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.

import UserDict

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

from _weakrefset import WeakSet, _IterationGuard

from exceptions import ReferenceError


ProxyTypes = (ProxyType, CallableProxyType)

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


class WeakValueDictionary(UserDict.UserDict):
    """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__(*args, **kw):
        if not args:
            raise TypeError("descriptor '__init__' of 'WeakValueDictionary' "
                            "object needs an argument")
        self = args[0]
        args = args[1:]
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        def remove(wr, selfref=ref(self)):
            self = selfref()
            if self is not None:
                if self._iterating:
                    self._pending_removals.append(wr.key)
                else:
                    # Atomic removal is necessary since this function
                    # can be called asynchronously by the GC
                    _remove_dead_weakref(self.data, wr.key)
        self._remove = remove
        # A list of keys to be removed
        self._pending_removals = []
        self._iterating = set()
        UserDict.UserDict.__init__(self, *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:
            key = l.pop()
            _remove_dead_weakref(d, key)

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

    def has_key(self, key):
        if self._pending_removals:
            self._commit_removals()
        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 clear(self):
        if self._pending_removals:
            self._commit_removals()
        self.data.clear()

    def copy(self):
        if self._pending_removals:
            self._commit_removals()
        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
        if self._pending_removals:
            self._commit_removals()
        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):
        if self._pending_removals:
            self._commit_removals()
        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):
        if self._pending_removals:
            self._commit_removals()
        L = []
        for key, wr in self.data.items():
            o = wr()
            if o is not None:
                L.append((key, o))
        return L

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

    def iterkeys(self):
        if self._pending_removals:
            self._commit_removals()
        with _IterationGuard(self):
            for k in self.data.iterkeys():
                yield k

    __iter__ = iterkeys

    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.

        """
        if self._pending_removals:
            self._commit_removals()
        with _IterationGuard(self):
            for wr in self.data.itervalues():
                yield wr

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

    def popitem(self):
        if self._pending_removals:
            self._commit_removals()
        while 1:
            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:
            o = None
        if o is None:
            if args:
                return args[0]
            else:
                raise KeyError, key
        else:
            return o

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

    def update(*args, **kwargs):
        if not args:
            raise TypeError("descriptor 'update' of 'WeakValueDictionary' "
                            "object needs an argument")
        self = args[0]
        args = args[1:]
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        dict = args[0] if args else None
        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.

        """
        if self._pending_removals:
            self._commit_removals()
        return self.data.values()

    def values(self):
        if self._pending_removals:
            self._commit_removals()
        L = []
        for wr in self.data.values():
            o = wr()
            if o is not None:
                L.append(o)
        return L


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(KeyedRef,  self).__init__(ob, callback)


class WeakKeyDictionary(UserDict.UserDict):
    """ 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 __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 has_key(self, key):
        try:
            wr = ref(key)
        except TypeError:
            return 0
        return wr in self.data

    def __contains__(self, key):
        try:
            wr = ref(key)
        except TypeError:
            return 0
        return wr in self.data

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

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

    def iterkeyrefs(self):
        """Return an iterator that yields the 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.

        """
        with _IterationGuard(self):
            for wr in self.data.iterkeys():
                yield wr

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

    __iter__ = iterkeys

    def itervalues(self):
        with _IterationGuard(self):
            for value in self.data.itervalues():
                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 self.data.keys()

    def keys(self):
        L = []
        for wr in self.data.keys():
            o = wr()
            if o is not None:
                L.append(o)
        return L

    def popitem(self):
        while 1:
            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)