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# Greenlet Objects

`gevent.Greenlet` is a light-weight cooperatively-scheduled execution unit. It is a more powerful version of `greenlet.greenlet`. For general information, see Lightweight pseudothreads.

You can retrieve the current greenlet at any time using `gevent.getcurrent()`.

## Starting Greenlets

To start a new greenlet, pass the target function and its arguments to `Greenlet` constructor and call `Greenlet.start()`:

    >>> from gevent import Greenlet
    >>> def myfunction(arg1, arg2, kwarg1=None):
    ...    pass
    >>> g = Greenlet(myfunction, 'arg1', 'arg2', kwarg1=1)
    >>> g.start()

or use classmethod `Greenlet.spawn()` which is a shortcut that does the same:

    >>> g = Greenlet.spawn(myfunction, 'arg1', 'arg2', kwarg1=1)

There are also various spawn helpers in `gevent`, including:

- `gevent.spawn()`

- `gevent.spawn_later()`

- `gevent.spawn_raw()`

## Waiting For Greenlets

You can wait for a greenlet to finish with its `Greenlet.join()` method. There are helper functions to join multiple greenlets or heterogenous collections of objects:

- `gevent.joinall()`

- `gevent.wait()`

- `gevent.iwait()`

## Stopping Greenlets

You can forcibly stop a `Greenlet` using its `Greenlet.kill()` method. There are also helper functions that can be useful in limited circumstances (if you might have a `raw greenlet`):

- `gevent.kill()`

- `gevent.killall()`

## Context Managers

New in version 21.1.0.

Greenlets also function as context managers, so you can combine spawning and waiting for a greenlet to finish in a single line:

    >>> def in_greenlet():
    ...     print("In the greenlet")
    ...     return 42
    >>> with Greenlet.spawn(in_greenlet) as g:
    ...     print("In the with suite")
    In the with suite
    In the greenlet
    >>> g.get(block=False)
    42

Normally, the greenlet is joined to wait for it to finish, but if the body of the suite raises an exception, the greenlet is killed with that exception.

    >>> import gevent
    >>> try:
    ...    with Greenlet.spawn(gevent.sleep, 0.1) as g:
    ...       raise Exception("From with body")
    ... except Exception:
    ...     pass
    >>> g.dead
    True
    >>> g.successful()
    False
    >>> g.get(block=False)
    Traceback (most recent call last):
    ...
    Exception: From with body

## Subclassing Greenlet

To subclass a `Greenlet`, override its `_run()` method and call `Greenlet.__init__(self)` in the subclass `__init__`. This can be done to override `Greenlet.__str__()`: if `_run` raises an exception, its string representation will be printed after the traceback it generated.

    class MyNoopGreenlet(Greenlet):

        def __init__(self, seconds):
            Greenlet.__init__(self)
            self.seconds = seconds

        def _run(self):
            gevent.sleep(self.seconds)

        def __str__(self):
            return 'MyNoopGreenlet(%s)' % self.seconds

Important

You SHOULD NOT attempt to override the `run()` method.

## Boolean Contexts

Greenlet objects have a boolean value (`__nonzero__` or `__bool__`) which is true if it’s active: started but not dead yet.

It’s possible to use it like this:

    >>> g = gevent.spawn(...)
    >>> while g:
           # do something while g is alive

The Greenlet’s boolean value is an improvement on the raw `greenlet's` boolean value. The raw greenlet’s boolean value returns False if the greenlet has not been switched to yet or is already dead. While the latter is OK, the former is not good, because a just spawned Greenlet has not been switched to yet and thus would evaluate to False.

exception GreenletExit

  A special exception that kills the greenlet silently.

  When a greenlet raises `GreenletExit` or a subclass, the traceback is not printed and the greenlet is considered `successful`. The exception instance is available under `value` property as if it was returned by the greenlet, not raised.

class Greenlet

Greenlet.__init__(run=None, *args, **kwargs)
  Parameters:

    - args – The arguments passed to the `run` function.

    - kwargs – The keyword arguments passed to the `run` function.

