path: root/lib/sqlalchemy/util.py

# util.py
# Copyright (C) 2005, 2006, 2007, 2008 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

import inspect, itertools, new, sets, sys, warnings, weakref
import __builtin__
types = __import__('types')

from sqlalchemy import exceptions

try:
    import thread, threading
except ImportError:
    import dummy_thread as thread
    import dummy_threading as threading

try:
    Set = set
    set_types = set, sets.Set
except NameError:
    set_types = sets.Set,
    # layer some of __builtin__.set's binop behavior onto sets.Set
    class Set(sets.Set):
        def _binary_sanity_check(self, other):
            pass

        def issubset(self, iterable):
            other = type(self)(iterable)
            return sets.Set.issubset(self, other)
        def __le__(self, other):
            sets.Set._binary_sanity_check(self, other)
            return sets.Set.__le__(self, other)
        def issuperset(self, iterable):
            other = type(self)(iterable)
            return sets.Set.issuperset(self, other)
        def __ge__(self, other):
            sets.Set._binary_sanity_check(self, other)
            return sets.Set.__ge__(self, other)

        # lt and gt still require a BaseSet
        def __lt__(self, other):
            sets.Set._binary_sanity_check(self, other)
            return sets.Set.__lt__(self, other)
        def __gt__(self, other):
            sets.Set._binary_sanity_check(self, other)
            return sets.Set.__gt__(self, other)

        def __ior__(self, other):
            if not isinstance(other, sets.BaseSet):
                return NotImplemented
            return sets.Set.__ior__(self, other)
        def __iand__(self, other):
            if not isinstance(other, sets.BaseSet):
                return NotImplemented
            return sets.Set.__iand__(self, other)
        def __ixor__(self, other):
            if not isinstance(other, sets.BaseSet):
                return NotImplemented
            return sets.Set.__ixor__(self, other)
        def __isub__(self, other):
            if not isinstance(other, sets.BaseSet):
                return NotImplemented
            return sets.Set.__isub__(self, other)

try:
    import cPickle as pickle
except ImportError:
    import pickle

try:
    reversed = __builtin__.reversed
except AttributeError:
    def reversed(seq):
        i = len(seq) -1
        while  i >= 0:
            yield seq[i]
            i -= 1
        raise StopIteration()

try:
    # Try the standard decimal for > 2.3 or the compatibility module
    # for 2.3, if installed.
    from decimal import Decimal
    decimal_type = Decimal
except ImportError:
    def Decimal(arg):
        if Decimal.warn:
            warn("True Decimal types not available on this Python, "
                "falling back to floats.")
            Decimal.warn = False
        return float(arg)
    Decimal.warn = True
    decimal_type = float

try:
    from operator import attrgetter
except:
    def attrgetter(attribute):
        return lambda value: getattr(value, attribute)

if sys.version_info >= (2, 5):
    class PopulateDict(dict):
        """a dict which populates missing values via a creation function.

        note the creation function takes a key, unlike collections.defaultdict.
        """

        def __init__(self, creator):
            self.creator = creator
        def __missing__(self, key):
            self[key] = val = self.creator(key)
            return val
else:
    class PopulateDict(dict):
        """a dict which populates missing values via a creation function."""

        def __init__(self, creator):
            self.creator = creator
        def __getitem__(self, key):
            try:
                return dict.__getitem__(self, key)
            except KeyError:
                self[key] = value = self.creator(key)
                return value

try:
    from collections import defaultdict
except ImportError:
    class defaultdict(dict):
        def __init__(self, default_factory=None, *a, **kw):
            if (default_factory is not None and
                not hasattr(default_factory, '__call__')):
                raise TypeError('first argument must be callable')
            dict.__init__(self, *a, **kw)
            self.default_factory = default_factory
        def __getitem__(self, key):
            try:
                return dict.__getitem__(self, key)
            except KeyError:
                return self.__missing__(key)
        def __missing__(self, key):
            if self.default_factory is None:
                raise KeyError(key)
            self[key] = value = self.default_factory()
            return value
        def __reduce__(self):
            if self.default_factory is None:
                args = tuple()
            else:
                args = self.default_factory,
            return type(self), args, None, None, self.iteritems()
        def copy(self):
            return self.__copy__()
        def __copy__(self):
            return type(self)(self.default_factory, self)
        def __deepcopy__(self, memo):
            import copy
            return type(self)(self.default_factory,
                              copy.deepcopy(self.items()))
        def __repr__(self):
            return 'defaultdict(%s, %s)' % (self.default_factory,
                                            dict.__repr__(self))

