emitwrapper.py

"""Generate CPython API wrapper functions for native functions.

The wrapper functions are used by the CPython runtime when calling
native functions from interpreted code, and when the called function
can't be determined statically in compiled code. They validate, match,
unbox and type check function arguments, and box return values as
needed. All wrappers accept and return 'PyObject *' (boxed) values.

The wrappers aren't used for most calls between two native functions
or methods in a single compilation unit.
"""

from typing import List, Dict, Optional, Sequence

from mypy.nodes import ArgKind, ARG_POS, ARG_OPT, ARG_NAMED_OPT, ARG_NAMED, ARG_STAR, ARG_STAR2
from mypy.operators import op_methods_to_symbols, reverse_op_methods, reverse_op_method_names

from mypyc.common import PREFIX, NATIVE_PREFIX, DUNDER_PREFIX, use_vectorcall
from mypyc.codegen.emit import Emitter, ErrorHandler, GotoHandler, AssignHandler, ReturnHandler
from mypyc.ir.rtypes import (
    RType, RInstance, is_object_rprimitive, is_int_rprimitive, is_bool_rprimitive,
    object_rprimitive
)
from mypyc.ir.func_ir import FuncIR, RuntimeArg, FUNC_STATICMETHOD
from mypyc.ir.class_ir import ClassIR
from mypyc.namegen import NameGenerator


# Generic vectorcall wrapper functions (Python 3.7+)
#
# A wrapper function has a signature like this:
#
# PyObject *fn(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
#
# The function takes a self object, pointer to an array of arguments,
# the number of positional arguments, and a tuple of keyword argument
# names (that are stored starting in args[nargs]).
#
# It returns the returned object, or NULL on an exception.
#
# These are more efficient than legacy wrapper functions, since
# usually no tuple or dict objects need to be created for the
# arguments. Vectorcalls also use pre-constructed str objects for
# keyword argument names and other pre-computed information, instead
# of processing the argument format string on each call.


def wrapper_function_header(fn: FuncIR, names: NameGenerator) -> str:
    """Return header of a vectorcall wrapper function.

    See comment above for a summary of the arguments.
    """
    return (
        'PyObject *{prefix}{name}('
        'PyObject *self, PyObject *const *args, size_t nargs, PyObject *kwnames)').format(
            prefix=PREFIX,
            name=fn.cname(names))


def generate_traceback_code(fn: FuncIR,
                            emitter: Emitter,
                            source_path: str,
                            module_name: str) -> str:
    # If we hit an error while processing arguments, then we emit a
    # traceback frame to make it possible to debug where it happened.
    # Unlike traceback frames added for exceptions seen in IR, we do this
    # even if there is no `traceback_name`. This is because the error will
    # have originated here and so we need it in the traceback.
    globals_static = emitter.static_name('globals', module_name)
    traceback_code = 'CPy_AddTraceback("%s", "%s", %d, %s);' % (
        source_path.replace("\\", "\\\\"),
        fn.traceback_name or fn.name,
        fn.line,
        globals_static)
    return traceback_code


def make_arg_groups(args: List[RuntimeArg]) -> Dict[ArgKind, List[RuntimeArg]]:
    """Group arguments by kind."""
    return {k: [arg for arg in args if arg.kind == k] for k in ArgKind}


def reorder_arg_groups(groups: Dict[ArgKind, List[RuntimeArg]]) -> List[RuntimeArg]:
    """Reorder argument groups to match their order in a format string."""
    return groups[ARG_POS] + groups[ARG_OPT] + groups[ARG_NAMED_OPT] + groups[ARG_NAMED]


def make_static_kwlist(args: List[RuntimeArg]) -> str:
    arg_names = ''.join('"{}", '.format(arg.name) for arg in args)
    return 'static const char * const kwlist[] = {{{}0}};'.format(arg_names)


def make_format_string(func_name: Optional[str], groups: Dict[ArgKind, List[RuntimeArg]]) -> str:
    """Return a format string that specifies the accepted arguments.

    The format string is an extended subset of what is supported by
    PyArg_ParseTupleAndKeywords(). Only the type 'O' is used, and we
    also support some extensions:

    - Required keyword-only arguments are introduced after '@'
    - If the function receives *args or **kwargs, we add a '%' prefix

    Each group requires the previous groups' delimiters to be present
    first.

