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-
-Concurrency-related challenges regarding embedding of ptpython in asyncio code
-==============================================================================
-
-Things we want to be possible
------------------------------
-
-- Embed blocking ptpython in non-asyncio code (the normal use case).
-- Embed blocking ptpython in asyncio code (the event loop will block).
-- Embed awaitable ptpython in asyncio code (the loop will continue).
-- React to resize events (SIGWINCH).
-- Support top-level await.
-- Be able to patch_stdout, so that logging messages from another thread will be
-  printed above the prompt.
-- It should be possible to handle `KeyboardInterrupt` during evaluation of an
-  expression.
-- The "eval" should happen in the same thread from where embed() was called.
-
-
-Limitations of asyncio/python
------------------------------
-
-- We can only listen to SIGWINCH signal (resize) events in the main thread.
-
-- Usage of Control-C for triggering a `KeyboardInterrupt` only works for code
-  running in the main thread. (And only if the terminal was not set in raw
-  input mode).
-
-- Spawning a new event loop from within a coroutine, that's being executed in
-  an existing event loop is not allowed in asyncio. We can however spawn any
-  event loop in a separate thread, and wait for that thread to finish.
-
-- For patch_stdout to work correctly, we have to know what prompt_toolkit
-  application is running on the terminal, then tell that application to print
-  the output and redraw itself.
-
-
-Additional challenges for IPython
----------------------------------
-
-IPython supports integration of 3rd party event loops (for various GUI
-toolkits). These event loops are supposed to continue running while we are
-prompting for input. In an asyncio environment, it means that there are
-situations where we have to juggle three event loops:
-
-- The asyncio loop in which the code was embedded.
-- The asyncio loop from the prompt.
-- The 3rd party GUI loop. 
-
-Approach taken in ptpython 3.0.11
----------------------------------
-
-For ptpython, the most reliable solution is to to run the prompt_toolkit input
-prompt in a separate background thread. This way it can use its own asyncio
-event loop without ever having to interfere with whatever runs in the main
-thread.
-
-Then, depending on how we embed, we do the following:
-When a normal blocking embed is used:
-    * We start the UI thread for the input, and do a blocking wait on
-      `thread.join()` here.
-    * The "eval" happens in the main thread.
-    * The "print" happens also in the main thread. Unless a pager is shown,
-      which is also a prompt_toolkit application, then the pager itself is runs
-      also in another thread, similar to the way we do the input.
-
-When an awaitable embed is used, for embedding in a coroutine, but having the
-event loop continue:
-    * We run the input method from the blocking embed in an asyncio executor
-      and do an `await loop.run_in_excecutor(...)`.
-    * The "eval" happens again in the main thread.
-    * "print" is also similar, except that the pager code (if used) runs in an
-      executor too.
-
-This means that the prompt_toolkit application code will always run in a
-different thread. It means it won't be able to respond to SIGWINCH (window
-resize events), but prompt_toolkit's 3.0.11 has now terminal size polling which
-solves this.
-
-Control-C key presses won't interrupt the main thread while we wait for input,
-because the prompt_toolkit application turns the terminal in raw mode, while
-it's reading, which means that it will receive control-c key presses as raw
-data in its own thread.
-
-Top-level await works in most situations as expected.
-- If a blocking embed is used. We execute ``loop.run_until_complete(code)``.
-  This assumes that the blocking embed is not used in a coroutine of a running
-  event loop, otherwise, this will attempt to start a nested event loop, which
-  asyncio does not support. In that case we will get an exception.
-- If an awaitable embed is used. We literally execute ``await code``. This will
-  integrate nicely in the current event loop.