    - run (callable) – The callable object to run. If not given, this object’s `_run` method will be invoked (typically defined by subclasses).

  Changed in version 1.1b1: The `run` argument to the constructor is now verified to be a callable object. Previously, passing a non-callable object would fail after the greenlet was spawned.

  Changed in version 1.3b1: The `GEVENT_TRACK_GREENLET_TREE` configuration value may be set to a false value to disable `spawn_tree_locals`, `spawning_greenlet`, and `spawning_stack`. The first two will be None in that case, and the latter will be empty.

  Changed in version 1.5: Greenlet objects are now more careful to verify that their `parent` is really a gevent hub, raising a `TypeError` earlier instead of an `AttributeError` later.

  Changed in version 20.12.1: Greenlet objects now function as context managers. Exiting the `with` suite ensures that the greenlet has completed by `joining` the greenlet (blocking, with no timeout). If the body of the suite raises an exception, the greenlet is `killed` with the default arguments and not joined in that case.

Attributes

Greenlet.exception

  Holds the exception instance raised by the function if the greenlet has finished with an error. Otherwise `None`.

Greenlet.minimal_ident

  A small, unique non-negative integer that identifies this object.

  This is similar to `threading.Thread.ident` (and `id`) in that as long as this object is alive, no other greenlet in this hub will have the same id, but it makes a stronger guarantee that the assigned values will be small and sequential. Sometime after this object has died, the value will be available for reuse.

  To get ids that are unique across all hubs, combine this with the hub’s (`self.parent`) `minimal_ident`.

  Accessing this property from threads other than the thread running this greenlet is not defined.

  New in version 1.3a2.

Greenlet.name = <gevent._util.readproperty object>

Greenlet.dead

  Boolean indicating that the greenlet is dead and will not run again.

  This is true if:

  1. We were never started, but were `killed` immediately after creation (not possible with `spawn()`); OR

  2. We were started, but were killed before running; OR

  3. We have run and terminated (by raising an exception out of the started function or by reaching the end of the started function).

Greenlet.value

  Holds the value returned by the function if the greenlet has finished successfully. Until then, or if it finished in error, `None`.

  Tip

  Recall that a greenlet killed with the default `GreenletExit` is considered to have finished successfully, and the `GreenletExit` exception will be its value.

Greenlet.spawn_tree_locals

  A dictionary that is shared between all the greenlets in a “spawn tree”, that is, a spawning greenlet and all its descendent greenlets. All children of the main (root) greenlet start their own spawn trees. Assign a new dictionary to this attribute on an instance of this class to create a new spawn tree (as far as locals are concerned).

  New in version 1.3a2.

Greenlet.spawning_greenlet

  A weak-reference to the greenlet that was current when this object was created. Note that the `parent` attribute is always the hub.

  New in version 1.3a2.

Greenlet.spawning_stack

  A lightweight `frame`-like object capturing the stack when this greenlet was created as well as the stack when the spawning greenlet was created (if applicable). This can be passed to `traceback.print_stack()`.

  New in version 1.3a2.

Greenlet.spawning_stack_limit

  A class attribute specifying how many levels of the spawning stack will be kept. Specify a smaller number for higher performance, spawning greenlets, specify a larger value for improved debugging.

  New in version 1.3a2.

Methods

classmethod Greenlet.spawn(function, *args, **kwargs) → Greenlet

  Create a new `Greenlet` object and schedule it to run `function(*args, **kwargs)`. This can be used as `gevent.spawn` or `Greenlet.spawn`.

  The arguments are passed to `Greenlet.__init__()`.

  Changed in version 1.1b1: If a function is given that is not callable, immediately raise a `TypeError` instead of spawning a greenlet that will raise an uncaught TypeError.

Greenlet.ready()

  Return a true value if and only if the greenlet has finished execution.

  Changed in version 1.1: This function is only guaranteed to return true or false values, not necessarily the literal constants `True` or `False`.

Greenlet.successful()

  Return a true value if and only if the greenlet has finished execution successfully, that is, without raising an error.

  Tip

  A greenlet that has been killed with the default `GreenletExit` exception is considered successful. That is, `GreenletExit` is not considered an error.