    These are used by both vectorcall and legacy wrapper functions.
    """
    format = ''
    if groups[ARG_STAR] or groups[ARG_STAR2]:
        format += '%'
    format += 'O' * len(groups[ARG_POS])
    if groups[ARG_OPT] or groups[ARG_NAMED_OPT] or groups[ARG_NAMED]:
        format += '|' + 'O' * len(groups[ARG_OPT])
    if groups[ARG_NAMED_OPT] or groups[ARG_NAMED]:
        format += '$' + 'O' * len(groups[ARG_NAMED_OPT])
    if groups[ARG_NAMED]:
        format += '@' + 'O' * len(groups[ARG_NAMED])
    if func_name is not None:
        format += ':{}'.format(func_name)
    return format


def generate_wrapper_function(fn: FuncIR,
                              emitter: Emitter,
                              source_path: str,
                              module_name: str) -> None:
    """Generate a CPython-compatible vectorcall wrapper for a native function.

    In particular, this handles unboxing the arguments, calling the native function, and
    then boxing the return value.
    """
    emitter.emit_line('{} {{'.format(wrapper_function_header(fn, emitter.names)))

    # If fn is a method, then the first argument is a self param
    real_args = list(fn.args)
    if fn.class_name and not fn.decl.kind == FUNC_STATICMETHOD:
        arg = real_args.pop(0)
        emitter.emit_line('PyObject *obj_{} = self;'.format(arg.name))

    # Need to order args as: required, optional, kwonly optional, kwonly required
    # This is because CPyArg_ParseStackAndKeywords format string requires
    # them grouped in that way.
    groups = make_arg_groups(real_args)
    reordered_args = reorder_arg_groups(groups)

    emitter.emit_line(make_static_kwlist(reordered_args))
    fmt = make_format_string(fn.name, groups)
    # Define the arguments the function accepts (but no types yet)
    emitter.emit_line('static CPyArg_Parser parser = {{"{}", kwlist, 0}};'.format(fmt))

    for arg in real_args:
        emitter.emit_line('PyObject *obj_{}{};'.format(
                          arg.name, ' = NULL' if arg.optional else ''))

    cleanups = ['CPy_DECREF(obj_{});'.format(arg.name)
                for arg in groups[ARG_STAR] + groups[ARG_STAR2]]

    arg_ptrs: List[str] = []
    if groups[ARG_STAR] or groups[ARG_STAR2]:
        arg_ptrs += ['&obj_{}'.format(groups[ARG_STAR][0].name) if groups[ARG_STAR] else 'NULL']
        arg_ptrs += ['&obj_{}'.format(groups[ARG_STAR2][0].name) if groups[ARG_STAR2] else 'NULL']
    arg_ptrs += ['&obj_{}'.format(arg.name) for arg in reordered_args]

    if fn.name == '__call__' and use_vectorcall(emitter.capi_version):
        nargs = 'PyVectorcall_NARGS(nargs)'
    else:
        nargs = 'nargs'
    parse_fn = 'CPyArg_ParseStackAndKeywords'
    # Special case some common signatures
    if len(real_args) == 0:
        # No args
        parse_fn = 'CPyArg_ParseStackAndKeywordsNoArgs'
    elif len(real_args) == 1 and len(groups[ARG_POS]) == 1:
        # Single positional arg
        parse_fn = 'CPyArg_ParseStackAndKeywordsOneArg'
    elif len(real_args) == len(groups[ARG_POS]) + len(groups[ARG_OPT]):
        # No keyword-only args, *args or **kwargs
        parse_fn = 'CPyArg_ParseStackAndKeywordsSimple'
    emitter.emit_lines(
        'if (!{}(args, {}, kwnames, &parser{})) {{'.format(
            parse_fn, nargs, ''.join(', ' + n for n in arg_ptrs)),
        'return NULL;',
        '}')
    traceback_code = generate_traceback_code(fn, emitter, source_path, module_name)
    generate_wrapper_core(fn, emitter, groups[ARG_OPT] + groups[ARG_NAMED_OPT],
                          cleanups=cleanups,
                          traceback_code=traceback_code)

    emitter.emit_line('}')


# Legacy generic wrapper functions
#
# These take a self object, a Python tuple of positional arguments,
# and a dict of keyword arguments. These are a lot slower than
# vectorcall wrappers, especially in calls involving keyword
# arguments.


def legacy_wrapper_function_header(fn: FuncIR, names: NameGenerator) -> str:
    return 'PyObject *{prefix}{name}(PyObject *self, PyObject *args, PyObject *kw)'.format(
        prefix=PREFIX,
        name=fn.cname(names))


def generate_legacy_wrapper_function(fn: FuncIR,
                                     emitter: Emitter,
                                     source_path: str,
                                     module_name: str) -> None:
    """Generates a CPython-compatible legacy wrapper for a native function.

