cpython/Lib/weakref.py

"""Weak reference support for Python.

This module is an implementation of PEP 205:

http://www.python.org/dev/peps/pep-0205/
"""

# Naming convention: Variables named "wr" are weak reference objects;
# they are called this instead of "ref" to avoid name collisions with
# the module-global ref() function imported from _weakref.

from _weakref import (
     getweakrefcount,
     getweakrefs,
     ref,
     proxy,
     CallableProxyType,
     ProxyType,
     ReferenceType)

from _weakrefset import WeakSet, _IterationGuard

import collections  # Import after _weakref to avoid circular import.

ProxyTypes = (ProxyType, CallableProxyType)

__all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
           "WeakKeyDictionary", "ReferenceType", "ProxyType",
           "CallableProxyType", "ProxyTypes", "WeakValueDictionary",
           "WeakSet", "WeakMethod"]


class WeakMethod(ref):
    """
    A custom `weakref.ref` subclass which simulates a weak reference to
    a bound method, working around the lifetime problem of bound methods.
    """

    __slots__ = "_func_ref", "_meth_type", "_alive", "__weakref__"

    def __new__(cls, meth, callback=None):
        try:
            obj = meth.__self__
            func = meth.__func__
        except AttributeError:
            raise TypeError("argument should be a bound method, not {}"
                            .format(type(meth))) from None
        def _cb(arg):
            # The self-weakref trick is needed to avoid creating a reference
            # cycle.
            self = self_wr()
            if self._alive:
                self._alive = False
                if callback is not None:
                    callback(self)
        self = ref.__new__(cls, obj, _cb)
        self._func_ref = ref(func, _cb)
        self._meth_type = type(meth)
        self._alive = True
        self_wr = ref(self)
        return self

    def __call__(self):
        obj = super().__call__()
        func = self._func_ref()
        if obj is None or func is None:
            return None
        return self._meth_type(func, obj)

    def __eq__(self, other):
        if isinstance(other, WeakMethod):
            if not self._alive or not other._alive:
                return self is other
            return ref.__eq__(self, other) and self._func_ref == other._func_ref
        return False

    def __ne__(self, other):
        if isinstance(other, WeakMethod):
            if not self._alive or not other._alive:
                return self is not other
            return ref.__ne__(self, other) or self._func_ref != other._func_ref
        return True

    __hash__ = ref.__hash__


class WeakValueDictionary(collections.MutableMapping):
    """Mapping class that references values weakly.

    Entries in the dictionary will be discarded when no strong
    reference to the value exists anymore
    """
    # We inherit the constructor without worrying about the input
    # dictionary; since it uses our .update() method, we get the right
    # checks (if the other dictionary is a WeakValueDictionary,
    # objects are unwrapped on the way out, and we always wrap on the
    # way in).

    def __init__(self, *args, **kw):
        def remove(wr, selfref=ref(self)):
            self = selfref()
            if self is not None:
                if self._iterating:
                    self._pending_removals.append(wr.key)
                else:
                    del self.data[wr.key]
        self._remove = remove
        # A list of keys to be removed
        self._pending_removals = []
        self._iterating = set()
        self.data = d = {}
        self.update(*args, **kw)

    def _commit_removals(self):
        l = self._pending_removals
        d = self.data
        # We shouldn't encounter any KeyError, because this method should
        # always be called *before* mutating the dict.
        while l:
            del d[l.pop()]

    def __getitem__(self, key):
        o = self.data[key]()
        if o is None:
            raise KeyError(key)
        else:
            return o

    def __delitem__(self, key):
        if self._pending_removals:
            self._commit_removals()
        del self.data[key]

    def __len__(self):
        return len(self.data) - len(self._pending_removals)

    def __contains__(self, key):
        try:
            o = self.data[key]()
        except KeyError:
            return False
        return o is not None

    def __repr__(self):
        return "<WeakValueDictionary at %s>" % id(self)

    def __setitem__(self, key, value):
        if self._pending_removals:
            self._commit_removals()
        self.data[key] = KeyedRef(value, self._remove, key)

    def copy(self):
        new = WeakValueDictionary()
        for key, wr in self.data.items():
            o = wr()
            if o is not None:
                new[key] = o
        return new

    __copy__ = copy

    def __deepcopy__(self, memo):
        from copy import deepcopy
        new = self.__class__()
        for key, wr in self.data.items():
            o = wr()
            if o is not None:
                new[deepcopy(key, memo)] = o
        return new

    def get(self, key, default=None):
        try:
            wr = self.data[key]
        except KeyError:
            return default
        else:
            o = wr()
            if o is None:
                # This should only happen
                return default
            else:
                return o

    def items(self):
        with _IterationGuard(self):
            for k, wr in self.data.items():
                v = wr()
                if v is not None:
                    yield k, v

    def keys(self):
        with _IterationGuard(self):
            for k, wr in self.data.items():
                if wr() is not None:
                    yield k

    __iter__ = keys

    def itervaluerefs(self):
        """Return an iterator that yields the weak references to the values.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the values around longer than needed.

