Implement the once method on Evented.

Author: hansthen

folium/elements.py

@@ -107,19 +107,20 @@ class EventHandler(MacroElement):
     _template = Template(
         """
         {% macro script(this, kwargs) %}
-            {{ this._parent.get_name()}}.on(
+            {{ this._parent.get_name()}}.{{ this.method }}(
                 {{ this.event|tojson}},
                 {{ this.handler.js_code }}
             );
         {% endmacro %}
         """
     )
 
-    def __init__(self, event: str, handler: JsCode):
+    def __init__(self, event: str, handler: JsCode, once=False):
         super().__init__()
         self._name = "EventHandler"
         self.event = event
         self.handler = handler
+        self.method = "once" if once else "on"
 
 
 class ElementAddToElement(MacroElement):

folium/map.py

@@ -25,7 +25,7 @@
 class Evented(MacroElement):
     """The base class for Layer and Map
 
-    Adds the `on` method for event handling capabilities.
+    Adds the `on` and `once` methods for event handling capabilities.
 
     See https://leafletjs.com/reference.html#evented for
     more in depth documentation. Please note that we have
@@ -34,8 +34,14 @@ class Evented(MacroElement):
     """
 
     def on(self, **event_map: JsCode):
+        self._add(False, **event_map)
+
+    def once(self, **event_map: JsCode):
+        self._add(True, **event_map)
+
+    def _add(self, once, **event_map: JsCode):
         for event_type, handler in event_map.items():
-            self.add_child(EventHandler(event_type, handler))
+            self.add_child(EventHandler(event_type, handler, once))
 
 
 class Layer(Evented):