Task scopes for AnyIO
Project description
This library implements scoped taskgroups / nurseries.
Rationale
Large programs often consist of building blocks which depend on each other. Crucially, these blocks may get started in FIFO order: you start the main program which starts a mid-level support task which starts a low-level connection. Then it stats another mid-level task which starts a low-level task which uses the same connection … you get the idea: these relationships typically form a DAG (directed acyclic graph).
In order to cleanly terminate this, the main program must be cancelled first (so that it may clean up after itself), then the mid-level stuff (data conversion, error reporting …) gets torn down, then the low-level connection (database, MQTT, …).
Usage
Wrap your main code in async with main_scope(): ....
Start a service task (i.e. something you depend on) with await spawn_service(async_proc, ...). Service tasks are cancelled automatically when your main code exits. Don’t do it yourself, you’ll create a deadlock. if the service scope wants to create a new
Start some other subtask which runs in parallel with your main task with cancel_scope = await spawn(async_proc, ...). You can cancel them like any other task if necessary.
The current scope is available as the scope context variable.
The examples directory has sample code.
Cancellation semantics
When a scope exits (either cleanly or when it raises an error that escapes its taskgroup), all scopes depending on it are cancelled immediately, in parallel. Then those it itself depends on are terminated cleanly and in-order.
This also happens when a scope’s main task ends.
“Clean termination” means that the scope’s task is cancelles unless it waits for no_more_dependents() to return.
Code structure
A scope’s main code typically looks like this:
start the services it depends on
call “register(serice_object)”
await no_more_dependents() (subordinate task) or wait for SIGTERM (daemon main task) or terminate (main task’s job is done)
cleanly stop itself
If no_more_dependents is not called, the scope will be cancelled; as this typically affects all subtasks, clean shutdown is unnecessarily difficult.
Scopes typically don’t need to access its own scope object. It’s stored in the asyncscope.scope contextvar if you need it. asyncscope’s global procedures access the current scope transparently.
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