python3-mypy 0.560-1 / usr / lib / python3 / dist-packages / mypy / fastparse2.py

"""
This file is nearly identical to `fastparse.py`, except that it works with a Python 2
AST instead of a Python 3 AST.

Previously, how we handled Python 2 code was by first obtaining the Python 2 AST via
typed_ast, converting it into a Python 3 AST by using typed_ast.conversion, then
running it through mypy.fastparse.

While this worked, it did add some overhead, especially in larger Python 2 codebases.
This module allows us to skip the conversion step, saving us some time.

The reason why this file is not easily merged with mypy.fastparse despite the large amount
of redundancy is because the Python 2 AST and the Python 3 AST nodes belong to two completely
different class heirarchies, which made it difficult to write a shared visitor between the
two in a typesafe way.
"""
from functools import wraps
import sys

from typing import Tuple, Union, TypeVar, Callable, Sequence, Optional, Any, cast, List, Set
from mypy.sharedparse import (
    special_function_elide_names, argument_elide_name,
)
from mypy.nodes import (
    MypyFile, Node, ImportBase, Import, ImportAll, ImportFrom, FuncDef, OverloadedFuncDef,
    ClassDef, Decorator, Block, Var, OperatorAssignmentStmt,
    ExpressionStmt, AssignmentStmt, ReturnStmt, RaiseStmt, AssertStmt,
    DelStmt, BreakStmt, ContinueStmt, PassStmt, GlobalDecl,
    WhileStmt, ForStmt, IfStmt, TryStmt, WithStmt,
    TupleExpr, GeneratorExpr, ListComprehension, ListExpr, ConditionalExpr,
    DictExpr, SetExpr, NameExpr, IntExpr, StrExpr, BytesExpr, UnicodeExpr,
    FloatExpr, CallExpr, SuperExpr, MemberExpr, IndexExpr, SliceExpr, OpExpr,
    UnaryExpr, LambdaExpr, ComparisonExpr, DictionaryComprehension,
    SetComprehension, ComplexExpr, EllipsisExpr, YieldExpr, Argument,
    Expression, Statement, BackquoteExpr, PrintStmt, ExecStmt,
    ARG_POS, ARG_OPT, ARG_STAR, ARG_NAMED, ARG_STAR2, OverloadPart, check_arg_names,
)
from mypy.types import (
    Type, CallableType, AnyType, UnboundType, EllipsisType, TypeOfAny
)
from mypy import experiments
from mypy import messages
from mypy.errors import Errors
from mypy.fastparse import TypeConverter, parse_type_comment
from mypy.options import Options

try:
    from typed_ast import ast27
    from typed_ast import ast3
except ImportError:
    if sys.version_info.minor > 2:
        try:
            from typed_ast import ast35  # type: ignore
        except ImportError:
            print('The typed_ast package is not installed.\n'
                  'You can install it with `python3 -m pip install typed-ast`.',
                  file=sys.stderr)
        else:
            print('You need a more recent version of the typed_ast package.\n'
                  'You can update to the latest version with '
                  '`python3 -m pip install -U typed-ast`.',
                  file=sys.stderr)
    else:
        print('Mypy requires the typed_ast package, which is only compatible with\n'
              'Python 3.3 and greater.', file=sys.stderr)
    sys.exit(1)

T = TypeVar('T', bound=Union[ast27.expr, ast27.stmt])
U = TypeVar('U', bound=Node)
V = TypeVar('V')

# There is no way to create reasonable fallbacks at this stage,
# they must be patched later.
_dummy_fallback = None  # type: Any

TYPE_COMMENT_SYNTAX_ERROR = 'syntax error in type comment'
TYPE_COMMENT_AST_ERROR = 'invalid type comment'


def parse(source: Union[str, bytes],
          fnam: str,
          module: Optional[str],
          errors: Optional[Errors] = None,
          options: Options = Options()) -> MypyFile:
    """Parse a source file, without doing any semantic analysis.

