get the body of a message, with the fallback part stripped
+
+        :param fallback_for: namespace of the fallback to strip
+        :param element: set this to "subject" get the stripped subject instead
+            of body
+
+        :return: body (or subject) content minus the fallback part
+        """
+        fallback = self._find_fallback(fallback_for)
+        start = fallback[element]["start"]
+        end = fallback[element]["end"]
+        body = self.parent()[element]
+        if start == end == 0:
+            return ""
+        if start <= end < len(body):
+            return body[:start] + body[end:]
+        else:
+            return body
+
+
+class FallbackMixin(ABC):
+    namespace = NS
+    name = NotImplemented
+    plugin_attrib = NotImplemented
+    interfaces = {"start", "end"}
+
+    def set_start(self, v: int):
+        self._set_attr("start", str(v))
+
+    def get_start(self):
+        return _int_or_zero(self._get_attr("start"))
+
+    def set_end(self, v: int):
+        self._set_attr("end", str(v))
+
+    def get_end(self):
+        return _int_or_zero(self._get_attr("end"))
+
+
+class FallbackBody(FallbackMixin, ElementBase):
+    name = plugin_attrib = "body"
+
+
+class FallbackSubject(FallbackMixin, ElementBase):
+    name = plugin_attrib = "subject"
+
+
+def _int_or_zero(v: str):
+    try:
+        return int(v)
+    except ValueError:
+        return 0
 
 
 def register_plugins():
-    register_stanza_plugin(Message, Fallback)
+    register_stanza_plugin(Message, Fallback, iterable=True)
+    register_stanza_plugin(Fallback, FallbackBody)
+    register_stanza_plugin(Fallback, FallbackSubject)