    In particular, this handles unboxing the arguments, calling the native function, and
    then boxing the return value.
    """
    emitter.emit_line('{} {{'.format(legacy_wrapper_function_header(fn, emitter.names)))

    # If fn is a method, then the first argument is a self param
    real_args = list(fn.args)
    if fn.class_name and not fn.decl.kind == FUNC_STATICMETHOD:
        arg = real_args.pop(0)
        emitter.emit_line('PyObject *obj_{} = self;'.format(arg.name))

    # Need to order args as: required, optional, kwonly optional, kwonly required
    # This is because CPyArg_ParseTupleAndKeywords format string requires
    # them grouped in that way.
    groups = make_arg_groups(real_args)
    reordered_args = reorder_arg_groups(groups)

    emitter.emit_line(make_static_kwlist(reordered_args))
    for arg in real_args:
        emitter.emit_line('PyObject *obj_{}{};'.format(
                          arg.name, ' = NULL' if arg.optional else ''))

    cleanups = ['CPy_DECREF(obj_{});'.format(arg.name)
                for arg in groups[ARG_STAR] + groups[ARG_STAR2]]

    arg_ptrs: List[str] = []
    if groups[ARG_STAR] or groups[ARG_STAR2]:
        arg_ptrs += ['&obj_{}'.format(groups[ARG_STAR][0].name) if groups[ARG_STAR] else 'NULL']
        arg_ptrs += ['&obj_{}'.format(groups[ARG_STAR2][0].name) if groups[ARG_STAR2] else 'NULL']
    arg_ptrs += ['&obj_{}'.format(arg.name) for arg in reordered_args]

    emitter.emit_lines(
        'if (!CPyArg_ParseTupleAndKeywords(args, kw, "{}", "{}", kwlist{})) {{'.format(
            make_format_string(None, groups), fn.name, ''.join(', ' + n for n in arg_ptrs)),
        'return NULL;',
        '}')
    traceback_code = generate_traceback_code(fn, emitter, source_path, module_name)
    generate_wrapper_core(fn, emitter, groups[ARG_OPT] + groups[ARG_NAMED_OPT],
                          cleanups=cleanups,
                          traceback_code=traceback_code)

    emitter.emit_line('}')


# Specialized wrapper functions


def generate_dunder_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __dunder__ methods to be able to fit into the mapping
    protocol slot. This specifically means that the arguments are taken as *PyObjects and returned
    as *PyObjects.
    """
    gen = WrapperGenerator(cl, emitter)
    gen.set_target(fn)
    gen.emit_header()
    gen.emit_arg_processing()
    gen.emit_call()
    gen.finish()
    return gen.wrapper_name()


def generate_bin_op_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for a native binary dunder method.

    The same wrapper that handles the forward method (e.g. __add__) also handles
    the corresponding reverse method (e.g. __radd__), if defined.

    Both arguments and the return value are PyObject *.
    """
    gen = WrapperGenerator(cl, emitter)
    gen.set_target(fn)
    gen.arg_names = ['left', 'right']
    wrapper_name = gen.wrapper_name()

    gen.emit_header()
    if fn.name not in reverse_op_methods and fn.name in reverse_op_method_names:
        # There's only a reverse operator method.
        generate_bin_op_reverse_only_wrapper(cl, fn, emitter, gen)
    else:
        rmethod = reverse_op_methods[fn.name]
        fn_rev = cl.get_method(rmethod)
        if fn_rev is None:
            # There's only a forward operator method.
            generate_bin_op_forward_only_wrapper(cl, fn, emitter, gen)
        else:
            # There's both a forward and a reverse operator method.
            generate_bin_op_both_wrappers(cl, fn, fn_rev, emitter, gen)
    return wrapper_name


def generate_bin_op_forward_only_wrapper(cl: ClassIR,
                                         fn: FuncIR,
                                         emitter: Emitter,
                                         gen: 'WrapperGenerator') -> None:
    gen.emit_arg_processing(error=GotoHandler('typefail'), raise_exception=False)
    gen.emit_call(not_implemented_handler='goto typefail;')
    gen.emit_error_handling()
    emitter.emit_label('typefail')
    # If some argument has an incompatible type, treat this the same as
    # returning NotImplemented, and try to call the reverse operator method.
    #
    # Note that in normal Python you'd instead of an explicit
    # return of NotImplemented, but it doesn't generally work here
    # the body won't be executed at all if there is an argument
    # type check failure.
    #
    # The recommended way is to still use a type check in the
    # body. This will only be used in interpreted mode:
    #
    #    def __add__(self, other: int) -> Foo:
    #        if not isinstance(other, int):
    #            return NotImplemented
    #        ...
    rmethod = reverse_op_methods[fn.name]
    emitter.emit_line('_Py_IDENTIFIER({});'.format(rmethod))
    emitter.emit_line(
        'return CPy_CallReverseOpMethod(obj_left, obj_right, "{}", &PyId_{});'.format(
            op_methods_to_symbols[fn.name],
            rmethod))
    gen.finish()