        """
        with _IterationGuard(self):
            yield from self.data.values()

    def values(self):
        with _IterationGuard(self):
            for wr in self.data.values():
                obj = wr()
                if obj is not None:
                    yield obj

    def popitem(self):
        if self._pending_removals:
            self._commit_removals()
        while True:
            key, wr = self.data.popitem()
            o = wr()
            if o is not None:
                return key, o

    def pop(self, key, *args):
        if self._pending_removals:
            self._commit_removals()
        try:
            o = self.data.pop(key)()
        except KeyError:
            if args:
                return args[0]
            raise
        if o is None:
            raise KeyError(key)
        else:
            return o

    def setdefault(self, key, default=None):
        try:
            wr = self.data[key]
        except KeyError:
            if self._pending_removals:
                self._commit_removals()
            self.data[key] = KeyedRef(default, self._remove, key)
            return default
        else:
            return wr()

    def update(self, dict=None, **kwargs):
        if self._pending_removals:
            self._commit_removals()
        d = self.data
        if dict is not None:
            if not hasattr(dict, "items"):
                dict = type({})(dict)
            for key, o in dict.items():
                d[key] = KeyedRef(o, self._remove, key)
        if len(kwargs):
            self.update(kwargs)

    def valuerefs(self):
        """Return a list of weak references to the values.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the values around longer than needed.

        """
        return list(self.data.values())


class KeyedRef(ref):
    """Specialized reference that includes a key corresponding to the value.

    This is used in the WeakValueDictionary to avoid having to create
    a function object for each key stored in the mapping.  A shared
    callback object can use the 'key' attribute of a KeyedRef instead
    of getting a reference to the key from an enclosing scope.

    """

    __slots__ = "key",

    def __new__(type, ob, callback, key):
        self = ref.__new__(type, ob, callback)
        self.key = key
        return self

    def __init__(self, ob, callback, key):
        super().__init__(ob, callback)


class WeakKeyDictionary(collections.MutableMapping):
    """ Mapping class that references keys weakly.

    Entries in the dictionary will be discarded when there is no
    longer a strong reference to the key. This can be used to
    associate additional data with an object owned by other parts of
    an application without adding attributes to those objects. This
    can be especially useful with objects that override attribute
    accesses.
    """

    def __init__(self, dict=None):
        self.data = {}
        def remove(k, selfref=ref(self)):
            self = selfref()
            if self is not None:
                if self._iterating:
                    self._pending_removals.append(k)
                else:
                    del self.data[k]
        self._remove = remove
        # A list of dead weakrefs (keys to be removed)
        self._pending_removals = []
        self._iterating = set()
        if dict is not None:
            self.update(dict)

    def _commit_removals(self):
        # NOTE: We don't need to call this method before mutating the dict,
        # because a dead weakref never compares equal to a live weakref,
        # even if they happened to refer to equal objects.
        # However, it means keys may already have been removed.
        l = self._pending_removals
        d = self.data
        while l:
            try:
                del d[l.pop()]
            except KeyError:
                pass

    def __delitem__(self, key):
        del self.data[ref(key)]

    def __getitem__(self, key):
        return self.data[ref(key)]

    def __len__(self):
        return len(self.data) - len(self._pending_removals)

    def __repr__(self):
        return "<WeakKeyDictionary at %s>" % id(self)

    def __setitem__(self, key, value):
        self.data[ref(key, self._remove)] = value

    def copy(self):
        new = WeakKeyDictionary()
        for key, value in self.data.items():
            o = key()
            if o is not None:
                new[o] = value
        return new

    __copy__ = copy

    def __deepcopy__(self, memo):
        from copy import deepcopy
        new = self.__class__()
        for key, value in self.data.items():
            o = key()
            if o is not None:
                new[o] = deepcopy(value, memo)
        return new

    def get(self, key, default=None):
        return self.data.get(ref(key),default)

    def __contains__(self, key):
        try:
            wr = ref(key)
        except TypeError:
            return False
        return wr in self.data

    def items(self):
        with _IterationGuard(self):
            for wr, value in self.data.items():
                key = wr()
                if key is not None:
                    yield key, value

    def keys(self):
        with _IterationGuard(self):
            for wr in self.data:
                obj = wr()
                if obj is not None:
                    yield obj

    __iter__ = keys

    def values(self):
        with _IterationGuard(self):
            for wr, value in self.data.items():
                if wr() is not None:
                    yield value

    def keyrefs(self):
        """Return a list of weak references to the keys.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the keys around longer than needed.

        """
        return list(self.data)

    def popitem(self):
        while True:
            key, value = self.data.popitem()
            o = key()
            if o is not None:
                return o, value

    def pop(self, key, *args):
        return self.data.pop(ref(key), *args)

    def setdefault(self, key, default=None):
        return self.data.setdefault(ref(key, self._remove),default)

    def update(self, dict=None, **kwargs):
        d = self.data
        if dict is not None:
            if not hasattr(dict, "items"):
                dict = type({})(dict)
            for key, value in dict.items():
                d[ref(key, self._remove)] = value
        if len(kwargs):
            self.update(kwargs)