pypy / pypy / annotation / description.py

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from __future__ import absolute_import
import types, py
from pypy.annotation.signature import enforce_signature_args, enforce_signature_return
from pypy.objspace.flow.model import Constant, FunctionGraph
from pypy.objspace.flow.bytecode import cpython_code_signature
from pypy.objspace.flow.argument import rawshape, ArgErr
from pypy.tool.sourcetools import valid_identifier
from pypy.tool.pairtype import extendabletype

class CallFamily(object):
    """A family of Desc objects that could be called from common call sites.
    The call families are conceptually a partition of all (callable) Desc
    objects, where the equivalence relation is the transitive closure of
    'd1~d2 if d1 and d2 might be called at the same call site'.
    """
    overridden = False
    normalized = False
    modified   = True

    def __init__(self, desc):
        self.descs = { desc: True }
        self.calltables = {}  # see calltable_lookup_row()
        self.total_calltable_size = 0

    def update(self, other):
        self.modified = True
        self.normalized = self.normalized or other.normalized
        self.descs.update(other.descs)
        for shape, table in other.calltables.items():
            for row in table:
                self.calltable_add_row(shape, row)
    absorb = update # UnionFind API

    def calltable_lookup_row(self, callshape, row):
        # this code looks up a table of which graph to
        # call at which call site.  Each call site gets a row of graphs,
        # sharable with other call sites.  Each column is a FunctionDesc.
        # There is one such table per "call shape".
        table = self.calltables.get(callshape, [])
        for i, existing_row in enumerate(table):
            if existing_row == row:   # XXX maybe use a dict again here?
                return i
        raise LookupError

    def calltable_add_row(self, callshape, row):
        try:
            self.calltable_lookup_row(callshape, row)
        except LookupError:
            self.modified = True
            table = self.calltables.setdefault(callshape, [])
            table.append(row)
            self.total_calltable_size += 1


class FrozenAttrFamily(object):
    """A family of FrozenDesc objects that have any common 'getattr' sites.
    The attr families are conceptually a partition of FrozenDesc objects,
    where the equivalence relation is the transitive closure of:
    d1~d2 if d1 and d2 might have some attribute read on them by the same
    getattr operation.
    """
    def __init__(self, desc):
        self.descs = {desc: True}
        self.read_locations = {}     # set of position_keys
        self.attrs = {}              # { attr: s_value }

    def update(self, other):
        self.descs.update(other.descs)
        self.read_locations.update(other.read_locations)
        self.attrs.update(other.attrs)
    absorb = update # UnionFind API

    def get_s_value(self, attrname):
        try:
            return self.attrs[attrname]
        except KeyError:
            from pypy.annotation.model import s_ImpossibleValue
            return s_ImpossibleValue

    def set_s_value(self, attrname, s_value):
        self.attrs[attrname] = s_value


class ClassAttrFamily(object):
    """A family of ClassDesc objects that have common 'getattr' sites for a
    given attribute name.  The attr families are conceptually a partition
    of ClassDesc objects, where the equivalence relation is the transitive
    closure of:  d1~d2 if d1 and d2 might have a common attribute 'attrname'
    read on them by the same getattr operation.

    The 'attrname' is not explicitly stored here, but is the key used
    in the dictionary bookkeeper.pbc_maximal_access_sets_map.
    """
    # The difference between ClassAttrFamily and FrozenAttrFamily is that
    # FrozenAttrFamily is the union for all attribute names, but
    # ClassAttrFamily is more precise: it is only about one attribut name.

    def __init__(self, desc):
        from pypy.annotation.model import s_ImpossibleValue
        self.descs = { desc: True }
        self.read_locations = {}     # set of position_keys
        self.s_value = s_ImpossibleValue    # union of possible values

    def update(self, other):
        from pypy.annotation.model import unionof
        self.descs.update(other.descs)
        self.read_locations.update(other.read_locations)
        self.s_value = unionof(self.s_value, other.s_value)
    absorb = update # UnionFind API

    def get_s_value(self, attrname):
        return self.s_value

    def set_s_value(self, attrname, s_value):
        self.s_value = s_value

# ____________________________________________________________

class Desc(object):
    __metaclass__ = extendabletype

    def __init__(self, bookkeeper, pyobj=None):
        self.bookkeeper = bookkeeper
        # 'pyobj' is non-None if there is an associated underlying Python obj
        self.pyobj = pyobj

    def __repr__(self):
        pyobj = self.pyobj
        if pyobj is None:
            return object.__repr__(self)
        return '<%s for %r>' % (self.__class__.__name__, pyobj)

