grass.pydispatch.saferef — GRASS Python Library documentation

# Source code for grass.pydispatch.saferef

    """Refactored "safe reference" from dispatcher.py"""

    import weakref
    import traceback
    import sys

    im_func = "__func__"
    im_self = "__self__"

    def safeRef(target, onDelete=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.
        onDelete -- 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 or a BoundMethodWeakref) as argument.
        """
        if hasattr(target, im_self):
            if getattr(target, im_self) is not None:
                # Turn a bound method into a BoundMethodWeakref instance.
                # Keep track of these instances for lookup by disconnect().
                assert hasattr(target, im_func), (
                    """safeRef target %r has %s, """
                    """but no %s, don't know how """
                    """to create reference""" % (target, im_self, im_func)
                )
                reference = BoundMethodWeakref(target=target, onDelete=onDelete)
                return reference
        if onDelete is not None:
            return weakref.ref(target, onDelete)
        else:
            return weakref.ref(target)

    class BoundMethodWeakref:
        """'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 -- 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.
            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.

        """

        _allInstances = weakref.WeakValueDictionary()

        def __new__(cls, target, onDelete=None, *arguments, **named):
            """Create new instance or return current instance

            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 is 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.calculateKey(target)
            current = cls._allInstances.get(key)
            if current is not None:
                current.deletionMethods.append(onDelete)
                return current
            else:
                base = super().__new__(cls)
                cls._allInstances[key] = base
                base.__init__(target, onDelete, *arguments, **named)
                return base

        def __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:
                    target.<im_func>.__get__( target.<im_self> )
                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.
            """

            def remove(weak, self=self):
                """Set self.isDead to true when method or instance is destroyed"""
                methods = self.deletionMethods[:]
                del self.deletionMethods[:]
                try:
                    del self.__class__._allInstances[self.key]
                except KeyError:
                    pass
                for function in methods:
                    try:
                        if hasattr(function, "__call__"):
                            function(self)
                    except Exception as e:
                        try:
                            traceback.print_exc()
                        except AttributeError:
                            print(
                                """Exception during saferef %s cleanup """
                                """function %s: %s""" % (self, function, e),
                                file=sys.stderr,
                            )

            self.deletionMethods = [onDelete]
            self.key = self.calculateKey(target)
            self.weakSelf = weakref.ref(getattr(target, im_self), remove)
            self.weakFunc = weakref.ref(getattr(target, im_func), remove)
            self.selfName = getattr(target, im_self).__class__.__name__
            self.funcName = str(getattr(target, im_func).__name__)

        def 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.
            """
            return (id(getattr(target, im_self)), id(getattr(target, im_func)))

        calculateKey = classmethod(calculateKey)

        def __str__(self):
            """Give a friendly representation of the object"""
            return """%s( %s.%s )""" % (
                self.__class__.__name__,
                self.selfName,
                self.funcName,
            )

        __repr__ = __str__

        def __nonzero__(self):
            """Whether we are still a valid reference"""
            return self() is not None

        def __cmp__(self, other):
            """Compare with another reference"""
            if not isinstance(other, self.__class__):
                return cmp(self.__class__, type(other))
            return cmp(self.key, other.key)

        def __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.
            """
            target = self.weakSelf()
            if target is not None:
                function = self.weakFunc()
                if function is not None:
                    return function.__get__(target)
            return None

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