chore: import upstream snapshot with attribution

docs/md/how_to/python/callbacks.md

@@ -0,0 +1,34 @@
+# Callbacks and Events
+
+`perspective.Table` allows for `on_update` and `on_delete` callbacks to be
+set—simply call `on_update` or `on_delete` with a reference to a function or a
+lambda without any parameters:
+
+```python
+def update_callback():
+    print("Updated!")
+
+# set the update callback
+on_update_id = view.on_update(update_callback)
+
+
+def delete_callback():
+    print("Deleted!")
+
+# set the delete callback
+on_delete_id = view.on_delete(delete_callback)
+
+# set a lambda as a callback
+view.on_delete(lambda: print("Deleted x2!"))
+```
+
+If the callback is a named reference to a function, it can be removed with
+`remove_update` or `remove_delete`:
+
+```python
+view.remove_update(on_update_id)
+view.remove_delete(on_delete_id)
+```
+
+Callbacks defined with a lambda function cannot be removed, as lambda functions
+have no identifier.