try:
    from collections import deque
except ImportError:
    class deque(list):
        def appendleft(self, x):
            self.insert(0, x)
        
        def extendleft(self, iterable):
            self[0:0] = list(iterable)

        def popleft(self):
            return self.pop(0)
            
        def rotate(self, n):
            for i in xrange(n):
                self.appendleft(self.pop())
                
def to_list(x, default=None):
    if x is None:
        return default
    if not isinstance(x, (list, tuple)):
        return [x]
    else:
        return x

def array_as_starargs_decorator(func):
    """Interpret a single positional array argument as
    *args for the decorated method.
    
    """
    def starargs_as_list(self, *args, **kwargs):
        if len(args) == 1:
            return func(self, *to_list(args[0], []), **kwargs)
        else:
            return func(self, *args, **kwargs)
    return starargs_as_list
    
def to_set(x):
    if x is None:
        return Set()
    if not isinstance(x, Set):
        return Set(to_list(x))
    else:
        return x

def to_ascii(x):
    """Convert Unicode or a string with unknown encoding into ASCII."""

    if isinstance(x, str):
        return x.encode('string_escape')
    elif isinstance(x, unicode):
        return x.encode('unicode_escape')
    else:
        raise TypeError

def flatten_iterator(x):
    """Given an iterator of which further sub-elements may also be
    iterators, flatten the sub-elements into a single iterator.
    """

    for elem in x:
        if hasattr(elem, '__iter__'):
            for y in flatten_iterator(elem):
                yield y
        else:
            yield elem

class ArgSingleton(type):
    instances = weakref.WeakValueDictionary()

    def dispose(cls):
        for key in list(ArgSingleton.instances):
            if key[0] is cls:
                del ArgSingleton.instances[key]
    dispose = staticmethod(dispose)

    def __call__(self, *args):
        hashkey = (self, args)
        try:
            return ArgSingleton.instances[hashkey]
        except KeyError:
            instance = type.__call__(self, *args)
            ArgSingleton.instances[hashkey] = instance
            return instance

def get_cls_kwargs(cls):
    """Return the full set of inherited kwargs for the given `cls`.

    Probes a class's __init__ method, collecting all named arguments.  If the
    __init__ defines a **kwargs catch-all, then the constructor is presumed to
    pass along unrecognized keywords to it's base classes, and the collection
    process is repeated recursively on each of the bases.
    """

    for c in cls.__mro__:
        if '__init__' in c.__dict__:
            stack = Set([c])
            break
    else:
        return []

    args = Set()
    while stack:
        class_ = stack.pop()
        ctr = class_.__dict__.get('__init__', False)
        if not ctr or not isinstance(ctr, types.FunctionType):
            continue
        names, _, has_kw, _ = inspect.getargspec(ctr)
        args.update(names)
        if has_kw:
            stack.update(class_.__bases__)
    args.discard('self')
    return list(args)

def get_func_kwargs(func):
    """Return the full set of legal kwargs for the given `func`."""
    return inspect.getargspec(func)[0]

def unbound_method_to_callable(func_or_cls):
    """Adjust the incoming callable such that a 'self' argument is not required."""
    
    if isinstance(func_or_cls, types.MethodType) and not func_or_cls.im_self:
        return func_or_cls.im_func
    else:
        return func_or_cls

# from paste.deploy.converters
def asbool(obj):
    if isinstance(obj, (str, unicode)):
        obj = obj.strip().lower()
        if obj in ['true', 'yes', 'on', 'y', 't', '1']:
            return True
        elif obj in ['false', 'no', 'off', 'n', 'f', '0']:
            return False
        else:
            raise ValueError("String is not true/false: %r" % obj)
    return bool(obj)

def coerce_kw_type(kw, key, type_, flexi_bool=True):
    """If 'key' is present in dict 'kw', coerce its value to type 'type_' if
    necessary.  If 'flexi_bool' is True, the string '0' is considered false
    when coercing to boolean.
    """

