This document describes Celery 2.4. For development docs, go here.


“Safe weakrefs”, originally from pyDispatcher.

Provides a way to safely weakref any function, including bound methods (which aren’t handled by the core weakref module).

class celery.utils.dispatch.saferef.BoundMethodWeakref(target, on_delete=None)

‘Safe’ and reusable weak references to instance methods.

BoundMethodWeakref objects provide a mechanism for referencing a bound method without requiring that the method object itself (which is normally a transient object) is kept alive. Instead, the BoundMethodWeakref object keeps weak references to both the object and the function which together define the instance method.


the identity key for the reference, calculated by the class’s calculate_key() method applied to the target instance method


sequence of callable objects taking single argument, a reference to this object which will be called when either the target object or target function is garbage collected (i.e. when this object becomes invalid). These are specified as the on_delete parameters of safe_ref() calls.


weak reference to the target object


weak reference to the target function


class attribute pointing to all live BoundMethodWeakref objects indexed by the class’s calculate_key(target) method applied to the target objects. This weak value dictionary is used to short-circuit creation so that multiple references to the same (object, function) pair produce the same BoundMethodWeakref instance.

classmethod calculate_key(target)

Calculate the reference key for this reference

Currently this is a two-tuple of the id()‘s of the target object and the target function respectively.

class celery.utils.dispatch.saferef.BoundNonDescriptorMethodWeakref(target, on_delete=None)

A specialized BoundMethodWeakref, for platforms where instance methods are not descriptors.

It assumes that the function name and the target attribute name are the same, instead of assuming that the function is a descriptor. This approach is equally fast, but not 100% reliable because functions can be stored on an attribute named differenty than the function’s name such as in:

>>> class A(object):
...     pass

>>> def foo(self):
...     return "foo"
>>> = foo

But this shouldn’t be a common use case. So, on platforms where methods aren’t descriptors (such as Jython) this implementation has the advantage of working in the most cases.

celery.utils.dispatch.saferef.get_bound_method_weakref(target, on_delete)

Instantiates the appropiate BoundMethodWeakRef, depending on the details of the underlying class method implementation.

celery.utils.dispatch.saferef.safe_ref(target, on_delete=None)

Return a safe weak reference to a callable target

  • target – the object to be weakly referenced, if it’s a bound method reference, will create a BoundMethodWeakref, otherwise creates a simple weakref.ref.
  • on_delete – if provided, will have a hard reference stored to the callable to be called after the safe reference goes out of scope with the reference object, (either a weakref.ref or a BoundMethodWeakref) as argument.