    Return the parse tree. If errors is not provided, raise ParseError
    on failure. Otherwise, use the errors object to report parse errors.
    """
    raise_on_error = False
    if errors is None:
        errors = Errors()
        raise_on_error = True
    errors.set_file(fnam, module)
    is_stub_file = fnam.endswith('.pyi')
    try:
        assert options.python_version[0] < 3 and not is_stub_file
        ast = ast27.parse(source, fnam, 'exec')
        tree = ASTConverter(options=options,
                            is_stub=is_stub_file,
                            errors=errors,
                            ).visit(ast)
        assert isinstance(tree, MypyFile)
        tree.path = fnam
        tree.is_stub = is_stub_file
    except SyntaxError as e:
        errors.report(e.lineno, e.offset, e.msg, blocker=True)
        tree = MypyFile([], [], False, set())

    if raise_on_error and errors.is_errors():
        errors.raise_error()

    return tree


def with_line(f: Callable[['ASTConverter', T], U]) -> Callable[['ASTConverter', T], U]:
    @wraps(f)
    def wrapper(self: 'ASTConverter', ast: T) -> U:
        node = f(self, ast)
        node.set_line(ast.lineno, ast.col_offset)
        return node
    return wrapper


def find(f: Callable[[V], bool], seq: Sequence[V]) -> Optional[V]:
    for item in seq:
        if f(item):
            return item
    return None


def is_no_type_check_decorator(expr: ast27.expr) -> bool:
    if isinstance(expr, ast27.Name):
        return expr.id == 'no_type_check'
    elif isinstance(expr, ast27.Attribute):
        if isinstance(expr.value, ast27.Name):
            return expr.value.id == 'typing' and expr.attr == 'no_type_check'
    return False


class ASTConverter(ast27.NodeTransformer):
    def __init__(self,
                 options: Options,
                 is_stub: bool,
                 errors: Errors) -> None:
        self.class_nesting = 0
        self.imports = []  # type: List[ImportBase]

        self.options = options
        self.is_stub = is_stub
        self.errors = errors

    def fail(self, msg: str, line: int, column: int) -> None:
        self.errors.report(line, column, msg, blocker=True)

    def generic_visit(self, node: ast27.AST) -> None:
        raise RuntimeError('AST node not implemented: ' + str(type(node)))

    def visit(self, node: Optional[ast27.AST]) -> Any:  # same as in typed_ast stub
        if node is None:
            return None
        return super().visit(node)

    def translate_expr_list(self, l: Sequence[ast27.AST]) -> List[Expression]:
        res = []  # type: List[Expression]
        for e in l:
            exp = self.visit(e)
            assert isinstance(exp, Expression)
            res.append(exp)
        return res

    def translate_stmt_list(self, l: Sequence[ast27.AST]) -> List[Statement]:
        res = []  # type: List[Statement]
        for e in l:
            stmt = self.visit(e)
            assert isinstance(stmt, Statement)
            res.append(stmt)
        return res

    op_map = {
        ast27.Add: '+',
        ast27.Sub: '-',
        ast27.Mult: '*',
        ast27.Div: '/',
        ast27.Mod: '%',
        ast27.Pow: '**',
        ast27.LShift: '<<',
        ast27.RShift: '>>',
        ast27.BitOr: '|',
        ast27.BitXor: '^',
        ast27.BitAnd: '&',
        ast27.FloorDiv: '//'
    }

    def from_operator(self, op: ast27.operator) -> str:
        op_name = ASTConverter.op_map.get(type(op))
        if op_name is None:
            raise RuntimeError('Unknown operator ' + str(type(op)))
        elif op_name == '@':
            raise RuntimeError('mypy does not support the MatMult operator')
        else:
            return op_name

    comp_op_map = {
        ast27.Gt: '>',
        ast27.Lt: '<',
        ast27.Eq: '==',
        ast27.GtE: '>=',
        ast27.LtE: '<=',
        ast27.NotEq: '!=',
        ast27.Is: 'is',
        ast27.IsNot: 'is not',
        ast27.In: 'in',
        ast27.NotIn: 'not in'
    }

    def from_comp_operator(self, op: ast27.cmpop) -> str:
        op_name = ASTConverter.comp_op_map.get(type(op))
        if op_name is None:
            raise RuntimeError('Unknown comparison operator ' + str(type(op)))
        else:
            return op_name

    def as_block(self, stmts: List[ast27.stmt], lineno: int) -> Optional[Block]:
        b = None
        if stmts:
            b = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
            b.set_line(lineno)
        return b