def generate_bin_op_reverse_only_wrapper(cl: ClassIR,
                                         fn_rev: FuncIR,
                                         emitter: Emitter,
                                         gen: 'WrapperGenerator') -> None:
    gen.arg_names = ['right', 'left']
    gen.emit_arg_processing(error=GotoHandler('typefail'), raise_exception=False)
    gen.emit_call()
    gen.emit_error_handling()
    emitter.emit_label('typefail')
    emitter.emit_line('Py_INCREF(Py_NotImplemented);')
    emitter.emit_line('return Py_NotImplemented;')
    gen.finish()


def generate_bin_op_both_wrappers(cl: ClassIR,
                                  fn: FuncIR,
                                  fn_rev: FuncIR,
                                  emitter: Emitter,
                                  gen: 'WrapperGenerator') -> None:
    # There's both a forward and a reverse operator method. First
    # check if we should try calling the forward one. If the
    # argument type check fails, fall back to the reverse method.
    #
    # Similar to above, we can't perfectly match Python semantics.
    # In regular Python code you'd return NotImplemented if the
    # operand has the wrong type, but in compiled code we'll never
    # get to execute the type check.
    emitter.emit_line('if (PyObject_IsInstance(obj_left, (PyObject *){})) {{'.format(
        emitter.type_struct_name(cl)))
    gen.emit_arg_processing(error=GotoHandler('typefail'), raise_exception=False)
    gen.emit_call(not_implemented_handler='goto typefail;')
    gen.emit_error_handling()
    emitter.emit_line('}')
    emitter.emit_label('typefail')
    emitter.emit_line('if (PyObject_IsInstance(obj_right, (PyObject *){})) {{'.format(
        emitter.type_struct_name(cl)))
    gen.set_target(fn_rev)
    gen.arg_names = ['right', 'left']
    gen.emit_arg_processing(error=GotoHandler('typefail2'), raise_exception=False)
    gen.emit_call()
    gen.emit_error_handling()
    emitter.emit_line('} else {')
    emitter.emit_line('_Py_IDENTIFIER({});'.format(fn_rev.name))
    emitter.emit_line(
        'return CPy_CallReverseOpMethod(obj_left, obj_right, "{}", &PyId_{});'.format(
            op_methods_to_symbols[fn.name],
            fn_rev.name))
    emitter.emit_line('}')
    emitter.emit_label('typefail2')
    emitter.emit_line('Py_INCREF(Py_NotImplemented);')
    emitter.emit_line('return Py_NotImplemented;')
    gen.finish()


RICHCOMPARE_OPS = {
    '__lt__': 'Py_LT',
    '__gt__': 'Py_GT',
    '__le__': 'Py_LE',
    '__ge__': 'Py_GE',
    '__eq__': 'Py_EQ',
    '__ne__': 'Py_NE',
}


def generate_richcompare_wrapper(cl: ClassIR, emitter: Emitter) -> Optional[str]:
    """Generates a wrapper for richcompare dunder methods."""
    # Sort for determinism on Python 3.5
    matches = sorted([name for name in RICHCOMPARE_OPS if cl.has_method(name)])
    if not matches:
        return None

    name = '{}_RichCompare_{}'.format(DUNDER_PREFIX, cl.name_prefix(emitter.names))
    emitter.emit_line(
        'static PyObject *{name}(PyObject *obj_lhs, PyObject *obj_rhs, int op) {{'.format(
            name=name)
    )
    emitter.emit_line('switch (op) {')
    for func in matches:
        emitter.emit_line('case {}: {{'.format(RICHCOMPARE_OPS[func]))
        method = cl.get_method(func)
        assert method is not None
        generate_wrapper_core(method, emitter, arg_names=['lhs', 'rhs'])
        emitter.emit_line('}')
    emitter.emit_line('}')

    emitter.emit_line('Py_INCREF(Py_NotImplemented);')
    emitter.emit_line('return Py_NotImplemented;')

    emitter.emit_line('}')

