# `BoundMethodWeakref`

#### 'Safe' and reusable weak references to instance methods

The BoundMethodWeakref class is accessible via the `sqlobject.include.pydispatch.saferef` module.

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 -- the identity key for the reference, calculated
    by the class's calculateKey method applied to the target instance method
  deletionMethods -- 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 onDelete parameters of safeRef calls.

  System Message: WARNING/2 (`sqlobject.include.pydispatch.saferef.BoundMethodWeakref`, line 18)

  Definition list ends without a blank line; unexpected unindent.

  weakSelf -- weak reference to the target object weakFunc -- weak reference to the target function

Class Attributes:
  _allInstances -- class attribute pointing to all live
    BoundMethodWeakref objects indexed by the class's calculateKey(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.

## Methods

#### f `__format__(...)` ...

default object formatter

#### f `__subclasshook__(...)` ...

Abstract classes can override this to customize issubclass().

This is invoked early on by abc.ABCMeta.__subclasscheck__(). It should return True, False or NotImplemented. If it returns NotImplemented, the normal algorithm is used. Otherwise, it overrides the normal algorithm (and the outcome is cached).

#### f `__sizeof__(...)` ...

__sizeof__() -> int size of object in memory, in bytes

#### f `__init__(self, target, onDelete=None)` ...

Return a weak-reference-like instance for a bound method

target -- the instance-method target for the weak

  reference, must have im_self and im_func attributes and be reconstructable via:

  System Message: ERROR/3 (`sqlobject.include.pydispatch.saferef.BoundMethodWeakref.__init__`, line 4)

  Unexpected indentation.

  > target.im_func.__get__( target.im_self )

  System Message: WARNING/2 (`sqlobject.include.pydispatch.saferef.BoundMethodWeakref.__init__`, line 5)

  Block quote ends without a blank line; unexpected unindent.

  which is true of built-in instance methods.

onDelete -- 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.

#### f `calculateKey(cls, 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.

#### f `__nonzero__(self)` ...

Whether we are still a valid reference

#### f `__cmp__(self, other)` ...

Compare with another reference

#### f `__call__(self)` ...

Return a strong reference to the bound method

If the target cannot be retrieved, then will return None, otherwise returns 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.

See the source for more information.