    def as_required_block(self, stmts: List[ast27.stmt], lineno: int) -> Block:
        assert stmts  # must be non-empty
        b = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
        b.set_line(lineno)
        return b

    def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
        ret = []  # type: List[Statement]
        current_overload = []  # type: List[OverloadPart]
        current_overload_name = None
        for stmt in stmts:
            if (current_overload_name is not None
                    and isinstance(stmt, (Decorator, FuncDef))
                    and stmt.name() == current_overload_name):
                current_overload.append(stmt)
            else:
                if len(current_overload) == 1:
                    ret.append(current_overload[0])
                elif len(current_overload) > 1:
                    ret.append(OverloadedFuncDef(current_overload))

                if isinstance(stmt, Decorator):
                    current_overload = [stmt]
                    current_overload_name = stmt.name()
                else:
                    current_overload = []
                    current_overload_name = None
                    ret.append(stmt)

        if len(current_overload) == 1:
            ret.append(current_overload[0])
        elif len(current_overload) > 1:
            ret.append(OverloadedFuncDef(current_overload))
        return ret

    def in_class(self) -> bool:
        return self.class_nesting > 0

    def translate_module_id(self, id: str) -> str:
        """Return the actual, internal module id for a source text id.

        For example, translate '__builtin__' in Python 2 to 'builtins'.
        """
        if id == self.options.custom_typing_module:
            return 'typing'
        elif id == '__builtin__':
            # HACK: __builtin__ in Python 2 is aliases to builtins. However, the implementation
            #   is named __builtin__.py (there is another layer of translation elsewhere).
            return 'builtins'
        return id

    def visit_Module(self, mod: ast27.Module) -> MypyFile:
        body = self.fix_function_overloads(self.translate_stmt_list(mod.body))

        return MypyFile(body,
                        self.imports,
                        False,
                        {ti.lineno for ti in mod.type_ignores},
                        )

    # --- stmt ---
    # FunctionDef(identifier name, arguments args,
    #             stmt* body, expr* decorator_list, expr? returns, string? type_comment)
    # arguments = (arg* args, arg? vararg, arg* kwonlyargs, expr* kw_defaults,
    #              arg? kwarg, expr* defaults)
    @with_line
    def visit_FunctionDef(self, n: ast27.FunctionDef) -> Statement:
        converter = TypeConverter(self.errors, line=n.lineno)
        args, decompose_stmts = self.transform_args(n.args, n.lineno)

        arg_kinds = [arg.kind for arg in args]
        arg_names = [arg.variable.name() for arg in args]  # type: List[Optional[str]]
        arg_names = [None if argument_elide_name(name) else name for name in arg_names]
        if special_function_elide_names(n.name):
            arg_names = [None] * len(arg_names)

        arg_types = []  # type: List[Optional[Type]]
        if (n.decorator_list and any(is_no_type_check_decorator(d) for d in n.decorator_list)):
            arg_types = [None] * len(args)
            return_type = None
        elif n.type_comment is not None and len(n.type_comment) > 0:
            try:
                func_type_ast = ast3.parse(n.type_comment, '<func_type>', 'func_type')
                assert isinstance(func_type_ast, ast3.FunctionType)
                # for ellipsis arg
                if (len(func_type_ast.argtypes) == 1 and
                        isinstance(func_type_ast.argtypes[0], ast3.Ellipsis)):
                    arg_types = [a.type_annotation
                                 if a.type_annotation is not None
                                 else AnyType(TypeOfAny.unannotated)
                                 for a in args]
                else:
                    # PEP 484 disallows both type annotations and type comments
                    if any(a.type_annotation is not None for a in args):
                        self.fail(messages.DUPLICATE_TYPE_SIGNATURES, n.lineno, n.col_offset)
                    arg_types = [a if a is not None else AnyType(TypeOfAny.unannotated) for
                                 a in converter.translate_expr_list(func_type_ast.argtypes)]
                return_type = converter.visit(func_type_ast.returns)

