Tasklets — Lightweight threads

Tasklets wrap functions, allowing them to be launched as microthreads to be run within the scheduler.

Launching a tasklet:

stackless.tasklet(callable)(*args, **kwargs)

That is the most common way of launching a tasklet. This does not just create a tasklet, but it also automatically inserts the created tasklet into the scheduler.

Example - launching a more concrete tasklet:

>>> def func(*args, **kwargs):
...     print "scheduled with", args, "and", kwargs
>>> stackless.tasklet(func)(1, 2, 3, string="test")
<stackless.tasklet object at 0x01C58030>
>>> stackless.run()
scheduled with (1, 2, 3) and {'string': 'test'}

Tasklets, main, current and more

There are two especially notable tasklets, the main tasklet and the current tasklet.

The main tasklet is fixed, and it is the initial thread of execution of your application. Chances are that it is running the scheduler.

The current tasklet however, is the tasklet that is currently running. It might be the main tasklet, if no other tasklets are being run. Otherwise, it is the entry in the scheduler’s chain of runnable tasklets, that is currently executing.

Example - is the main tasklet the current tasklet:

stackless.main == stackless.current

Example - is the current tasklet the main tasklet:

stackless.current.is_main == 1

Example - how many tasklets are scheduled:



The main tasklet factors into the stackless.runcount value. If you are checking how many tasklets are in the scheduler from your main loop, you need to keep in mind that there will be another tasklet in there over and above the ones you explicitly created.

The tasklet class

class tasklet(callable=None)

This class exposes the form of lightweight thread (the tasklet) provided by Stackless Python. Wrapping a callable object and arguments to pass into it when it is invoked, the callable is run within the tasklet.

Tasklets are usually created in the following manner:

>>> stackless.tasklet(func)(1, 2, 3, name="test")

Note that there is no need to hold a reference to the created tasklet. This is because the act of passing the arguments to it, implicitly inserts it into the scheduler.


Bind the tasklet to the given callable object, callable:

>>> t = stackless.tasklet()
>>> t.bind(func)

In most every case, programmers will instead pass func into the tasklet constructor:

>>> t = stackless.tasklet(func)

Note that the tasklet cannot be run until it has been provided with arguments to call callable with, through a subsequent call to :meth:tasklet.setup.

tasklet.setup(*args, **kwargs)

Provide the tasklet with arguments to pass into its bound callable:

>>> t = stackless.tasklet()
>>> t.bind(func)
>>> t.setup(1, 2, name="test")

In most every case, programmers will instead pass the arguments and callable into the tasklet constructor instead:

>>> t = stackless.tasklet(func)(1, 2, name="test")

Note that when tasklets have been bound to a callable object and provided with arguments to pass to it, they are implicitly scheduled and will be run in turn when the scheduler is next run.

Insert the tasklet into the scheduler.
Remove the tasklet from the scheduler.

If the tasklet is alive and not blocked on a channel, then it will be run immediately. However, this behaves differently depending on whether the tasklet is in the scheduler’s chain of runnable tasklets.

Example - running a tasklet that is scheduled:

>>> def f():
...     while 1:
...             print id(stackless.current)
...             stackless.schedule()
>>> t1 = stackless.tasklet(f)()
>>> t2 = stackless.tasklet(f)()
>>> t3 = stackless.tasklet(f)()
>>> t1.run()

What you see here is that t1 is not the only tasklet that ran. When t1 yields, the next tasklet in the chain is scheduled and so forth until the tasklet that actually ran t1 is scheduled and resumes execution.

If you were to run t2 instead of t1, then we would have only seen the output of t2 and t3, because the tasklet calling run is before t1 in the chain.

Removing the tasklet to be run from the scheduler before it is actually run, gives more predictable results as shown in the following example. But keep in mind that the scheduler is still being run and the chain is still involved, the only reason it looks correct is tht the act of removing the tasklet effectively moves it before the tasklet that calls remove.

Example - running a tasklet that is not scheduled:

>>> t2.remove()
<stackless.tasklet object at 0x01C287B0>
>>> t2.run()

While the ability to run a tasklet directly is useful on occasion, that the scheduler is still involved and that this is merely directing its operation in limited ways, is something you need to be aware of.

tasklet.raise_exception(exc_class, *args)

Raise an exception on the given tasklet. exc_class is required to be a sub-class of Exception. It is instantiated with the given arguments args and raised within the given tasklet.

In order to make best use of this function, you should be familiar with how tasklets and the scheduler deal with exceptions, and the purpose of the TaskletExit exception.


This method is used to construct a block of code within which the tasklet will not be auto-scheduled when pre-emptive scheduling. It is useful for wrapping critical sections that should not be interrupted:

old_value = t.set_atomic(1)
# Implement unsafe logic here.

It is probably best not to use this until you understand nesting levels:

old_value = t.set_ignore_nesting(1)
# Implement unsafe logic here.

The following (read-only) attributes allow tasklet state to be checked:

This attribute is True while a tasklet is still running. Tasklets that are not running will most likely have either run to completion and exited, or will have unexpectedly exited through an exception of some kind.
This attribute is True when a tasklet is alive, but not scheduled or blocked on a channel.
This attribute is True when a tasklet is blocked on a channel.
This attribute is True when the tasklet is either scheduled or blocked on a channel.

This attribute is relevant to tasklets that were created through unpickling.

If it is possible to continue running the unpickled tasklet from whatever point in execution it may be, then this attribute will be True. For the tasklet to be runnable, it must not have lost runtime information (C stack usage for instance).

The following attributes allow checking of user set situations:

This attribute is True while this tasklet is within a tasklet.set_atomic() block
Setting this attribute to True prevents the tasklet from being blocked on a channel.
This attribute is True while this tasklet is within a tasklet.set_ignore_nesting() block

The following attributes allow identification of tasklet place:

This attribute is True if the tasklet is the current tasklet.
This attribute is True if the tasklet is the main tasklet.

This attribute is the id of the thread the tasklet belongs to.

The relationship between tasklets and threads is covered elsewhere.

In almost every case, tasklets will be linked into a chain of tasklets. This might be the scheduler itself, otherwise it will be a channel the tasklet is blocked on.

The following attributes allow a tasklets place in a chain to be identified:

The previous tasklet in the chain that this tasklet is linked into.
The next tasklet in the chain that this tasklet is linked into.

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