    if key in kw and type(kw[key]) is not type_ and kw[key] is not None:
        if type_ is bool and flexi_bool:
            kw[key] = asbool(kw[key])
        else:
            kw[key] = type_(kw[key])

def duck_type_collection(specimen, default=None):
    """Given an instance or class, guess if it is or is acting as one of
    the basic collection types: list, set and dict.  If the __emulates__
    property is present, return that preferentially.
    """

    if hasattr(specimen, '__emulates__'):
        # canonicalize set vs sets.Set to a standard: util.Set
        if (specimen.__emulates__ is not None and
            issubclass(specimen.__emulates__, set_types)):
            return Set
        else:
            return specimen.__emulates__

    isa = isinstance(specimen, type) and issubclass or isinstance
    if isa(specimen, list): return list
    if isa(specimen, set_types): return Set
    if isa(specimen, dict): return dict

    if hasattr(specimen, 'append'):
        return list
    elif hasattr(specimen, 'add'):
        return Set
    elif hasattr(specimen, 'set'):
        return dict
    else:
        return default

def dictlike_iteritems(dictlike):
    """Return a (key, value) iterator for almost any dict-like object."""

    if hasattr(dictlike, 'iteritems'):
        return dictlike.iteritems()
    elif hasattr(dictlike, 'items'):
        return iter(dictlike.items())

    getter = getattr(dictlike, '__getitem__', getattr(dictlike, 'get', None))
    if getter is None:
        raise TypeError(
            "Object '%r' is not dict-like" % dictlike)

    if hasattr(dictlike, 'iterkeys'):
        def iterator():
            for key in dictlike.iterkeys():
                yield key, getter(key)
        return iterator()
    elif hasattr(dictlike, 'keys'):
        return iter([(key, getter(key)) for key in dictlike.keys()])
    else:
        raise TypeError(
            "Object '%r' is not dict-like" % dictlike)

def assert_arg_type(arg, argtype, name):
    if isinstance(arg, argtype):
        return arg
    else:
        if isinstance(argtype, tuple):
            raise exceptions.ArgumentError("Argument '%s' is expected to be one of type %s, got '%s'" % (name, ' or '.join(["'%s'" % str(a) for a in argtype]), str(type(arg))))
        else:
            raise exceptions.ArgumentError("Argument '%s' is expected to be of type '%s', got '%s'" % (name, str(argtype), str(type(arg))))

def warn_exception(func, *args, **kwargs):
    """executes the given function, catches all exceptions and converts to a warning."""
    try:
        return func(*args, **kwargs)
    except:
        warn("%s('%s') ignored" % sys.exc_info()[0:2])

def monkeypatch_proxied_specials(into_cls, from_cls, skip=None, only=None,
                                 name='self.proxy', from_instance=None):
    """Automates delegation of __specials__ for a proxying type."""

    if only:
        dunders = only
    else:
        if skip is None:
            skip = ('__slots__', '__del__', '__getattribute__',
                    '__metaclass__', '__getstate__', '__setstate__')
        dunders = [m for m in dir(from_cls)
                   if (m.startswith('__') and m.endswith('__') and
                       not hasattr(into_cls, m) and m not in skip)]
    for method in dunders:
        try:
            spec = inspect.getargspec(getattr(from_cls, method))
            fn_args = inspect.formatargspec(spec[0])
            d_args = inspect.formatargspec(spec[0][1:])
        except TypeError:
            fn_args = '(self, *args, **kw)'
            d_args = '(*args, **kw)'

        py = ("def %(method)s%(fn_args)s: "
              "return %(name)s.%(method)s%(d_args)s" % locals())

        env = from_instance is not None and {name: from_instance} or {}
        exec py in env
        setattr(into_cls, method, env[method])

class SimpleProperty(object):
    """A *default* property accessor."""

    def __init__(self, key):
        self.key = key

    def __set__(self, obj, value):
        setattr(obj, self.key, value)

    def __delete__(self, obj):
        delattr(obj, self.key)

    def __get__(self, obj, owner):
        if obj is None:
            return self
        else:
            return getattr(obj, self.key)

class NotImplProperty(object):
  """a property that raises ``NotImplementedError``."""

  def __init__(self, doc):
      self.__doc__ = doc

  def __set__(self, obj, value):
      raise NotImplementedError()

  def __delete__(self, obj):
      raise NotImplementedError()

  def __get__(self, obj, owner):
      if obj is None:
          return self
      else:
          raise NotImplementedError()

class OrderedProperties(object):
    """An object that maintains the order in which attributes are set upon it.