    return name


def generate_get_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __get__ methods."""
    name = '{}{}{}'.format(DUNDER_PREFIX, fn.name, cl.name_prefix(emitter.names))
    emitter.emit_line(
        'static PyObject *{name}(PyObject *self, PyObject *instance, PyObject *owner) {{'.
        format(name=name))
    emitter.emit_line('instance = instance ? instance : Py_None;')
    emitter.emit_line('return {}{}(self, instance, owner);'.format(
        NATIVE_PREFIX,
        fn.cname(emitter.names)))
    emitter.emit_line('}')

    return name


def generate_hash_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __hash__ methods."""
    name = '{}{}{}'.format(DUNDER_PREFIX, fn.name, cl.name_prefix(emitter.names))
    emitter.emit_line('static Py_ssize_t {name}(PyObject *self) {{'.format(
        name=name
    ))
    emitter.emit_line('{}retval = {}{}{}(self);'.format(emitter.ctype_spaced(fn.ret_type),
                                                        emitter.get_group_prefix(fn.decl),
                                                        NATIVE_PREFIX,
                                                        fn.cname(emitter.names)))
    emitter.emit_error_check('retval', fn.ret_type, 'return -1;')
    if is_int_rprimitive(fn.ret_type):
        emitter.emit_line('Py_ssize_t val = CPyTagged_AsSsize_t(retval);')
    else:
        emitter.emit_line('Py_ssize_t val = PyLong_AsSsize_t(retval);')
    emitter.emit_dec_ref('retval', fn.ret_type)
    emitter.emit_line('if (PyErr_Occurred()) return -1;')
    # We can't return -1 from a hash function..
    emitter.emit_line('if (val == -1) return -2;')
    emitter.emit_line('return val;')
    emitter.emit_line('}')

    return name


def generate_len_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __len__ methods."""
    name = '{}{}{}'.format(DUNDER_PREFIX, fn.name, cl.name_prefix(emitter.names))
    emitter.emit_line('static Py_ssize_t {name}(PyObject *self) {{'.format(
        name=name
    ))
    emitter.emit_line('{}retval = {}{}{}(self);'.format(emitter.ctype_spaced(fn.ret_type),
                                                        emitter.get_group_prefix(fn.decl),
                                                        NATIVE_PREFIX,
                                                        fn.cname(emitter.names)))
    emitter.emit_error_check('retval', fn.ret_type, 'return -1;')
    if is_int_rprimitive(fn.ret_type):
        emitter.emit_line('Py_ssize_t val = CPyTagged_AsSsize_t(retval);')
    else:
        emitter.emit_line('Py_ssize_t val = PyLong_AsSsize_t(retval);')
    emitter.emit_dec_ref('retval', fn.ret_type)
    emitter.emit_line('if (PyErr_Occurred()) return -1;')
    emitter.emit_line('return val;')
    emitter.emit_line('}')

    return name


def generate_bool_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __bool__ methods."""
    name = '{}{}{}'.format(DUNDER_PREFIX, fn.name, cl.name_prefix(emitter.names))
    emitter.emit_line('static int {name}(PyObject *self) {{'.format(
        name=name
    ))
    emitter.emit_line('{}val = {}{}(self);'.format(emitter.ctype_spaced(fn.ret_type),
                                                   NATIVE_PREFIX,
                                                   fn.cname(emitter.names)))
    emitter.emit_error_check('val', fn.ret_type, 'return -1;')
    # This wouldn't be that hard to fix but it seems unimportant and
    # getting error handling and unboxing right would be fiddly. (And
    # way easier to do in IR!)
    assert is_bool_rprimitive(fn.ret_type), "Only bool return supported for __bool__"
    emitter.emit_line('return val;')
    emitter.emit_line('}')

    return name


def generate_del_item_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __delitem__.

    This is only called from a combined __delitem__/__setitem__ wrapper.
    """
    name = '{}{}{}'.format(DUNDER_PREFIX, '__delitem__', cl.name_prefix(emitter.names))
    input_args = ', '.join('PyObject *obj_{}'.format(arg.name) for arg in fn.args)
    emitter.emit_line('static int {name}({input_args}) {{'.format(
        name=name,
        input_args=input_args,
    ))
    generate_set_del_item_wrapper_inner(fn, emitter, fn.args)
    return name


def generate_set_del_item_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for native __setitem__ method (also works for __delitem__).

    This is used with the mapping protocol slot. Arguments are taken as *PyObjects and we
    return a negative C int on error.