    def querycallfamily(self):
        """Retrieve the CallFamily object if there is one, otherwise
           return None."""
        call_families = self.bookkeeper.pbc_maximal_call_families
        try:
            return call_families[self]
        except KeyError:
            return None

    def getcallfamily(self):
        """Get the CallFamily object. Possibly creates one."""
        call_families = self.bookkeeper.pbc_maximal_call_families
        _, _, callfamily = call_families.find(self.rowkey())
        return callfamily

    def mergecallfamilies(self, *others):
        """Merge the call families of the given Descs into one."""
        call_families = self.bookkeeper.pbc_maximal_call_families
        changed, rep, callfamily = call_families.find(self.rowkey())
        for desc in others:
            changed1, rep, callfamily = call_families.union(rep, desc.rowkey())
            changed = changed or changed1
        return changed

    def queryattrfamily(self):
        # no attributes supported by default;
        # overriden in FrozenDesc and ClassDesc
        return None

    def bind_under(self, classdef, name):
        return self

    def simplify_desc_set(descs):
        pass
    simplify_desc_set = staticmethod(simplify_desc_set)


class NoStandardGraph(Exception):
    """The function doesn't have a single standard non-specialized graph."""

class FunctionDesc(Desc):
    knowntype = types.FunctionType
    overridden = False

    def __init__(self, bookkeeper, pyobj=None,
                 name=None, signature=None, defaults=None,
                 specializer=None):
        super(FunctionDesc, self).__init__(bookkeeper, pyobj)
        if name is None:
            name = pyobj.func_name
        if signature is None:
            if hasattr(pyobj, '_generator_next_method_of_'):
                from pypy.objspace.flow.argument import Signature
                signature = Signature(['entry'])     # haaaaaack
                defaults = ()
            else:
                signature = cpython_code_signature(pyobj.func_code)
        if defaults is None:
            defaults = pyobj.func_defaults
        self.name = name
        self.signature = signature
        self.defaults = defaults or ()
        # 'specializer' is a function with the following signature:
        #      specializer(funcdesc, args_s) => graph
        #                                 or => s_result (overridden/memo cases)
        self.specializer = specializer
        self._cache = {}     # convenience for the specializer

    def buildgraph(self, alt_name=None, builder=None):
        translator = self.bookkeeper.annotator.translator
        if builder:
            graph = builder(translator, self.pyobj)
        else:
            graph = translator.buildflowgraph(self.pyobj)
        if alt_name:
            graph.name = alt_name
        return graph

    def getgraphs(self):
        return self._cache.values()

    def getuniquegraph(self):
        if len(self._cache) != 1:
            raise NoStandardGraph(self)
        [graph] = self._cache.values()
        relax_sig_check = getattr(self.pyobj, "relax_sig_check", False)
        if (graph.signature != self.signature or
            graph.defaults  != self.defaults) and not relax_sig_check:
            raise NoStandardGraph(self)
        return graph

    def cachedgraph(self, key, alt_name=None, builder=None):
        try:
            return self._cache[key]
        except KeyError:
            def nameof(thing):
                if isinstance(thing, str):
                    return thing
                elif hasattr(thing, '__name__'): # mostly types and functions
                    return thing.__name__
                elif hasattr(thing, 'name') and isinstance(thing.name, str):
                    return thing.name            # mostly ClassDescs
                elif isinstance(thing, tuple):
                    return '_'.join(map(nameof, thing))
                else:
                    return str(thing)[:30]

            if key is not None and alt_name is None:
                postfix = valid_identifier(nameof(key))
                alt_name = "%s__%s"%(self.name, postfix)
            graph = self.buildgraph(alt_name, builder)
            self._cache[key] = graph
            return graph

    def parse_arguments(self, args, graph=None):
        defs_s = []
        if graph is None:
            signature = self.signature
            defaults = self.defaults
        else:
            signature = graph.signature
            defaults = graph.defaults
        if defaults:
            for x in defaults:
                if x is NODEFAULT:
                    defs_s.append(None)
                else:
                    defs_s.append(self.bookkeeper.immutablevalue(x))
        try:
            inputcells = args.match_signature(signature, defs_s)
        except ArgErr, e:
            raise TypeError("signature mismatch: %s() %s" %
                            (self.name, e.getmsg()))
        return inputcells