                # add implicit self type
                if self.in_class() and len(arg_types) < len(args):
                    arg_types.insert(0, AnyType(TypeOfAny.special_form))
            except SyntaxError:
                self.fail(TYPE_COMMENT_SYNTAX_ERROR, n.lineno, n.col_offset)
                arg_types = [AnyType(TypeOfAny.from_error)] * len(args)
                return_type = AnyType(TypeOfAny.from_error)
        else:
            arg_types = [a.type_annotation for a in args]
            return_type = converter.visit(None)

        for arg, arg_type in zip(args, arg_types):
            self.set_type_optional(arg_type, arg.initializer)

        func_type = None
        if any(arg_types) or return_type:
            if len(arg_types) != 1 and any(isinstance(t, EllipsisType) for t in arg_types):
                self.fail("Ellipses cannot accompany other argument types "
                          "in function type signature.", n.lineno, 0)
            elif len(arg_types) > len(arg_kinds):
                self.fail('Type signature has too many arguments', n.lineno, 0)
            elif len(arg_types) < len(arg_kinds):
                self.fail('Type signature has too few arguments', n.lineno, 0)
            else:
                any_type = AnyType(TypeOfAny.unannotated)
                func_type = CallableType([a if a is not None else any_type for a in arg_types],
                                        arg_kinds,
                                        arg_names,
                                        return_type if return_type is not None else any_type,
                                        _dummy_fallback)

        body = self.as_required_block(n.body, n.lineno)
        if decompose_stmts:
            body.body = decompose_stmts + body.body
        func_def = FuncDef(n.name,
                       args,
                       body,
                       func_type)
        if func_type is not None:
            func_type.definition = func_def
            func_type.line = n.lineno

        if n.decorator_list:
            var = Var(func_def.name())
            var.is_ready = False
            var.set_line(n.decorator_list[0].lineno)

            func_def.is_decorated = True
            func_def.set_line(n.lineno + len(n.decorator_list))
            func_def.body.set_line(func_def.get_line())
            return Decorator(func_def, self.translate_expr_list(n.decorator_list), var)
        else:
            return func_def

    def set_type_optional(self, type: Optional[Type], initializer: Optional[Expression]) -> None:
        if self.options.no_implicit_optional:
            return
        # Indicate that type should be wrapped in an Optional if arg is initialized to None.
        optional = isinstance(initializer, NameExpr) and initializer.name == 'None'
        if isinstance(type, UnboundType):
            type.optional = optional

    def transform_args(self,
                       n: ast27.arguments,
                       line: int,
                       ) -> Tuple[List[Argument], List[Statement]]:
        type_comments = n.type_comments
        converter = TypeConverter(self.errors, line=line)
        decompose_stmts = []  # type: List[Statement]

        def extract_names(arg: ast27.expr) -> List[str]:
            if isinstance(arg, ast27.Name):
                return [arg.id]
            elif isinstance(arg, ast27.Tuple):
                return [name for elt in arg.elts for name in extract_names(elt)]
            else:
                return []

        def convert_arg(index: int, arg: ast27.expr) -> Var:
            if isinstance(arg, ast27.Name):
                v = arg.id
            elif isinstance(arg, ast27.Tuple):
                v = '__tuple_arg_{}'.format(index + 1)
                rvalue = NameExpr(v)
                rvalue.set_line(line)
                assignment = AssignmentStmt([self.visit(arg)], rvalue)
                assignment.set_line(line)
                decompose_stmts.append(assignment)
            else:
                raise RuntimeError("'{}' is not a valid argument.".format(ast27.dump(arg)))
            return Var(v)

        def get_type(i: int) -> Optional[Type]:
            if i < len(type_comments) and type_comments[i] is not None:
                return converter.visit_raw_str(type_comments[i])
            return None

        args = [(convert_arg(i, arg), get_type(i)) for i, arg in enumerate(n.args)]
        defaults = self.translate_expr_list(n.defaults)
        names = [name for arg in n.args for name in extract_names(arg)]  # type: List[str]

        new_args = []  # type: List[Argument]
        num_no_defaults = len(args) - len(defaults)
        # positional arguments without defaults
        for a, annotation in args[:num_no_defaults]:
            new_args.append(Argument(a, annotation, None, ARG_POS))

        # positional arguments with defaults
        for (a, annotation), d in zip(args[num_no_defaults:], defaults):
            new_args.append(Argument(a, annotation, d, ARG_OPT))

