Modify ThreadedHistory example(s) for new API

Author: Bob Hyman

examples/prompts/history/slow-history.py

@@ -6,6 +6,7 @@
 without blocking any user interaction.
 """
 import time
+import asyncio
 
 from prompt_toolkit import PromptSession
 from prompt_toolkit.history import History, ThreadedHistory
@@ -32,11 +33,22 @@ def main():
         "Even when the input is accepted, loading will continue in the "
         "background and when the next prompt is displayed.\n"
     )
-    our_history = ThreadedHistory(SlowHistory())
+
+    my_loop = asyncio.get_event_loop()  # creates loop if needed
+
+    # Inform ThreadedHistory which event loop to use
+    # when passing lines of history to the prompt.
+
+    our_history = ThreadedHistory(SlowHistory(), my_loop)
 
     # The history needs to be passed to the `PromptSession`. It can't be passed
     # to the `prompt` call because only one history can be used during a
     # session.
+    # Note that PromptSession runs on the thread's current event loop because
+    # it was created above and is therefore in synch with ThreadedHistory.
+    # PromptSession would create and event loop if it didn't find one
+    # already running, but then ThreadedHistory would not work.
+
     session = PromptSession(history=our_history)
 
     while True:

prompt_toolkit/history.py

@@ -103,7 +103,7 @@ class ThreadedHistory(History):
     """
 
     def __init__(
-        self, history: History, event_loop: asyncio.BaseEventLoop = None
+        self, history: History, event_loop: Optional[asyncio.BaseEventLoop] = None
     ) -> None:
         """Create instance of ThreadedHistory
 

prompt_toolkit/shortcuts/prompt.py

@@ -556,8 +556,7 @@ def display_placeholder() -> bool:
             # Users can insert processors here.
             DynamicProcessor(lambda: merge_processors(self.input_processors or [])),
             ConditionalProcessor(
-                AfterInput(lambda: self.placeholder),
-                filter=display_placeholder,
+                AfterInput(lambda: self.placeholder), filter=display_placeholder,
             ),
         ]