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File: //usr/local/lib/python3.9/site-packages/vine/__pycache__/promises.cpython-39.pyc
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�Zdd�Zdd�Z
ddd�Zddd�Zddd�Zedd��ZdS)ra~Promise of future evaluation.

    This is a special implementation of promises in that it can
    be used both for "promise of a value" and lazy evaluation.
    The biggest upside for this is that everything in a promise can also be
    a promise, e.g. filters, callbacks and errbacks can all be promises.

    Usage examples:

    .. code-block:: python

        >>> p = promise()
        >>> p.then(promise(print, ('OK',)))  # noqa
        >>> p.on_error = promise(print, ('ERROR',))  # noqa
        >>> p(20)
        OK, 20
        >>> p.then(promise(print, ('hello',)))  # noqa
        hello, 20


        >>> p.throw(KeyError('foo'))
        ERROR, KeyError('foo')


        >>> p2 = promise()
        >>> p2.then(print)  # noqa
        >>> p2.cancel()
        >>> p(30)

    Example:
    .. code-block:: python

        from vine import promise, wrap

        class Protocol:

            def __init__(self):
                self.buffer = []

            def receive_message(self):
                return self.read_header().then(
                    self.read_body).then(
                        wrap(self.prepare_body))

            def read(self, size, callback=None):
                callback = callback or promise()
                tell_eventloop_to_read(size, callback)
                return callback

            def read_header(self, callback=None):
                return self.read(4, callback)

            def read_body(self, header, callback=None):
                body_size, = unpack('>L', header)
                return self.read(body_size, callback)

            def prepare_body(self, value):
                self.buffer.append(value)
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