    def specialize(self, inputcells, op=None):
        if (op is None and
            getattr(self.bookkeeper, "position_key", None) is not None):
            _, block, i = self.bookkeeper.position_key
            op = block.operations[i]
        if self.specializer is None:
            # get the specializer based on the tag of the 'pyobj'
            # (if any), according to the current policy
            tag = getattr(self.pyobj, '_annspecialcase_', None)
            policy = self.bookkeeper.annotator.policy
            self.specializer = policy.get_specializer(tag)
        enforceargs = getattr(self.pyobj, '_annenforceargs_', None)
        signature = getattr(self.pyobj, '_signature_', None)
        if enforceargs and signature:
            raise Exception("%r: signature and enforceargs cannot both be used" % (self,))
        if enforceargs:
            if not callable(enforceargs):
                from pypy.annotation.policy import Sig
                enforceargs = Sig(*enforceargs)
                self.pyobj._annenforceargs_ = enforceargs
            enforceargs(self, inputcells) # can modify inputcells in-place
        if signature:
            enforce_signature_args(self, signature[0], inputcells) # mutates inputcells
        if getattr(self.pyobj, '_annspecialcase_', '').endswith("call_location"):
            return self.specializer(self, inputcells, op)
        else:
            return self.specializer(self, inputcells)

    def pycall(self, schedule, args, s_previous_result, op=None):
        inputcells = self.parse_arguments(args)
        result = self.specialize(inputcells, op)
        if isinstance(result, FunctionGraph):
            graph = result         # common case
            # if that graph has a different signature, we need to re-parse
            # the arguments.
            # recreate the args object because inputcells may have been changed
            new_args = args.unmatch_signature(self.signature, inputcells)
            inputcells = self.parse_arguments(new_args, graph)
            result = schedule(graph, inputcells)
            signature = getattr(self.pyobj, '_signature_', None)
            if signature:
                result = enforce_signature_return(self, signature[1], result)
                self.bookkeeper.annotator.addpendingblock(graph, graph.returnblock, [result])
        # Some specializations may break the invariant of returning
        # annotations that are always more general than the previous time.
        # We restore it here:
        from pypy.annotation.model import unionof
        result = unionof(result, s_previous_result)
        return result

    def bind_under(self, classdef, name):
        # XXX static methods
        return self.bookkeeper.getmethoddesc(self,
                                             classdef,   # originclassdef,
                                             None,       # selfclassdef
                                             name)

    def consider_call_site(bookkeeper, family, descs, args, s_result, op):
        shape = rawshape(args)
        row = FunctionDesc.row_to_consider(descs, args, op)
        family.calltable_add_row(shape, row)
    consider_call_site = staticmethod(consider_call_site)

    def variant_for_call_site(bookkeeper, family, descs, args, op):
        shape = rawshape(args)
        bookkeeper.enter(None)
        try:
            row = FunctionDesc.row_to_consider(descs, args, op)
        finally:
            bookkeeper.leave()
        index = family.calltable_lookup_row(shape, row)
        return shape, index
    variant_for_call_site = staticmethod(variant_for_call_site)

    def rowkey(self):
        return self

    def row_to_consider(descs, args, op):
        # see comments in CallFamily
        from pypy.annotation.model import s_ImpossibleValue
        row = {}
        for desc in descs:
            def enlist(graph, ignore):
                row[desc.rowkey()] = graph
                return s_ImpossibleValue   # meaningless
            desc.pycall(enlist, args, s_ImpossibleValue, op)
        return row
    row_to_consider = staticmethod(row_to_consider)

    def get_s_signatures(self, shape):
        family = self.getcallfamily()
        table = family.calltables.get(shape)
        if table is None:
            return []
        else:
            graph_seen = {}
            s_sigs = []

            binding = self.bookkeeper.annotator.binding

            def enlist(graph):
                if graph in graph_seen:
                    return
                graph_seen[graph] = True
                s_sig = ([binding(v) for v in graph.getargs()],
                         binding(graph.getreturnvar()))
                if s_sig in s_sigs:
                    return
                s_sigs.append(s_sig)

            for row in table:
                for graph in row.itervalues():
                    enlist(graph)

            return s_sigs

NODEFAULT = object()

class ClassDesc(Desc):
    knowntype = type
    instance_level = False
    all_enforced_attrs = None   # or a set
    settled = False

    def __init__(self, bookkeeper, pyobj=None,
                 name=None, basedesc=None, classdict=None,
                 specialize=None):
        super(ClassDesc, self).__init__(bookkeeper, pyobj)

        if name is None:
            name = pyobj.__module__ + '.' + pyobj.__name__
        self.name = name
        self.basedesc = basedesc
        if classdict is None:
            classdict = {}    # populated below
        self.classdict = classdict     # {attr: Constant-or-Desc}
        if specialize is None:
            specialize = pyobj.__dict__.get('_annspecialcase_', '')
        self.specialize = specialize
        self._classdefs = {}

        if pyobj is not None:
            assert pyobj.__module__ != '__builtin__'
            cls = pyobj
            base = object
            baselist = list(cls.__bases__)