    Also provides an iterator and a very basic getitem/setitem
    interface to those attributes.

    (Not really a dict, since it iterates over values, not keys.  Not really
    a list, either, since each value must have a key associated; hence there is
    no append or extend.)
    """

    def __init__(self):
        self.__dict__['_data'] = OrderedDict()

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return self._data.itervalues()

    def __add__(self, other):
        return list(self) + list(other)

    def __setitem__(self, key, object):
        self._data[key] = object

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

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

    def __setattr__(self, key, object):
        self._data[key] = object

    def __getstate__(self):
        return {'_data': self.__dict__['_data']}

    def __setstate__(self, state):
        self.__dict__['_data'] = state['_data']

    def __getattr__(self, key):
        try:
            return self._data[key]
        except KeyError:
            raise AttributeError(key)

    def __contains__(self, key):
        return key in self._data
    
    def update(self, value):
        self._data.update(value)
        
    def get(self, key, default=None):
        if key in self:
            return self[key]
        else:
            return default

    def keys(self):
        return self._data.keys()

    def has_key(self, key):
        return self._data.has_key(key)

    def clear(self):
        self._data.clear()

class OrderedDict(dict):
    """A Dictionary that returns keys/values/items in the order they were added."""

    def __init__(self, ____sequence=None, **kwargs):
        self._list = []
        if ____sequence is None:
            if kwargs:
                self.update(**kwargs)
        else:
            self.update(____sequence, **kwargs)

    def clear(self):
        self._list = []
        dict.clear(self)

    def update(self, ____sequence=None, **kwargs):
        if ____sequence is not None:
            if hasattr(____sequence, 'keys'):
                for key in ____sequence.keys():
                    self.__setitem__(key, ____sequence[key])
            else:
                for key, value in ____sequence:
                    self[key] = value
        if kwargs:
            self.update(kwargs)

    def setdefault(self, key, value):
        if key not in self:
            self.__setitem__(key, value)
            return value
        else:
            return self.__getitem__(key)

    def __iter__(self):
        return iter(self._list)

    def values(self):
        return [self[key] for key in self._list]

    def itervalues(self):
        return iter(self.values())

    def keys(self):
        return list(self._list)

    def iterkeys(self):
        return iter(self.keys())

    def items(self):
        return [(key, self[key]) for key in self.keys()]

    def iteritems(self):
        return iter(self.items())

    def __setitem__(self, key, object):
        if key not in self:
            self._list.append(key)
        dict.__setitem__(self, key, object)

    def __delitem__(self, key):
        dict.__delitem__(self, key)
        self._list.remove(key)

    def pop(self, key):
        value = dict.pop(self, key)
        self._list.remove(key)
        return value

    def popitem(self):
        item = dict.popitem(self)
        self._list.remove(item[0])
        return item

try:
    from threading import local as ThreadLocal
except ImportError:
    try:
        from dummy_threading import local as ThreadLocal
    except ImportError:
        class ThreadLocal(object):
            """An object in which attribute access occurs only within the context of the current thread."""

            def __init__(self):
                self.__dict__['_tdict'] = {}

            def __delattr__(self, key):
                try:
                    del self._tdict[(thread.get_ident(), key)]
                except KeyError:
                    raise AttributeError(key)

            def __getattr__(self, key):
                try:
                    return self._tdict[(thread.get_ident(), key)]
                except KeyError:
                    raise AttributeError(key)

            def __setattr__(self, key, value):
                self._tdict[(thread.get_ident(), key)] = value

class OrderedSet(Set):
    def __init__(self, d=None):
        Set.__init__(self)
        self._list = []
        if d is not None:
            self.update(d)

    def add(self, key):
        if key not in self:
            self._list.append(key)
        Set.add(self, key)

    def remove(self, element):
        Set.remove(self, element)
        self._list.remove(element)

    def discard(self, element):
        try:
            Set.remove(self, element)
        except KeyError:
            pass
        else:
            self._list.remove(element)

    def clear(self):
        Set.clear(self)
        self._list = []

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

    def __iter__(self):
        return iter(self._list)

    def __repr__(self):
      return '%s(%r)' % (self.__class__.__name__, self._list)

