So far we have only considered Future objects created by asynchronous computations started using the Future method. A failure category name and other further details may be provided that will be returned by the failure method in list context.
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There are two kinds of futures traders: hedgers and speculators. Hedgers do not usually seek a profit by trading commodities ev but rather seek to stabilize the revenues or costs of their business operations. Their gains or losses are usually offset to some degree by a corresponding loss or gain in the market for the underlying physical commodity.
Event loops use cooperative scheduling: an event loop runs one Task at a time. While a Task awaits for the completion of a Future, the event loop runs other Tasks, callbacks, or performs IO operations.
While futures are defined as a type of read-only placeholder object created for a result which doesn’t yet exist, a promise can be thought of as a writable, single-assignment container, which completes a future. That is, a promise can be used to successfully complete a future with a value (by completing” the promise) using the success method. Conversely, a promise can also be used to complete a future with an exception, by failing the promise, using the failure method.
which is a bit harder to grasp than the for-comprehension, but we analyze it to better understand the flatMap operation. The flatMap operation maps its own value into some other future. Once this different future is completed, the resulting future is completed with its value. In our example, flatMap uses the value of the usdQuote future to map the value of the chfQuote into a third future which sends a request to buy a certain amount of Swiss francs. The resulting future purchase is completed only once this third future returned from map completes.
The point of cancellation propagation is to trace backwards through stages of some larger sequence of operations that now no longer need to happen, because the final result is no longer required. But in each of these cases, just because $f1 has been cancelled, the initial future $f_initial is still required because there is another future ($f2) that will still require its result.
If the future is not yet ready, adds a callback to be invoked when the future is ready, if it fails. If the future has already failed, invokes it immediately. If it completed successfully or was cancelled, it is not invoked at all.
The following methods all return a new future to represent the combination of its invocant followed by another action given by a code reference. The combined activity waits for the first future to be ready, then may invoke the code depending on the success or failure of the first, or may run it regardless. The returned sequence future represents the entire combination of activity.
The recoverWith combinator creates a new future which holds the same result as the original future if it completed successfully. Otherwise, the partial function is applied to the Throwable which failed the original future. If it maps the Throwable to some future, then this future is completed with the result of that future. Its relation to recover is similar to that of flatMap to map.
An included script called futurize aids in converting code (from either Python 2 or Python 3) to code compatible with both platforms. It is similar to python-modernize but goes further in providing Python 3 compatibility through the use of the backported types and builtin functions in future.
A Future represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future> and return null as a result of the underlying task.
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Futures provide a way to reason about performing many operations in parallel- in an efficient and non-blocking way. A Future is a placeholder object for a value that may not yet exist. Generally, the value of the Future is supplied concurrently and can subsequently be used. Composing concurrent tasks in this way tends to result in faster, asynchronous, non-blocking parallel code.
If the computation has completed with a value or with an exception, we say that the Future is completed. If the Future is already done or cancelled, return False. Otherwise, change the Future’s state to cancelled, schedule the callbacks, and return True.
Return the result of the Task. Calls the provided asynchronous function getNewUsername() and returns its result. If an instance overrides the “block_until_ready” method, this will be called by get and failure if the instance is still pending.
If the future was cancelled an exception is thrown. a coroutine object: an object returned by calling a coroutine function. In summary, the combinators on futures are purely functional. Every combinator returns a new future which is related to the future it was derived from.
Returns the $future. Return True if the Task is cancelled. The function will return when any future finishes or is cancelled. Registering a foreach callback has the same semantics as onComplete, with the difference that the closure is only called if the future is completed successfully.
By default all tasks for the current event loop are returned. If loop is None, the get_event_loop() function is used to get the current loop. Convenient shortcuts to returning an immediate future from a then block, when the result is already known.
When given an empty list this constructor returns a new immediately-done future. Changed in version 3.7: When aw is cancelled due to a timeout, wait_for waits for aw to be cancelled. Previously, it raised asyncio.TimeoutError immediately.
The following methods query the internal state of a Future instance without modifying it or otherwise causing side-effects. unlike asyncio Futures, concurrent.futures.Future instances cannot be awaited. If the Task has been cancelled, this method raises a CancelledError exception.
But what happens if isProfitable returns false, hence causing an exception to be thrown? In that case purchase is failed with that exception. Furthermore, imagine that the connection was broken and that getCurrentValue threw an exception, failing rateQuote. In that case we’d have no value to map, so the purchase would automatically be failed with the same exception as rateQuote.
If successful, its result will be that of the first component future that succeeded. If it fails, its failure will be that of the last component future to fail. To access the other failures, use failed_futures.
In some cases the client may want to complete the promise only if it has not been completed yet (e.g., there are several HTTP requests being executed from several different futures and the client is interested only in the first HTTP response – corresponding to the first future to complete the promise). For these reasons methods tryComplete, trySuccess and tryFailure exist on promise. The client should be aware that using these methods results in programs which are not deterministic, but depend on the execution schedule.
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For example, the following wrapper saves the value of a package variable at the time the callback was saved, and restores that value at invocation time later on. This could be useful for preserving context during logging in a Future-based program.