            # special case: skip BaseException in Python 2.5, and pretend
            # that all exceptions ultimately inherit from Exception instead
            # of BaseException (XXX hack)
            if cls is Exception:
                baselist = []
            elif baselist == [py.builtin.BaseException]:
                baselist = [Exception]

            mixins_before = []
            mixins_after = []
            for b1 in baselist:
                if b1 is object:
                    continue
                if b1.__dict__.get('_mixin_', False):
                    if base is object:
                        mixins_before.append(b1)
                    else:
                        mixins_after.append(b1)
                else:
                    assert base is object, ("multiple inheritance only supported "
                                            "with _mixin_: %r" % (cls,))
                    base = b1
            if mixins_before and mixins_after:
                raise Exception("unsupported: class %r has mixin bases both"
                                " before and after the regular base" % (self,))
            self.add_mixins(mixins_after, check_not_in=base)
            self.add_mixins(mixins_before)
            self.add_sources_for_class(cls)

            if base is not object:
                self.basedesc = bookkeeper.getdesc(base)

            if '_settled_' in cls.__dict__:
                self.settled = bool(cls.__dict__['_settled_'])

            if '__slots__' in cls.__dict__ or '_attrs_' in cls.__dict__:
                attrs = {}
                for decl in ('__slots__', '_attrs_'):
                    decl = cls.__dict__.get(decl, [])
                    if isinstance(decl, str):
                        decl = (decl,)
                    decl = dict.fromkeys(decl)
                    attrs.update(decl)
                if self.basedesc is not None:
                    if self.basedesc.all_enforced_attrs is None:
                        raise Exception("%r has slots or _attrs_, "
                                        "but not its base class"
                                        % (pyobj,))
                    attrs.update(self.basedesc.all_enforced_attrs)
                self.all_enforced_attrs = attrs

            if (self.is_builtin_exception_class() and
                self.all_enforced_attrs is None):
                from pypy.annotation import classdef
                if self.pyobj not in classdef.FORCE_ATTRIBUTES_INTO_CLASSES:
                    self.all_enforced_attrs = []    # no attribute allowed

    def add_source_attribute(self, name, value, mixin=False):
        if isinstance(value, types.FunctionType):
            # for debugging
            if not hasattr(value, 'class_'):
                value.class_ = self.pyobj # remember that this is really a method
            if self.specialize:
                # make a custom funcdesc that specializes on its first
                # argument (i.e. 'self').
                from pypy.annotation.specialize import specialize_argtype
                def argtype0(funcdesc, args_s):
                    return specialize_argtype(funcdesc, args_s, 0)
                funcdesc = FunctionDesc(self.bookkeeper, value,
                                        specializer=argtype0)
                self.classdict[name] = funcdesc
                return
            if mixin:
                # make a new copy of the FunctionDesc for this class,
                # but don't specialize further for all subclasses
                funcdesc = FunctionDesc(self.bookkeeper, value)
                self.classdict[name] = funcdesc
                return
            # NB. if value is, say, AssertionError.__init__, then we
            # should not use getdesc() on it.  Never.  The problem is
            # that the py lib has its own AssertionError.__init__ which
            # is of type FunctionType.  But bookkeeper.immutablevalue()
            # will do the right thing in s_get_value().

        if type(value) in MemberDescriptorTypes:
            # skip __slots__, showing up in the class as 'member' objects
            return
        if name == '__init__' and self.is_builtin_exception_class():
            # pretend that built-in exceptions have no __init__,
            # unless explicitly specified in builtin.py
            from pypy.annotation.builtin import BUILTIN_ANALYZERS
            value = getattr(value, 'im_func', value)
            if value not in BUILTIN_ANALYZERS:
                return
        self.classdict[name] = Constant(value)

    def add_mixins(self, mixins, check_not_in=object):
        if not mixins:
            return
        A = type('tmp', tuple(mixins) + (object,), {})
        mro = A.__mro__
        assert mro[0] is A and mro[-1] is object
        mro = mro[1:-1]
        #
        skip = set()
        def add(cls):
            if cls is not object:
                for base in cls.__bases__:
                    add(base)
                for name in cls.__dict__:
                    skip.add(name)
        add(check_not_in)
        #
        for base in reversed(mro):
            assert base.__dict__.get("_mixin_", False), ("Mixin class %r has non"
                "mixin base class %r" % (mixins, base))
            for name, value in base.__dict__.items():
                if name in skip:
                    continue
                self.add_source_attribute(name, value, mixin=True)

    def add_sources_for_class(self, cls):
        for name, value in cls.__dict__.items():
            self.add_source_attribute(name, value)

    def getallclassdefs(self):
        return self._classdefs.values()

    def getclassdef(self, key):
        try:
            return self._classdefs[key]
        except KeyError:
            from pypy.annotation.classdef import ClassDef, FORCE_ATTRIBUTES_INTO_CLASSES
            classdef = ClassDef(self.bookkeeper, self)
            self.bookkeeper.classdefs.append(classdef)
            self._classdefs[key] = classdef