    __str__ = __repr__

    def update(self, iterable):
      add = self.add
      for i in iterable:
          add(i)
      return self

    __ior__ = update

    def union(self, other):
      result = self.__class__(self)
      result.update(other)
      return result

    __or__ = union

    def intersection(self, other):
        other = Set(other)
        return self.__class__([a for a in self if a in other])

    __and__ = intersection

    def symmetric_difference(self, other):
        other = Set(other)
        result = self.__class__([a for a in self if a not in other])
        result.update([a for a in other if a not in self])
        return result

    __xor__ = symmetric_difference

    def difference(self, other):
        other = Set(other)
        return self.__class__([a for a in self if a not in other])

    __sub__ = difference

    def intersection_update(self, other):
        other = Set(other)
        Set.intersection_update(self, other)
        self._list = [ a for a in self._list if a in other]
        return self

    __iand__ = intersection_update

    def symmetric_difference_update(self, other):
      Set.symmetric_difference_update(self, other)
      self._list =  [ a for a in self._list if a in self]
      self._list += [ a for a in other._list if a in self]
      return self

    __ixor__ = symmetric_difference_update

    def difference_update(self, other):
        Set.difference_update(self, other)
        self._list = [ a for a in self._list if a in self]
        return self

    __isub__ = difference_update

    if hasattr(Set, '__getstate__'):
        def __getstate__(self):
            base = Set.__getstate__(self)
            return base, self._list

        def __setstate__(self, state):
            Set.__setstate__(self, state[0])
            self._list = state[1]

class IdentitySet(object):
    """A set that considers only object id() for uniqueness.

    This strategy has edge cases for builtin types- it's possible to have
    two 'foo' strings in one of these sets, for example.  Use sparingly.
    """

    _working_set = Set

    def __init__(self, iterable=None):
        self._members = _IterableUpdatableDict()
        if iterable:
            for o in iterable:
                self.add(o)

    def add(self, value):
        self._members[id(value)] = value

    def __contains__(self, value):
        return id(value) in self._members

    def remove(self, value):
        del self._members[id(value)]

    def discard(self, value):
        try:
            self.remove(value)
        except KeyError:
            pass

    def pop(self):
        try:
            pair = self._members.popitem()
            return pair[1]
        except KeyError:
            raise KeyError('pop from an empty set')

    def clear(self):
        self._members.clear()

    def __cmp__(self, other):
        raise TypeError('cannot compare sets using cmp()')

    def __eq__(self, other):
        if isinstance(other, IdentitySet):
            return self._members == other._members
        else:
            return False

    def __ne__(self, other):
        if isinstance(other, IdentitySet):
            return self._members != other._members
        else:
            return True

    def issubset(self, iterable):
        other = type(self)(iterable)

        if len(self) > len(other):
            return False
        for m in itertools.ifilterfalse(other._members.has_key,
                                        self._members.iterkeys()):
            return False
        return True

    def __le__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.issubset(other)

    def __lt__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return len(self) < len(other) and self.issubset(other)

    def issuperset(self, iterable):
        other = type(self)(iterable)

        if len(self) < len(other):
            return False

        for m in itertools.ifilterfalse(self._members.has_key,
                                        other._members.iterkeys()):
            return False
        return True

    def __ge__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.issuperset(other)

    def __gt__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return len(self) > len(other) and self.issuperset(other)

    def union(self, iterable):
        result = type(self)()
        # testlib.pragma exempt:__hash__
        result._members.update(
            self._working_set(self._members.iteritems()).union(_iter_id(iterable)))
        return result

    def __or__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.union(other)

    def update(self, iterable):
        self._members = self.union(iterable)._members

    def __ior__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        self.update(other)
        return self

    def difference(self, iterable):
        result = type(self)()
        # testlib.pragma exempt:__hash__
        result._members.update(
            self._working_set(self._members.iteritems()).difference(_iter_id(iterable)))
        return result

    def __sub__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.difference(other)

    def difference_update(self, iterable):
        self._members = self.difference(iterable)._members

    def __isub__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        self.difference_update(other)
        return self

    def intersection(self, iterable):
        result = type(self)()
        # testlib.pragma exempt:__hash__
        result._members.update(
            self._working_set(self._members.iteritems()).intersection(_iter_id(iterable)))
        return result