        # *arg
        if n.vararg is not None:
            new_args.append(Argument(Var(n.vararg), get_type(len(args)), None, ARG_STAR))
            names.append(n.vararg)

        # **kwarg
        if n.kwarg is not None:
            typ = get_type(len(args) + (0 if n.vararg is None else 1))
            new_args.append(Argument(Var(n.kwarg), typ, None, ARG_STAR2))
            names.append(n.kwarg)

        # We don't have any context object to give, but we have closed around the line num
        def fail_arg(msg: str, arg: None) -> None:
            self.fail(msg, line, 0)
        check_arg_names(names, [None] * len(names), fail_arg)

        return new_args, decompose_stmts

    def stringify_name(self, n: ast27.AST) -> str:
        if isinstance(n, ast27.Name):
            return n.id
        elif isinstance(n, ast27.Attribute):
            return "{}.{}".format(self.stringify_name(n.value), n.attr)
        else:
            assert False, "can't stringify " + str(type(n))

    # ClassDef(identifier name,
    #  expr* bases,
    #  keyword* keywords,
    #  stmt* body,
    #  expr* decorator_list)
    @with_line
    def visit_ClassDef(self, n: ast27.ClassDef) -> ClassDef:
        self.class_nesting += 1

        cdef = ClassDef(n.name,
                        self.as_required_block(n.body, n.lineno),
                        None,
                        self.translate_expr_list(n.bases),
                        metaclass=None)
        cdef.decorators = self.translate_expr_list(n.decorator_list)
        self.class_nesting -= 1
        return cdef

    # Return(expr? value)
    @with_line
    def visit_Return(self, n: ast27.Return) -> ReturnStmt:
        return ReturnStmt(self.visit(n.value))

    # Delete(expr* targets)
    @with_line
    def visit_Delete(self, n: ast27.Delete) -> DelStmt:
        if len(n.targets) > 1:
            tup = TupleExpr(self.translate_expr_list(n.targets))
            tup.set_line(n.lineno)
            return DelStmt(tup)
        else:
            return DelStmt(self.visit(n.targets[0]))

    # Assign(expr* targets, expr value, string? type_comment)
    @with_line
    def visit_Assign(self, n: ast27.Assign) -> AssignmentStmt:
        typ = None
        if n.type_comment:
            typ = parse_type_comment(n.type_comment, n.lineno, self.errors)

        return AssignmentStmt(self.translate_expr_list(n.targets),
                              self.visit(n.value),
                              type=typ)

    # AugAssign(expr target, operator op, expr value)
    @with_line
    def visit_AugAssign(self, n: ast27.AugAssign) -> OperatorAssignmentStmt:
        return OperatorAssignmentStmt(self.from_operator(n.op),
                              self.visit(n.target),
                              self.visit(n.value))

    # For(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
    @with_line
    def visit_For(self, n: ast27.For) -> ForStmt:
        if n.type_comment is not None:
            target_type = parse_type_comment(n.type_comment, n.lineno, self.errors)
        else:
            target_type = None
        return ForStmt(self.visit(n.target),
                       self.visit(n.iter),
                       self.as_required_block(n.body, n.lineno),
                       self.as_block(n.orelse, n.lineno),
                       target_type)

    # While(expr test, stmt* body, stmt* orelse)
    @with_line
    def visit_While(self, n: ast27.While) -> WhileStmt:
        return WhileStmt(self.visit(n.test),
                         self.as_required_block(n.body, n.lineno),
                         self.as_block(n.orelse, n.lineno))

    # If(expr test, stmt* body, stmt* orelse)
    @with_line
    def visit_If(self, n: ast27.If) -> IfStmt:
        return IfStmt([self.visit(n.test)],
                      [self.as_required_block(n.body, n.lineno)],
                      self.as_block(n.orelse, n.lineno))

    # With(withitem* items, stmt* body, string? type_comment)
    @with_line
    def visit_With(self, n: ast27.With) -> WithStmt:
        if n.type_comment is not None:
            target_type = parse_type_comment(n.type_comment, n.lineno, self.errors)
        else:
            target_type = None
        return WithStmt([self.visit(n.context_expr)],
                        [self.visit(n.optional_vars)],
                        self.as_required_block(n.body, n.lineno),
                        target_type)