    Create a separate wrapper function for __delitem__ as needed and have the
    __setitem__ wrapper call it if the value is NULL. Return the name
    of the outer (__setitem__) wrapper.
    """
    method_cls = cl.get_method_and_class('__delitem__')
    del_name = None
    if method_cls and method_cls[1] == cl:
        # Generate a separate wrapper for __delitem__
        del_name = generate_del_item_wrapper(cl, method_cls[0], emitter)

    args = fn.args
    if fn.name == '__delitem__':
        # Add an extra argument for value that we expect to be NULL.
        args = list(args) + [RuntimeArg('___value', object_rprimitive, ARG_POS)]

    name = '{}{}{}'.format(DUNDER_PREFIX, '__setitem__', cl.name_prefix(emitter.names))
    input_args = ', '.join('PyObject *obj_{}'.format(arg.name) for arg in args)
    emitter.emit_line('static int {name}({input_args}) {{'.format(
        name=name,
        input_args=input_args,
    ))

    # First check if this is __delitem__
    emitter.emit_line('if (obj_{} == NULL) {{'.format(args[2].name))
    if del_name is not None:
        # We have a native implementation, so call it
        emitter.emit_line('return {}(obj_{}, obj_{});'.format(del_name,
                                                              args[0].name,
                                                              args[1].name))
    else:
        # Try to call superclass method instead
        emitter.emit_line(
            'PyObject *super = CPy_Super(CPyModule_builtins, obj_{});'.format(args[0].name))
        emitter.emit_line('if (super == NULL) return -1;')
        emitter.emit_line(
            'PyObject *result = PyObject_CallMethod(super, "__delitem__", "O", obj_{});'.format(
                args[1].name))
        emitter.emit_line('Py_DECREF(super);')
        emitter.emit_line('Py_XDECREF(result);')
        emitter.emit_line('return result == NULL ? -1 : 0;')
    emitter.emit_line('}')

    method_cls = cl.get_method_and_class('__setitem__')
    if method_cls and method_cls[1] == cl:
        generate_set_del_item_wrapper_inner(fn, emitter, args)
    else:
        emitter.emit_line(
            'PyObject *super = CPy_Super(CPyModule_builtins, obj_{});'.format(args[0].name))
        emitter.emit_line('if (super == NULL) return -1;')
        emitter.emit_line('PyObject *result;')

        if method_cls is None and cl.builtin_base is None:
            msg = "'{}' object does not support item assignment".format(cl.name)
            emitter.emit_line(
                'PyErr_SetString(PyExc_TypeError, "{}");'.format(msg))
            emitter.emit_line('result = NULL;')
        else:
            # A base class may have __setitem__
            emitter.emit_line(
                'result = PyObject_CallMethod(super, "__setitem__", "OO", obj_{}, obj_{});'.format(
                    args[1].name, args[2].name))
        emitter.emit_line('Py_DECREF(super);')
        emitter.emit_line('Py_XDECREF(result);')
        emitter.emit_line('return result == NULL ? -1 : 0;')
        emitter.emit_line('}')
    return name


def generate_set_del_item_wrapper_inner(fn: FuncIR, emitter: Emitter,
                                        args: Sequence[RuntimeArg]) -> None:
    for arg in args:
        generate_arg_check(arg.name, arg.type, emitter, GotoHandler('fail'))
    native_args = ', '.join('arg_{}'.format(arg.name) for arg in args)
    emitter.emit_line('{}val = {}{}({});'.format(emitter.ctype_spaced(fn.ret_type),
                                                 NATIVE_PREFIX,
                                                 fn.cname(emitter.names),
                                                 native_args))
    emitter.emit_error_check('val', fn.ret_type, 'goto fail;')
    emitter.emit_dec_ref('val', fn.ret_type)
    emitter.emit_line('return 0;')
    emitter.emit_label('fail')
    emitter.emit_line('return -1;')
    emitter.emit_line('}')


def generate_contains_wrapper(cl: ClassIR, fn: FuncIR, emitter: Emitter) -> str:
    """Generates a wrapper for a native __contains__ method."""
    name = '{}{}{}'.format(DUNDER_PREFIX, fn.name, cl.name_prefix(emitter.names))
    emitter.emit_line(
        'static int {name}(PyObject *self, PyObject *obj_item) {{'.
        format(name=name))
    generate_arg_check('item', fn.args[1].type, emitter, ReturnHandler('-1'))
    emitter.emit_line('{}val = {}{}(self, arg_item);'.format(emitter.ctype_spaced(fn.ret_type),
                                                             NATIVE_PREFIX,
                                                             fn.cname(emitter.names)))
    emitter.emit_error_check('val', fn.ret_type, 'return -1;')
    if is_bool_rprimitive(fn.ret_type):
        emitter.emit_line('return val;')
    else:
        emitter.emit_line('int boolval = PyObject_IsTrue(val);')
        emitter.emit_dec_ref('val', fn.ret_type)
        emitter.emit_line('return boolval;')
    emitter.emit_line('}')

