Class chain

celery/canvas.py:1308–1368

Chain tasks together. Each tasks follows one another, by being applied as a callback of the previous task. Note: If called with only one argument, then that argument must be an iterable of tasks to chain: this allows us to use generator expressions. Example

class chain(_chain):
    """Chain tasks together.

    Each tasks follows one another,
    by being applied as a callback of the previous task.

    Note:
        If called with only one argument, then that argument must
        be an iterable of tasks to chain: this allows us
        to use generator expressions.

    Example:
        This is effectively :math:`((2 + 2) + 4)`:

        .. code-block:: pycon

            >>> res = chain(add.s(2, 2), add.s(4))()
            >>> res.get()
            8

        Calling a chain will return the result of the last task in the chain.
        You can get to the other tasks by following the ``result.parent``'s:

        .. code-block:: pycon

            >>> res.parent.get()
            4

        Using a generator expression:

        .. code-block:: pycon

            >>> lazy_chain = chain(add.s(i) for i in range(10))
            >>> res = lazy_chain(3)

    Arguments:
        *tasks (Signature): List of task signatures to chain.
            If only one argument is passed and that argument is
            an iterable, then that'll be used as the list of signatures
            to chain instead.  This means that you can use a generator
            expression.

    Returns:
        ~celery.chain: A lazy signature that can be called to apply the first
            task in the chain.  When that task succeeds the next task in the
            chain is applied, and so on.
    """

    # could be function, but must be able to reference as :class:`chain`.
    def __new__(cls, *tasks, **kwargs):
        # This forces `chain(X, Y, Z)` to work the same way as `X | Y | Z`
        if not kwargs and tasks:
            if len(tasks) != 1 or is_list(tasks[0]):
                tasks = tasks[0] if len(tasks) == 1 else tasks
                # if is_list(tasks) and len(tasks) == 1:
                #     return super(chain, cls).__new__(cls, tasks, **kwargs)
                new_instance = reduce(operator.or_, tasks, _chain())
                if cls != chain and isinstance(new_instance, _chain) and not isinstance(new_instance, cls):