            # forced attributes
            if self.pyobj is not None:
                cls = self.pyobj
                if cls in FORCE_ATTRIBUTES_INTO_CLASSES:
                    for name, s_value in FORCE_ATTRIBUTES_INTO_CLASSES[cls].items():
                        classdef.generalize_attr(name, s_value)
                        classdef.find_attribute(name).modified(classdef)

            # register all class attributes as coming from this ClassDesc
            # (as opposed to prebuilt instances)
            classsources = {}
            for attr in self.classdict:
                classsources[attr] = self    # comes from this ClassDesc
            classdef.setup(classsources)
            # look for a __del__ method and annotate it if it's there
            if '__del__' in self.classdict:
                from pypy.annotation.model import s_None, SomeInstance
                s_func = self.s_read_attribute('__del__')
                args_s = [SomeInstance(classdef)]
                s = self.bookkeeper.emulate_pbc_call(classdef, s_func, args_s)
                assert s_None.contains(s)
            return classdef

    def getuniqueclassdef(self):
        if self.specialize:
            raise Exception("not supported on class %r because it needs "
                            "specialization" % (self.name,))
        return self.getclassdef(None)

    def pycall(self, schedule, args, s_previous_result, op=None):
        from pypy.annotation.model import SomeInstance, SomeImpossibleValue
        if self.specialize:
            if self.specialize == 'specialize:ctr_location':
                # We use the SomeInstance annotation returned the last time
                # to make sure we use the same ClassDef this time.
                if isinstance(s_previous_result, SomeInstance):
                    classdef = s_previous_result.classdef
                else:
                    classdef = self.getclassdef(object())
            else:
                raise Exception("unsupported specialization tag: %r" % (
                    self.specialize,))
        else:
            classdef = self.getuniqueclassdef()
        s_instance = SomeInstance(classdef)
        # look up __init__ directly on the class, bypassing the normal
        # lookup mechanisms ClassDef (to avoid influencing Attribute placement)
        s_init = self.s_read_attribute('__init__')
        if isinstance(s_init, SomeImpossibleValue):
            # no __init__: check that there are no constructor args
            if not self.is_exception_class():
                try:
                    args.fixedunpack(0)
                except ValueError:
                    raise Exception("default __init__ takes no argument"
                                    " (class %s)" % (self.name,))
            elif self.pyobj is Exception:
                # check explicitly against "raise Exception, x" where x
                # is a low-level exception pointer
                try:
                    [s_arg] = args.fixedunpack(1)
                except ValueError:
                    pass
                else:
                    from pypy.annotation.model import SomePtr
                    assert not isinstance(s_arg, SomePtr)
        else:
            # call the constructor
            args = args.prepend(s_instance)
            s_init.call(args)
        return s_instance

    def is_exception_class(self):
        return self.pyobj is not None and issubclass(self.pyobj,
                                                     py.builtin.BaseException)

    def is_builtin_exception_class(self):
        if self.is_exception_class():
            if self.pyobj.__module__ == 'exceptions':
                return True
            if issubclass(self.pyobj, AssertionError):
                return True
        return False

    def lookup(self, name):
        cdesc = self
        while name not in cdesc.classdict:
            cdesc = cdesc.basedesc
            if cdesc is None:
                return None
        else:
            return cdesc

    def read_attribute(self, name, default=NODEFAULT):
        cdesc = self.lookup(name)
        if cdesc is None:
            if default is NODEFAULT:
                raise AttributeError
            else:
                return default
        else:
            return cdesc.classdict[name]

    def s_read_attribute(self, name):
        # look up an attribute in the class
        cdesc = self.lookup(name)
        if cdesc is None:
            from pypy.annotation.model import s_ImpossibleValue
            return s_ImpossibleValue
        else:
            # delegate to s_get_value to turn it into an annotation
            return cdesc.s_get_value(None, name)

