Python API Reference#


exception greenlet.GreenletExit#

Bases: BaseException

exception greenlet.error#

Bases: Exception


greenlet.getcurrent() greenlet#

Returns the current greenlet (i.e. the one which called this function).

class greenlet.greenlet(run=None, parent=None)#

Bases: object

Creates a new greenlet object (without running it).

  • run – The callable to invoke.

  • parent – The parent greenlet. The default is the current greenlet.

Greenlets support boolean tests: bool(g) is true if g is active and false if it is dead or not yet started.

switch(*args, **kwargs)#

Switches execution to this greenlet. See Switching Between Greenlets: Passing Objects and Control.


Switches execution to this greenlet, but immediately raises the given exception in this greenlet. If no argument is provided, the exception defaults to greenlet.GreenletExit. The normal exception propagation rules apply, as described for switch. Note that calling this method is almost equivalent to the following:

def raiser():
    raise typ, val, tb
g_raiser = greenlet(raiser, parent=g)

except that this trick does not work for the greenlet.GreenletExit exception, which would not propagate from g_raiser to g.


True if this greenlet is dead (i.e., it finished its execution).


The contextvars.Context in which g will run. Writable; defaults to None, reflecting that a greenlet starts execution in an empty context unless told otherwise. Generally, this should only be set once, before a greenlet begins running. Accessing or modifying this attribute raises AttributeError on Python versions 3.6 and earlier (which don’t natively support the contextvars module) or if greenlet was built without contextvars support.

For more information, see Context Variables (asyncio).

New in version 1.0.0.


The frame that was active in this greenlet when it most recently called some_other_greenlet.switch(), and that will resume execution when this_greenlet.switch() is next called. The remainder of the greenlet’s stack can be accessed by following the frame object’s f_back attributes. gr_frame is non-None only for suspended greenlets; it is None if the greenlet is dead, not yet started, or currently executing.


Greenlet stack introspection is fragile on CPython 3.12 and later. The frame objects of a suspended greenlet are not safe to access as-is, but must be adjusted by the greenlet package in order to make traversing f_back links not crash the interpreter, and restored to their original state when resuming the greenlet. The intent is to handle this transparently, but it does introduce additional overhead to switching greenlets, and there may be obscure usage patterns that can still crash the interpreter; if you find one of these, please report it to the maintainer.


The parent greenlet. This is writable, but it is not allowed to create cycles of parents.

A greenlet without a parent is the main greenlet of its thread.

Cannot be set to anything except a greenlet.


The callable that this greenlet will run when it starts. After it is started, this attribute no longer exists.

Subclasses can define this as a method on the type.


For details on tracing, see Tracing And Profiling.

greenlet.gettrace() object#

Returns the currently set tracing function, or None.

greenlet.settrace(callback) object#

Sets a new tracing function and returns the previous one.


callback – A callable object with the signature callback(event, args).