This document describes the current stable version of Celery (4.0). For development docs, go here.
Source code for celery.utils.dispatch.saferef
# -*- coding: utf-8 -*-
"""Safe weakrefs, originally from :pypi:`pyDispatcher`.
Provides a way to safely weakref any function, including bound methods (which
aren't handled by the core weakref module).
"""
from __future__ import absolute_import, unicode_literals
import sys
import traceback
import weakref
from celery.five import python_2_unicode_compatible
__all__ = ['safe_ref']
PY3 = sys.version_info[0] == 3
[docs]def safe_ref(target, on_delete=None): # pragma: no cover
"""Return a *safe* weak reference to a callable target.
Arguments:
target (Any): The object to be weakly referenced, if it's a
bound method reference, will create a :class:`BoundMethodWeakref`,
otherwise creates a simple :class:`weakref.ref`.
on_delete (Callable): 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
:class:`weakref.ref` or a :class:`BoundMethodWeakref`) as argument.
"""
if getattr(target, '__self__', None) is not None:
# Turn a bound method into a BoundMethodWeakref instance.
# Keep track of these instances for lookup by disconnect().
assert hasattr(target, '__func__'), \
"""safe_ref target {0!r} has __self__, but no __func__: \
don't know how to create reference""".format(target)
return get_bound_method_weakref(target=target,
on_delete=on_delete)
if callable(on_delete):
return weakref.ref(target, on_delete)
else:
return weakref.ref(target)
@python_2_unicode_compatible
class BoundMethodWeakref(object): # pragma: no cover
"""'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.
Attributes:
key (str): the identity key for the reference, calculated
by the class's :meth:`calculate_key` method applied to the
target instance method.
deletion_methods (Sequence[Callable]): Callables 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 :func:`safe_ref` calls.
weak_self (weakref.ref): weak reference to the target object.
weak_fun (weakref.ref): weak reference to the target function
_all_instances (weakref.WeakValueDictionary):
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.
"""
_all_instances = weakref.WeakValueDictionary()
def __new__(cls, target, on_delete=None, *arguments, **named):
"""Create new instance or return current instance.
Note:
Basically this method of construction allows us to
short-circuit creation of references to already-
referenced instance methods. The key corresponding
to the target is calculated, and if there's already
an existing reference, that is returned, with its
deletionMethods attribute updated. Otherwise the
new instance is created and registered in the table
of already-referenced methods.
"""
key = cls.calculate_key(target)
current = cls._all_instances.get(key)
if current is not None:
current.deletion_methods.append(on_delete)
return current
else:
base = super(BoundMethodWeakref, cls).__new__(cls)
cls._all_instances[key] = base
base.__init__(target, on_delete, *arguments, **named)
return base
def __init__(self, target, on_delete=None):
"""Return a weak-reference-like instance for a bound method.
Arguments:
target (Any): The instance-method target for the weak
reference, must have `__self__` and `__func__` attributes
and be reconstructable via::
target.__func__.__get__(target.__self__)
which is true of built-in instance methods.
on_delete (Callable): Optional callback which will be called
when this weak reference ceases to be valid
(i.e., either the object or the function is garbage
collected). Should take a single argument,
which will be passed a pointer to this object.
"""
def remove(weak, self=self):
"""Set is_dead to true when method or instance is destroyed."""
methods = self.deletion_methods[:]
del(self.deletion_methods[:])
try:
del(self.__class__._all_instances[self.key])
except KeyError:
pass
for function in methods:
try:
if callable(function):
function(self)
except Exception as exc:
try:
traceback.print_exc()
except AttributeError:
print('Exception during saferef {0} cleanup function '
'{1}: {2}'.format(self, function, exc))
self.deletion_methods = [on_delete]
self.key = self.calculate_key(target)
self.weak_self = weakref.ref(target.__self__, remove)
self.weak_fun = weakref.ref(target.__func__, remove)
self.self_name = str(target.__self__)
self.fun_name = str(target.__func__.__name__)
def calculate_key(cls, target):
"""Calculate the reference key for this reference.
Returns:
Tuple[int, int]: Currently this is a two-tuple of
the `id()`'s of the target object and the target
function respectively.
"""
return id(target.__self__), id(target.__func__)
calculate_key = classmethod(calculate_key)
def __str__(self):
return '{0}( {1}.{2} )'.format(
type(self).__name__,
self.self_name,
self.fun_name,
)
def __repr__(self):
return str(self)
def __bool__(self):
"""Whether we're still a valid reference."""
return self() is not None
__nonzero__ = __bool__ # py2
if not PY3:
def __cmp__(self, other):
"""Compare with another reference."""
if not isinstance(other, self.__class__):
return cmp(self.__class__, type(other)) # noqa
return cmp(self.key, other.key) # noqa
def __call__(self):
"""Return a strong reference to the bound method.
If the target cannot be retrieved, then will
return None, otherwise return a bound instance
method for our object and function.
Note:
You may call this method any number of times,
as it does not invalidate the reference.
"""
target = self.weak_self()
if target is not None:
function = self.weak_fun()
if function is not None:
return function.__get__(target)
class BoundNonDescriptorMethodWeakref(BoundMethodWeakref): # pragma: no cover
"""A specialized :class:`BoundMethodWeakref`.
For platforms where instance methods are not descriptors.
Warning:
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'
>>> A.bar = foo
This shouldn't be a common use case. So, on platforms where methods
aren't descriptors (e.g., Jython) this implementation has the
advantage of working in the most cases.
"""
def __init__(self, target, on_delete=None):
"""Return a weak-reference-like instance for a bound method.
Arguments:
target (Any): the instance-method target for the weak
reference, must have `__self__` and `__func__` attributes
and be reconstructable via::
target.__func__.__get__(target.__self__)
which is true of built-in instance methods.
on_delete (Callable): Optional callback which will be called
when this weak reference ceases to be valid
(i.e., either the object or the function is garbage
collected). Should take a single argument,
which will be passed a pointer to this object.
"""
assert getattr(target.__self__, target.__name__) == target
super(BoundNonDescriptorMethodWeakref, self).__init__(target,
on_delete)
def __call__(self):
"""Return a strong reference to the bound method.
If the target cannot be retrieved, then will
return None, otherwise return a bound instance
method for our object and function.
Note:
You may call this method any number of times,
as it does not invalidate the reference.
"""
target = self.weak_self()
if target is not None:
function = self.weak_fun()
if function is not None:
# Using curry() would be another option, but it erases the
# "signature" of the function. That is, after a function is
# curried, the inspect module can't be used to determine how
# many arguments the function expects, nor what keyword
# arguments it supports, and pydispatcher needs this
# information.
return getattr(target, function.__name__)
def get_bound_method_weakref(target, on_delete): # pragma: no cover
"""Instantiate the appropiate :class:`BoundMethodWeakRef`.
Depending on the details of the underlying class method
implementation.
"""
if hasattr(target, '__get__'):
# target method is a descriptor, so the default implementation works:
return BoundMethodWeakref(target=target, on_delete=on_delete)
else:
# no luck, use the alternative implementation:
return BoundNonDescriptorMethodWeakref(
target=target, on_delete=on_delete)