    def s_get_value(self, classdef, name):
        obj = self.classdict[name]
        if isinstance(obj, Constant):
            value = obj.value
            if isinstance(value, staticmethod):   # special case
                value = value.__get__(42)
                classdef = None   # don't bind
            elif isinstance(value, classmethod):
                raise AssertionError("classmethods are not supported")
            s_value = self.bookkeeper.immutablevalue(value)
            if classdef is not None:
                s_value = s_value.bind_callables_under(classdef, name)
        elif isinstance(obj, Desc):
            from pypy.annotation.model import SomePBC
            if classdef is not None:
                obj = obj.bind_under(classdef, name)
            s_value = SomePBC([obj])
        else:
            raise TypeError("classdict should not contain %r" % (obj,))
        return s_value

    def create_new_attribute(self, name, value):
        assert name not in self.classdict, "name clash: %r" % (name,)
        self.classdict[name] = Constant(value)

    def find_source_for(self, name):
        if name in self.classdict:
            return self
        if self.pyobj is not None:
            # check whether in the case the classdesc corresponds to a real class
            # there is a new attribute
            cls = self.pyobj
            if name in cls.__dict__:
                self.add_source_attribute(name, cls.__dict__[name])
                if name in self.classdict:
                    return self
        return None

    def maybe_return_immutable_list(self, attr, s_result):
        # hack: 'x.lst' where lst is listed in _immutable_fields_ as
        # either 'lst[*]' or 'lst?[*]'
        # should really return an immutable list as a result.  Implemented
        # by changing the result's annotation (but not, of course, doing an
        # actual copy in the rtyper).  Tested in pypy.rpython.test.test_rlist,
        # test_immutable_list_out_of_instance.
        search1 = '%s[*]' % (attr,)
        search2 = '%s?[*]' % (attr,)
        cdesc = self
        while cdesc is not None:
            if '_immutable_fields_' in cdesc.classdict:
                if (search1 in cdesc.classdict['_immutable_fields_'].value or
                    search2 in cdesc.classdict['_immutable_fields_'].value):
                    s_result.listdef.never_resize()
                    s_copy = s_result.listdef.offspring()
                    s_copy.listdef.mark_as_immutable()
                    return s_copy
            cdesc = cdesc.basedesc
        return s_result     # common case

    def consider_call_site(bookkeeper, family, descs, args, s_result, op):
        from pypy.annotation.model import SomeInstance, SomePBC, s_None
        if len(descs) == 1:
            # call to a single class, look at the result annotation
            # in case it was specialized
            if not isinstance(s_result, SomeInstance):
                raise Exception("calling a class didn't return an instance??")
            classdefs = [s_result.classdef]
        else:
            # call to multiple classes: specialization not supported
            classdefs = [desc.getuniqueclassdef() for desc in descs]
            # If some of the classes have an __init__ and others not, then
            # we complain, even though in theory it could work if all the
            # __init__s take no argument.  But it's messy to implement, so
            # let's just say it is not RPython and you have to add an empty
            # __init__ to your base class.
            has_init = False
            for desc in descs:
                s_init = desc.s_read_attribute('__init__')
                has_init |= isinstance(s_init, SomePBC)
            basedesc = ClassDesc.getcommonbase(descs)
            s_init = basedesc.s_read_attribute('__init__')
            parent_has_init = isinstance(s_init, SomePBC)
            if has_init and not parent_has_init:
                raise Exception("some subclasses among %r declare __init__(),"
                                " but not the common parent class" % (descs,))
        # make a PBC of MethodDescs, one for the __init__ of each class
        initdescs = []
        for desc, classdef in zip(descs, classdefs):
            s_init = desc.s_read_attribute('__init__')
            if isinstance(s_init, SomePBC):
                assert len(s_init.descriptions) == 1, (
                    "unexpected dynamic __init__?")
                initfuncdesc, = s_init.descriptions
                if isinstance(initfuncdesc, FunctionDesc):
                    initmethdesc = bookkeeper.getmethoddesc(initfuncdesc,
                                                            classdef,
                                                            classdef,
                                                            '__init__')
                    initdescs.append(initmethdesc)
        # register a call to exactly these __init__ methods
        if initdescs:
            initdescs[0].mergecallfamilies(*initdescs[1:])
            initfamily = initdescs[0].getcallfamily()
            MethodDesc.consider_call_site(bookkeeper, initfamily, initdescs,
                                          args, s_None, op)
    consider_call_site = staticmethod(consider_call_site)

    def getallbases(self):
        desc = self
        while desc is not None:
            yield desc
            desc = desc.basedesc