    def __and__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.intersection(other)

    def intersection_update(self, iterable):
        self._members = self.intersection(iterable)._members

    def __iand__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        self.intersection_update(other)
        return self

    def symmetric_difference(self, iterable):
        result = type(self)()
        # testlib.pragma exempt:__hash__
        result._members.update(
            self._working_set(self._members.iteritems()).symmetric_difference(_iter_id(iterable)))
        return result

    def __xor__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        return self.symmetric_difference(other)

    def symmetric_difference_update(self, iterable):
        self._members = self.symmetric_difference(iterable)._members

    def __ixor__(self, other):
        if not isinstance(other, IdentitySet):
            return NotImplemented
        self.symmetric_difference(other)
        return self

    def copy(self):
        return type(self)(self._members.itervalues())

    __copy__ = copy

    def __len__(self):
        return len(self._members)

    def __iter__(self):
        return self._members.itervalues()

    def __hash__(self):
        raise TypeError('set objects are unhashable')

    def __repr__(self):
        return '%s(%r)' % (type(self).__name__, self._members.values())

if sys.version_info >= (2, 4):
    _IterableUpdatableDict = dict
else:
    class _IterableUpdatableDict(dict):
        """A dict that can update(iterable) like Python 2.4+'s dict."""
        def update(self, __iterable=None, **kw):
            if __iterable is not None:
                if not isinstance(__iterable, dict):
                    __iterable = dict(__iterable)
                dict.update(self, __iterable)
            if kw:
                dict.update(self, **kw)

def _iter_id(iterable):
    """Generator: ((id(o), o) for o in iterable)."""
    for item in iterable:
        yield id(item), item


class OrderedIdentitySet(IdentitySet):
    class _working_set(OrderedSet):
        # a testing pragma: exempt the OIDS working set from the test suite's
        # "never call the user's __hash__" assertions.  this is a big hammer,
        # but it's safe here: IDS operates on (id, instance) tuples in the
        # working set.
        __sa_hash_exempt__ = True

    def __init__(self, iterable=None):
        IdentitySet.__init__(self)
        self._members = OrderedDict()
        if iterable:
            for o in iterable:
                self.add(o)


class UniqueAppender(object):
    """Only adds items to a collection once.

    Additional appends() of the same object are ignored.  Membership is
    determined by identity (``is a``) not equality (``==``).
    """

    def __init__(self, data, via=None):
        self.data = data
        self._unique = IdentitySet()
        if via:
            self._data_appender = getattr(data, via)
        elif hasattr(data, 'append'):
            self._data_appender = data.append
        elif hasattr(data, 'add'):
            # TODO: we think its a set here.  bypass unneeded uniquing logic ?
            self._data_appender = data.add

    def append(self, item):
        if item not in self._unique:
            self._data_appender(item)
            self._unique.add(item)

    def __iter__(self):
        return iter(self.data)

class ScopedRegistry(object):
    """A Registry that can store one or multiple instances of a single
    class on a per-thread scoped basis, or on a customized scope.

    createfunc
      a callable that returns a new object to be placed in the registry

    scopefunc
      a callable that will return a key to store/retrieve an object,
      defaults to ``thread.get_ident`` for thread-local objects.  Use
      a value like ``lambda: True`` for application scope.
    """

    def __init__(self, createfunc, scopefunc=None):
        self.createfunc = createfunc
        if scopefunc is None:
            self.scopefunc = thread.get_ident
        else:
            self.scopefunc = scopefunc
        self.registry = {}

    def __call__(self):
        key = self._get_key()
        try:
            return self.registry[key]
        except KeyError:
            return self.registry.setdefault(key, self.createfunc())

    def has(self):
        return self._get_key() in self.registry

    def set(self, obj):
        self.registry[self._get_key()] = obj

    def clear(self):
        try:
            del self.registry[self._get_key()]
        except KeyError:
            pass

    def _get_key(self):
        return self.scopefunc()

class _symbol(object):
    def __init__(self, name):
        """Construct a new named symbol."""
        assert isinstance(name, str)
        self.name = name
    def __reduce__(self):
        return symbol, (self.name,)
    def __repr__(self):
        return "<symbol '%s>" % self.name
_symbol.__name__ = 'symbol'

class symbol(object):
    """A constant symbol.