    @with_line
    def visit_Raise(self, n: ast27.Raise) -> RaiseStmt:
        if n.type is None:
            e = None
        else:
            if n.inst is None:
                e = self.visit(n.type)
            else:
                if n.tback is None:
                    e = TupleExpr([self.visit(n.type), self.visit(n.inst)])
                else:
                    e = TupleExpr([self.visit(n.type), self.visit(n.inst), self.visit(n.tback)])

        return RaiseStmt(e, None)

    # TryExcept(stmt* body, excepthandler* handlers, stmt* orelse)
    @with_line
    def visit_TryExcept(self, n: ast27.TryExcept) -> TryStmt:
        return self.try_handler(n.body, n.handlers, n.orelse, [], n.lineno)

    @with_line
    def visit_TryFinally(self, n: ast27.TryFinally) -> TryStmt:
        if len(n.body) == 1 and isinstance(n.body[0], ast27.TryExcept):
            return self.try_handler([n.body[0]], [], [], n.finalbody, n.lineno)
        else:
            return self.try_handler(n.body, [], [], n.finalbody, n.lineno)

    def try_handler(self,
                    body: List[ast27.stmt],
                    handlers: List[ast27.ExceptHandler],
                    orelse: List[ast27.stmt],
                    finalbody: List[ast27.stmt],
                    lineno: int) -> TryStmt:
        vs = []  # type: List[Optional[NameExpr]]
        for item in handlers:
            if item.name is None:
                vs.append(None)
            elif isinstance(item.name, ast27.Name):
                vs.append(NameExpr(item.name.id))
            else:
                self.fail("Sorry, `except <expr>, <anything but a name>` is not supported",
                          item.lineno, item.col_offset)
                vs.append(None)
        types = [self.visit(h.type) for h in handlers]
        handlers_ = [self.as_required_block(h.body, h.lineno) for h in handlers]

        return TryStmt(self.as_required_block(body, lineno),
                       vs,
                       types,
                       handlers_,
                       self.as_block(orelse, lineno),
                       self.as_block(finalbody, lineno))

    @with_line
    def visit_Print(self, n: ast27.Print) -> PrintStmt:
        return PrintStmt(self.translate_expr_list(n.values), n.nl, self.visit(n.dest))

    @with_line
    def visit_Exec(self, n: ast27.Exec) -> ExecStmt:
        return ExecStmt(self.visit(n.body),
                        self.visit(n.globals),
                        self.visit(n.locals))

    @with_line
    def visit_Repr(self, n: ast27.Repr) -> BackquoteExpr:
        return BackquoteExpr(self.visit(n.value))

    # Assert(expr test, expr? msg)
    @with_line
    def visit_Assert(self, n: ast27.Assert) -> AssertStmt:
        return AssertStmt(self.visit(n.test), self.visit(n.msg))

    # Import(alias* names)
    @with_line
    def visit_Import(self, n: ast27.Import) -> Import:
        names = []  # type: List[Tuple[str, Optional[str]]]
        for alias in n.names:
            name = self.translate_module_id(alias.name)
            asname = alias.asname
            if asname is None and name != alias.name:
                # if the module name has been translated (and it's not already
                # an explicit import-as), make it an implicit import-as the
                # original name
                asname = alias.name
            names.append((name, asname))
        i = Import(names)
        self.imports.append(i)
        return i

    # ImportFrom(identifier? module, alias* names, int? level)
    @with_line
    def visit_ImportFrom(self, n: ast27.ImportFrom) -> ImportBase:
        assert n.level is not None
        if len(n.names) == 1 and n.names[0].name == '*':
            mod = n.module if n.module is not None else ''
            i = ImportAll(mod, n.level)  # type: ImportBase
        else:
            i = ImportFrom(self.translate_module_id(n.module) if n.module is not None else '',
                           n.level,
                           [(a.name, a.asname) for a in n.names])
        self.imports.append(i)
        return i

    # Global(identifier* names)
    @with_line
    def visit_Global(self, n: ast27.Global) -> GlobalDecl:
        return GlobalDecl(n.names)

    # Expr(expr value)
    @with_line
    def visit_Expr(self, n: ast27.Expr) -> ExpressionStmt:
        value = self.visit(n.value)
        return ExpressionStmt(value)