    def getcommonbase(descs):
        commondesc = descs[0]
        for desc in descs[1:]:
            allbases = set(commondesc.getallbases())
            while desc not in allbases:
                assert desc is not None, "no common base for %r" % (descs,)
                desc = desc.basedesc
            commondesc = desc
        return commondesc
    getcommonbase = staticmethod(getcommonbase)

    def rowkey(self):
        return self

    def getattrfamily(self, attrname):
        "Get the ClassAttrFamily object for attrname. Possibly creates one."
        access_sets = self.bookkeeper.get_classpbc_attr_families(attrname)
        _, _, attrfamily = access_sets.find(self)
        return attrfamily

    def queryattrfamily(self, attrname):
        """Retrieve the ClassAttrFamily object for attrname if there is one,
           otherwise return None."""
        access_sets = self.bookkeeper.get_classpbc_attr_families(attrname)
        try:
            return access_sets[self]
        except KeyError:
            return None

    def mergeattrfamilies(self, others, attrname):
        """Merge the attr families of the given Descs into one."""
        access_sets = self.bookkeeper.get_classpbc_attr_families(attrname)
        changed, rep, attrfamily = access_sets.find(self)
        for desc in others:
            changed1, rep, attrfamily = access_sets.union(rep, desc)
            changed = changed or changed1
        return changed


class MethodDesc(Desc):
    knowntype = types.MethodType

    def __init__(self, bookkeeper, funcdesc, originclassdef,
                 selfclassdef, name, flags={}):
        super(MethodDesc, self).__init__(bookkeeper)
        self.funcdesc = funcdesc
        self.originclassdef = originclassdef
        self.selfclassdef = selfclassdef
        self.name = name
        self.flags = flags

    def __repr__(self):
        if self.selfclassdef is None:
            return '<unbound MethodDesc %r of %r>' % (self.name,
                                                      self.originclassdef)
        else:
            return '<MethodDesc %r of %r bound to %r %r>' % (self.name,
                                                          self.originclassdef,
                                                          self.selfclassdef,
                                                          self.flags)

    def getuniquegraph(self):
        return self.funcdesc.getuniquegraph()

    def pycall(self, schedule, args, s_previous_result, op=None):
        from pypy.annotation.model import SomeInstance
        if self.selfclassdef is None:
            raise Exception("calling %r" % (self,))
        s_instance = SomeInstance(self.selfclassdef, flags = self.flags)
        args = args.prepend(s_instance)
        return self.funcdesc.pycall(schedule, args, s_previous_result, op)

    def bind_under(self, classdef, name):
        self.bookkeeper.warning("rebinding an already bound %r" % (self,))
        return self.funcdesc.bind_under(classdef, name)

    def bind_self(self, newselfclassdef, flags={}):
        return self.bookkeeper.getmethoddesc(self.funcdesc,
                                             self.originclassdef,
                                             newselfclassdef,
                                             self.name,
                                             flags)

    def consider_call_site(bookkeeper, family, descs, args, s_result, op):
        shape = rawshape(args, nextra=1)     # account for the extra 'self'
        funcdescs = [methoddesc.funcdesc for methoddesc in descs]
        row = FunctionDesc.row_to_consider(descs, args, op)
        family.calltable_add_row(shape, row)
    consider_call_site = staticmethod(consider_call_site)

    def rowkey(self):
        # we are computing call families and call tables that always contain
        # FunctionDescs, not MethodDescs.  The present method returns the
        # FunctionDesc to use as a key in that family.
        return self.funcdesc

    def simplify_desc_set(descs):
        # Some hacking needed to make contains() happy on SomePBC: if the
        # set of MethodDescs contains some "redundant" ones, i.e. ones that
        # are less general than others already in the set, then kill them.
        # This ensures that if 'a' is less general than 'b', then
        # SomePBC({a}) union SomePBC({b}) is again SomePBC({b}).
        #
        # Two cases:
        # 1. if two MethodDescs differ in their selfclassdefs, and if one
        #    of the selfclassdefs is a subclass of the other;
        # 2. if two MethodDescs differ in their flags, take the intersection.