    return name


# Helpers


def generate_wrapper_core(fn: FuncIR,
                          emitter: Emitter,
                          optional_args: Optional[List[RuntimeArg]] = None,
                          arg_names: Optional[List[str]] = None,
                          cleanups: Optional[List[str]] = None,
                          traceback_code: Optional[str] = None) -> None:
    """Generates the core part of a wrapper function for a native function.

    This expects each argument as a PyObject * named obj_{arg} as a precondition.
    It converts the PyObject *s to the necessary types, checking and unboxing if necessary,
    makes the call, then boxes the result if necessary and returns it.
    """

    optional_args = optional_args or []
    cleanups = cleanups or []
    use_goto = bool(cleanups or traceback_code)
    error = ReturnHandler('NULL') if not use_goto else GotoHandler('fail')

    arg_names = arg_names or [arg.name for arg in fn.args]
    for arg_name, arg in zip(arg_names, fn.args):
        # Suppress the argument check for *args/**kwargs, since we know it must be right.
        typ = arg.type if arg.kind not in (ARG_STAR, ARG_STAR2) else object_rprimitive
        generate_arg_check(arg_name,
                           typ,
                           emitter,
                           error,
                           optional=arg in optional_args)
    native_args = ', '.join('arg_{}'.format(arg) for arg in arg_names)
    if fn.ret_type.is_unboxed or use_goto:
        # TODO: The Py_RETURN macros return the correct PyObject * with reference count handling.
        #       Are they relevant?
        emitter.emit_line('{}retval = {}{}({});'.format(emitter.ctype_spaced(fn.ret_type),
                                                        NATIVE_PREFIX,
                                                        fn.cname(emitter.names),
                                                        native_args))
        emitter.emit_lines(*cleanups)
        if fn.ret_type.is_unboxed:
            emitter.emit_error_check('retval', fn.ret_type, 'return NULL;')
            emitter.emit_box('retval', 'retbox', fn.ret_type, declare_dest=True)

        emitter.emit_line('return {};'.format('retbox' if fn.ret_type.is_unboxed else 'retval'))
    else:
        emitter.emit_line('return {}{}({});'.format(NATIVE_PREFIX,
                                                    fn.cname(emitter.names),
                                                    native_args))
        # TODO: Tracebacks?

    if use_goto:
        emitter.emit_label('fail')
        emitter.emit_lines(*cleanups)
        if traceback_code:
            emitter.emit_lines(traceback_code)
        emitter.emit_lines('return NULL;')


def generate_arg_check(name: str,
                       typ: RType,
                       emitter: Emitter,
                       error: Optional[ErrorHandler] = None,
                       *,
                       optional: bool = False,
                       raise_exception: bool = True) -> None:
    """Insert a runtime check for argument and unbox if necessary.

    The object is named PyObject *obj_{}. This is expected to generate
    a value of name arg_{} (unboxed if necessary). For each primitive a runtime
    check ensures the correct type.
    """
    error = error or AssignHandler()
    if typ.is_unboxed:
        # Borrow when unboxing to avoid reference count manipulation.
        emitter.emit_unbox('obj_{}'.format(name),
                           'arg_{}'.format(name),
                           typ,
                           declare_dest=True,
                           raise_exception=raise_exception,
                           error=error,
                           borrow=True,
                           optional=optional)
    elif is_object_rprimitive(typ):
        # Object is trivial since any object is valid
        if optional:
            emitter.emit_line('PyObject *arg_{};'.format(name))
            emitter.emit_line('if (obj_{} == NULL) {{'.format(name))
            emitter.emit_line('arg_{} = {};'.format(name, emitter.c_error_value(typ)))
            emitter.emit_lines('} else {', 'arg_{} = obj_{}; '.format(name, name), '}')
        else:
            emitter.emit_line('PyObject *arg_{} = obj_{};'.format(name, name))
    else:
        emitter.emit_cast('obj_{}'.format(name),
                          'arg_{}'.format(name),
                          typ,
                          declare_dest=True,
                          raise_exception=raise_exception,
                          error=error,
                          optional=optional)


class WrapperGenerator:
    """Helper that simplifies the generation of wrapper functions."""