  Note

  This function is only guaranteed to return true or false values, not necessarily the literal constants `True` or `False`.

Greenlet.start()

  Schedule the greenlet to run in this loop iteration

Greenlet.start_later(seconds) → None

  Schedule the greenlet to run in the future loop iteration seconds later

Greenlet.join(timeout=None) → None

  Wait until the greenlet finishes or timeout expires. Return `None` regardless.

Greenlet.get(block=True, timeout=None) → object

  Return the result the greenlet has returned or re-raise the exception it has raised.

  If block is `False`, raise `gevent.Timeout` if the greenlet is still alive. If block is `True`, unschedule the current greenlet until the result is available or the timeout expires. In the latter case, `gevent.Timeout` is raised.

Greenlet.kill(exception=GreenletExit, block=True, timeout=None)

  Raise the `exception` in the greenlet.

  If `block` is `True` (the default), wait until the greenlet dies or the optional timeout expires; this may require switching greenlets. If block is `False`, the current greenlet is not unscheduled.

  This function always returns `None` and never raises an error. It may be called multpile times on the same greenlet object, and may be called on an unstarted or dead greenlet.

  Note

  Depending on what this greenlet is executing and the state of the event loop, the exception may or may not be raised immediately when this greenlet resumes execution. It may be raised on a subsequent green call, or, if this greenlet exits before making such a call, it may not be raised at all. As of 1.1, an example where the exception is raised later is if this greenlet had called `sleep(0)`; an example where the exception is raised immediately is if this greenlet had called `sleep(0.1)`.

  Caution

  Use care when killing greenlets. If the code executing is not exception safe (e.g., makes proper use of `finally`) then an unexpected exception could result in corrupted state. Using a `link()` or `rawlink()` (cheaper) may be a safer way to clean up resources.

  See also `gevent.kill()` and `gevent.killall()`.

  Parameters:

    exception (type) – The type of exception to raise in the greenlet. The default is `GreenletExit`, which indicates a `successful()` completion of the greenlet.

  Changed in version 0.13.0: block is now `True` by default.

  Changed in version 1.1a2: If this greenlet had never been switched to, killing it will prevent it from ever being switched to. Links (`rawlink()`) will still be executed, though.

  Changed in version 20.12.1: If this greenlet is `ready()`, immediately return instead of requiring a trip around the event loop.

Greenlet.link(callback)

  Link greenlet’s completion to a callable.

  The callback will be called with this instance as an argument once this greenlet is dead. A callable is called in its own `greenlet.greenlet` (not a `Greenlet`).

  The callback will be called even if linked after the greenlet is already ready().

Greenlet.link_value(callback)

  Like `link()` but callback is only notified when the greenlet has completed successfully.

Greenlet.link_exception(callback)

  Like `link()` but callback is only notified when the greenlet dies because of an unhandled exception.

Greenlet.rawlink(callback)

  Register a callable to be executed when the greenlet finishes execution.

  The callback will be called with this instance as an argument.

  The callback will be called even if linked after the greenlet is already ready().

  Caution

  The callback will be called in the hub and MUST NOT raise an exception.

Greenlet.unlink(callback)

  Remove the callback set by `link()` or `rawlink()`

Greenlet.__str__()

  Return str(self).

static Greenlet.add_spawn_callback(callback) → None

  Set up a callback to be invoked when `Greenlet` objects are started.

  The invocation order of spawn callbacks is unspecified. Adding the same callback more than one time will not cause it to be called more than once.

  New in version 1.4.0.

static Greenlet.remove_spawn_callback(callback) → None

  Remove callback function added with `Greenlet.add_spawn_callback()`. This function will not fail if callback has been already removed or if callback was never added.

  New in version 1.4.0.

## Raw greenlet Methods

Being a greenlet subclass, `Greenlet` also has switch() and throw() methods. However, these should not be used at the application level as they can very easily lead to greenlets that are forever unscheduled. Prefer higher-level safe classes, like `Event` and `Queue`, instead.

class greenlet.greenlet

  The base class from which `Greenlet` descends.