        # --- case 2 ---
        # only keep the intersection of all the flags, that's good enough
        lst = list(descs)
        commonflags = lst[0].flags.copy()
        for key, value in commonflags.items():
            for desc in lst[1:]:
                if key not in desc.flags or desc.flags[key] != value:
                    del commonflags[key]
                    break
        for desc in lst:
            if desc.flags != commonflags:
                newdesc = desc.bookkeeper.getmethoddesc(desc.funcdesc,
                                                        desc.originclassdef,
                                                        desc.selfclassdef,
                                                        desc.name,
                                                        commonflags)
                descs.remove(desc)
                descs.add(newdesc)

        # --- case 1 ---
        groups = {}
        for desc in descs:
            if desc.selfclassdef is not None:
                key = desc.funcdesc, desc.originclassdef, desc.name
                groups.setdefault(key, []).append(desc)
        for group in groups.values():
            if len(group) > 1:
                for desc1 in group:
                    cdef1 = desc1.selfclassdef
                    for desc2 in group:
                        cdef2 = desc2.selfclassdef
                        if cdef1 is not cdef2 and cdef1.issubclass(cdef2):
                            descs.remove(desc1)
                            break
    simplify_desc_set = staticmethod(simplify_desc_set)


def new_or_old_class(c):
    if hasattr(c, '__class__'):
        return c.__class__
    else:
        return type(c)

class FrozenDesc(Desc):

    def __init__(self, bookkeeper, pyobj, read_attribute=None):
        super(FrozenDesc, self).__init__(bookkeeper, pyobj)
        if read_attribute is None:
            read_attribute = lambda attr: getattr(pyobj, attr)
        self._read_attribute = read_attribute
        self.attrcache = {}
        self.knowntype = new_or_old_class(pyobj)
        assert bool(pyobj), "__nonzero__ unsupported on frozen PBC %r" %(pyobj,)

    def has_attribute(self, attr):
        if attr in self.attrcache:
            return True
        try:
            self._read_attribute(attr)
            return True
        except AttributeError:
            return False

    def warn_missing_attribute(self, attr):
        # only warn for missing attribute names whose name doesn't start
        # with '$', to silence the warnings about '$memofield_xxx'.
        return not self.has_attribute(attr) and not attr.startswith('$')

    def read_attribute(self, attr):
        try:
            return self.attrcache[attr]
        except KeyError:
            result = self.attrcache[attr] = self._read_attribute(attr)
            return result

    def s_read_attribute(self, attr):
        try:
            value = self.read_attribute(attr)
        except AttributeError:
            from pypy.annotation.model import s_ImpossibleValue
            return s_ImpossibleValue
        else:
            return self.bookkeeper.immutablevalue(value)

    def create_new_attribute(self, name, value):
        try:
            self.read_attribute(name)
        except AttributeError:
            pass
        else:
            raise AssertionError("name clash: %r" % (name,))
        self.attrcache[name] = value

    def getattrfamily(self, attrname=None):
        "Get the FrozenAttrFamily object for attrname. Possibly creates one."
        access_sets = self.bookkeeper.frozenpbc_attr_families
        _, _, attrfamily = access_sets.find(self)
        return attrfamily

    def queryattrfamily(self, attrname=None):
        """Retrieve the FrozenAttrFamily object for attrname if there is one,
           otherwise return None."""
        access_sets = self.bookkeeper.frozenpbc_attr_families
        try:
            return access_sets[self]
        except KeyError:
            return None

    def mergeattrfamilies(self, others, attrname=None):
        """Merge the attr families of the given Descs into one."""
        access_sets = self.bookkeeper.frozenpbc_attr_families
        changed, rep, attrfamily = access_sets.find(self)
        for desc in others:
            changed1, rep, attrfamily = access_sets.union(rep, desc)
            changed = changed or changed1
        return changed


class MethodOfFrozenDesc(Desc):
    knowntype = types.MethodType

    def __init__(self, bookkeeper, funcdesc, frozendesc):
        super(MethodOfFrozenDesc, self).__init__(bookkeeper)
        self.funcdesc = funcdesc
        self.frozendesc = frozendesc

    def __repr__(self):
        return '<MethodOfFrozenDesc %r of %r>' % (self.funcdesc,
                                                  self.frozendesc)

    def pycall(self, schedule, args, s_previous_result, op=None):
        from pypy.annotation.model import SomePBC
        s_self = SomePBC([self.frozendesc])
        args = args.prepend(s_self)
        return self.funcdesc.pycall(schedule, args, s_previous_result, op)

    def consider_call_site(bookkeeper, family, descs, args, s_result, op):
        shape = rawshape(args, nextra=1)    # account for the extra 'self'
        funcdescs = [mofdesc.funcdesc for mofdesc in descs]
        row = FunctionDesc.row_to_consider(descs, args, op)
        family.calltable_add_row(shape, row)
    consider_call_site = staticmethod(consider_call_site)

    def rowkey(self):
        return self.funcdesc

# ____________________________________________________________

class Sample(object):
    __slots__ = 'x'
MemberDescriptorTypes = [type(Sample.x)]
del Sample
try:
    MemberDescriptorTypes.append(type(OSError.errno))
except AttributeError:    # on CPython <= 2.4
    pass
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