    # TODO: Use this for more wrappers

    def __init__(self, cl: ClassIR, emitter: Emitter) -> None:
        self.cl = cl
        self.emitter = emitter
        self.cleanups: List[str] = []
        self.optional_args: List[RuntimeArg] = []
        self.traceback_code = ''

    def set_target(self, fn: FuncIR) -> None:
        """Set the wrapped function.

        It's fine to modify the attributes initialized here later to customize
        the wrapper function.
        """
        self.target_name = fn.name
        self.target_cname = fn.cname(self.emitter.names)
        self.arg_names = [arg.name for arg in fn.args]
        self.args = fn.args[:]
        self.ret_type = fn.ret_type

    def wrapper_name(self) -> str:
        """Return the name of the wrapper function."""
        return '{}{}{}'.format(DUNDER_PREFIX,
                               self.target_name,
                               self.cl.name_prefix(self.emitter.names))

    def use_goto(self) -> bool:
        """Do we use a goto for error handling (instead of straight return)?"""
        return bool(self.cleanups or self.traceback_code)

    def emit_header(self) -> None:
        """Emit the function header of the wrapper implementation."""
        input_args = ', '.join('PyObject *obj_{}'.format(arg) for arg in self.arg_names)
        self.emitter.emit_line('static PyObject *{name}({input_args}) {{'.format(
            name=self.wrapper_name(),
            input_args=input_args,
        ))

    def emit_arg_processing(self,
                            error: Optional[ErrorHandler] = None,
                            raise_exception: bool = True) -> None:
        """Emit validation and unboxing of arguments."""
        error = error or self.error()
        for arg_name, arg in zip(self.arg_names, self.args):
            # Suppress the argument check for *args/**kwargs, since we know it must be right.
            typ = arg.type if arg.kind not in (ARG_STAR, ARG_STAR2) else object_rprimitive
            generate_arg_check(arg_name,
                               typ,
                               self.emitter,
                               error,
                               raise_exception=raise_exception,
                               optional=arg in self.optional_args)

    def emit_call(self, not_implemented_handler: str = '') -> None:
        """Emit call to the wrapper function.

        If not_implemented_handler is non-empty, use this C code to handle
        a NotImplemented return value (if it's possible based on the return type).
        """
        native_args = ', '.join('arg_{}'.format(arg) for arg in self.arg_names)
        ret_type = self.ret_type
        emitter = self.emitter
        if ret_type.is_unboxed or self.use_goto():
            # TODO: The Py_RETURN macros return the correct PyObject * with reference count
            #       handling. Are they relevant?
            emitter.emit_line('{}retval = {}{}({});'.format(emitter.ctype_spaced(ret_type),
                                                            NATIVE_PREFIX,
                                                            self.target_cname,
                                                            native_args))
            emitter.emit_lines(*self.cleanups)
            if ret_type.is_unboxed:
                emitter.emit_error_check('retval', ret_type, 'return NULL;')
                emitter.emit_box('retval', 'retbox', ret_type, declare_dest=True)

            emitter.emit_line(
                'return {};'.format('retbox' if ret_type.is_unboxed else 'retval'))
        else:
            if not_implemented_handler and not isinstance(ret_type, RInstance):
                # The return value type may overlap with NotImplemented.
                emitter.emit_line('PyObject *retbox = {}{}({});'.format(NATIVE_PREFIX,
                                                                        self.target_cname,
                                                                        native_args))
                emitter.emit_lines('if (retbox == Py_NotImplemented) {',
                                   not_implemented_handler,
                                   '}',
                                   'return retbox;')
            else:
                emitter.emit_line('return {}{}({});'.format(NATIVE_PREFIX,
                                                            self.target_cname,
                                                            native_args))
            # TODO: Tracebacks?

    def error(self) -> ErrorHandler:
        """Figure out how to deal with errors in the wrapper."""
        if self.cleanups or self.traceback_code:
            # We'll have a label at the end with error handling code.
            return GotoHandler('fail')
        else:
            # Nothing special needs to done to handle errors, so just return.
            return ReturnHandler('NULL')

    def emit_error_handling(self) -> None:
        """Emit error handling block at the end of the wrapper, if needed."""
        emitter = self.emitter
        if self.use_goto():
            emitter.emit_label('fail')
            emitter.emit_lines(*self.cleanups)
            if self.traceback_code:
                emitter.emit_line(self.traceback_code)
            emitter.emit_line('return NULL;')

    def finish(self) -> None:
        self.emitter.emit_line('}')