tornado.stack_context — Exception handling across asynchronous callbacks

StackContext allows applications to maintain threadlocal-like state that follows execution as it moves to other execution contexts.

The motivating examples are to eliminate the need for explicit async_callback wrappers (as in tornado.web.RequestHandler), and to allow some additional context to be kept for logging.

This is slightly magic, but it’s an extension of the idea that an exception handler is a kind of stack-local state and when that stack is suspended and resumed in a new context that state needs to be preserved. StackContext shifts the burden of restoring that state from each call site (e.g. wrapping each AsyncHTTPClient callback in async_callback) to the mechanisms that transfer control from one context to another (e.g. AsyncHTTPClient itself, IOLoop, thread pools, etc).

Example usage:

@contextlib.contextmanager
def die_on_error():
    try:
        yield
    except Exception:
        logging.error("exception in asynchronous operation",exc_info=True)
        sys.exit(1)

with StackContext(die_on_error):
    # Any exception thrown here *or in callback and its desendents*
    # will cause the process to exit instead of spinning endlessly
    # in the ioloop.
    http_client.fetch(url, callback)
ioloop.start()

Most applications shouln’t have to work with StackContext directly. Here are a few rules of thumb for when it’s necessary:

  • If you’re writing an asynchronous library that doesn’t rely on a stack_context-aware library like tornado.ioloop or tornado.iostream (for example, if you’re writing a thread pool), use stack_context.wrap() before any asynchronous operations to capture the stack context from where the operation was started.
  • If you’re writing an asynchronous library that has some shared resources (such as a connection pool), create those shared resources within a with stack_context.NullContext(): block. This will prevent StackContexts from leaking from one request to another.
  • If you want to write something like an exception handler that will persist across asynchronous calls, create a new StackContext (or ExceptionStackContext), and make your asynchronous calls in a with block that references your StackContext.
class tornado.stack_context.StackContext(context_factory, _active_cell=None)[source]

Establishes the given context as a StackContext that will be transferred.

Note that the parameter is a callable that returns a context manager, not the context itself. That is, where for a non-transferable context manager you would say:

with my_context():

StackContext takes the function itself rather than its result:

with StackContext(my_context):

The result of with StackContext() as cb: is a deactivation callback. Run this callback when the StackContext is no longer needed to ensure that it is not propagated any further (note that deactivating a context does not affect any instances of that context that are currently pending). This is an advanced feature and not necessary in most applications.

class tornado.stack_context.ExceptionStackContext(exception_handler, _active_cell=None)[source]

Specialization of StackContext for exception handling.

The supplied exception_handler function will be called in the event of an uncaught exception in this context. The semantics are similar to a try/finally clause, and intended use cases are to log an error, close a socket, or similar cleanup actions. The exc_info triple (type, value, traceback) will be passed to the exception_handler function.

If the exception handler returns true, the exception will be consumed and will not be propagated to other exception handlers.

class tornado.stack_context.NullContext[source]

Resets the StackContext.

Useful when creating a shared resource on demand (e.g. an AsyncHTTPClient) where the stack that caused the creating is not relevant to future operations.

tornado.stack_context.wrap(fn)[source]

Returns a callable object that will restore the current StackContext when executed.

Use this whenever saving a callback to be executed later in a different execution context (either in a different thread or asynchronously in the same thread).

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