    >>> symbol('foo') is symbol('foo')
    True
    >>> symbol('foo')
    <symbol 'foo>

    A slight refinement of the MAGICCOOKIE=object() pattern.  The primary
    advantage of symbol() is its repr().  They are also singletons.

    Repeated calls of symbol('name') will all return the same instance.

    """
    symbols = {}
    _lock = threading.Lock()

    def __new__(cls, name):
        cls._lock.acquire()
        try:
            sym = cls.symbols.get(name)
            if sym is None:
                cls.symbols[name] = sym = _symbol(name)
            return sym
        finally:
            symbol._lock.release()


def function_named(fn, name):
    """Return a function with a given __name__.

    Will assign to __name__ and return the original function if possible on
    the Python implementation, otherwise a new function will be constructed.

    """
    try:
        fn.__name__ = name
    except TypeError:
        fn = new.function(fn.func_code, fn.func_globals, name,
                          fn.func_defaults, fn.func_closure)
    return fn

def conditional_cache_decorator(func):
    """apply conditional caching to the return value of a function."""

    return cache_decorator(func, conditional=True)

def cache_decorator(func, conditional=False):
    """apply caching to the return value of a function."""

    name = '_cached_' + func.__name__
    
    def do_with_cache(self, *args, **kwargs):
        if conditional:
            cache = kwargs.pop('cache', False)
            if not cache:
                return func(self, *args, **kwargs)
        try:
            return getattr(self, name)
        except AttributeError:
            value = func(self, *args, **kwargs)
            setattr(self, name, value)
            return value
    return do_with_cache
    
def reset_cached(instance, name):
    try:
        delattr(instance, '_cached_' + name)
    except AttributeError:
        pass

def warn(msg):
    if isinstance(msg, basestring):
        warnings.warn(msg, exceptions.SAWarning, stacklevel=3)
    else:
        warnings.warn(msg, stacklevel=3)

def warn_deprecated(msg):
    warnings.warn(msg, exceptions.SADeprecationWarning, stacklevel=3)

def deprecated(message=None, add_deprecation_to_docstring=True):
    """Decorates a function and issues a deprecation warning on use.

    message
      If provided, issue message in the warning.  A sensible default
      is used if not provided.

    add_deprecation_to_docstring
      Default True.  If False, the wrapped function's __doc__ is left
      as-is.  If True, the 'message' is prepended to the docs if
      provided, or sensible default if message is omitted.
    """

    if add_deprecation_to_docstring:
        header = message is not None and message or 'Deprecated.'
    else:
        header = None

    if message is None:
        message = "Call to deprecated function %(func)s"

    def decorate(fn):
        return _decorate_with_warning(
            fn, exceptions.SADeprecationWarning,
            message % dict(func=fn.__name__), header)
    return decorate

def pending_deprecation(version, message=None,
                        add_deprecation_to_docstring=True):
    """Decorates a function and issues a pending deprecation warning on use.

    version
      An approximate future version at which point the pending deprecation
      will become deprecated.  Not used in messaging.

    message
      If provided, issue message in the warning.  A sensible default
      is used if not provided.

    add_deprecation_to_docstring
      Default True.  If False, the wrapped function's __doc__ is left
      as-is.  If True, the 'message' is prepended to the docs if
      provided, or sensible default if message is omitted.
    """

    if add_deprecation_to_docstring:
        header = message is not None and message or 'Deprecated.'
    else:
        header = None

    if message is None:
        message = "Call to deprecated function %(func)s"

    def decorate(fn):
        return _decorate_with_warning(
            fn, exceptions.SAPendingDeprecationWarning,
            message % dict(func=fn.__name__), header)
    return decorate

def _decorate_with_warning(func, wtype, message, docstring_header=None):
    """Wrap a function with a warnings.warn and augmented docstring."""

    def func_with_warning(*args, **kwargs):
        warnings.warn(wtype(message), stacklevel=2)
        return func(*args, **kwargs)

    doc = func.__doc__ is not None and func.__doc__ or ''
    if docstring_header is not None:
        doc = '\n'.join((docstring_header.rstrip(), doc))

    func_with_warning.__doc__ = doc
    func_with_warning.__dict__.update(func.__dict__)

    return function_named(func_with_warning, func.__name__)