    # Pass
    @with_line
    def visit_Pass(self, n: ast27.Pass) -> PassStmt:
        return PassStmt()

    # Break
    @with_line
    def visit_Break(self, n: ast27.Break) -> BreakStmt:
        return BreakStmt()

    # Continue
    @with_line
    def visit_Continue(self, n: ast27.Continue) -> ContinueStmt:
        return ContinueStmt()

    # --- expr ---
    # BoolOp(boolop op, expr* values)
    @with_line
    def visit_BoolOp(self, n: ast27.BoolOp) -> OpExpr:
        # mypy translates (1 and 2 and 3) as (1 and (2 and 3))
        assert len(n.values) >= 2
        if isinstance(n.op, ast27.And):
            op = 'and'
        elif isinstance(n.op, ast27.Or):
            op = 'or'
        else:
            raise RuntimeError('unknown BoolOp ' + str(type(n)))

        # potentially inefficient!
        def group(vals: List[Expression]) -> OpExpr:
            if len(vals) == 2:
                return OpExpr(op, vals[0], vals[1])
            else:
                return OpExpr(op, vals[0], group(vals[1:]))

        return group(self.translate_expr_list(n.values))

    # BinOp(expr left, operator op, expr right)
    @with_line
    def visit_BinOp(self, n: ast27.BinOp) -> OpExpr:
        op = self.from_operator(n.op)

        if op is None:
            raise RuntimeError('cannot translate BinOp ' + str(type(n.op)))

        return OpExpr(op, self.visit(n.left), self.visit(n.right))

    # UnaryOp(unaryop op, expr operand)
    @with_line
    def visit_UnaryOp(self, n: ast27.UnaryOp) -> UnaryExpr:
        op = None
        if isinstance(n.op, ast27.Invert):
            op = '~'
        elif isinstance(n.op, ast27.Not):
            op = 'not'
        elif isinstance(n.op, ast27.UAdd):
            op = '+'
        elif isinstance(n.op, ast27.USub):
            op = '-'

        if op is None:
            raise RuntimeError('cannot translate UnaryOp ' + str(type(n.op)))

        return UnaryExpr(op, self.visit(n.operand))

    # Lambda(arguments args, expr body)
    @with_line
    def visit_Lambda(self, n: ast27.Lambda) -> LambdaExpr:
        args, decompose_stmts = self.transform_args(n.args, n.lineno)

        n_body = ast27.Return(n.body)
        n_body.lineno = n.lineno
        n_body.col_offset = n.col_offset
        body = self.as_required_block([n_body], n.lineno)
        if decompose_stmts:
            body.body = decompose_stmts + body.body

        return LambdaExpr(args, body)

    # IfExp(expr test, expr body, expr orelse)
    @with_line
    def visit_IfExp(self, n: ast27.IfExp) -> ConditionalExpr:
        return ConditionalExpr(self.visit(n.test),
                               self.visit(n.body),
                               self.visit(n.orelse))

    # Dict(expr* keys, expr* values)
    @with_line
    def visit_Dict(self, n: ast27.Dict) -> DictExpr:
        return DictExpr(list(zip(self.translate_expr_list(n.keys),
                                 self.translate_expr_list(n.values))))

    # Set(expr* elts)
    @with_line
    def visit_Set(self, n: ast27.Set) -> SetExpr:
        return SetExpr(self.translate_expr_list(n.elts))

    # ListComp(expr elt, comprehension* generators)
    @with_line
    def visit_ListComp(self, n: ast27.ListComp) -> ListComprehension:
        return ListComprehension(self.visit_GeneratorExp(cast(ast27.GeneratorExp, n)))

    # SetComp(expr elt, comprehension* generators)
    @with_line
    def visit_SetComp(self, n: ast27.SetComp) -> SetComprehension:
        return SetComprehension(self.visit_GeneratorExp(cast(ast27.GeneratorExp, n)))

    # DictComp(expr key, expr value, comprehension* generators)
    @with_line
    def visit_DictComp(self, n: ast27.DictComp) -> DictionaryComprehension:
        targets = [self.visit(c.target) for c in n.generators]
        iters = [self.visit(c.iter) for c in n.generators]
        ifs_list = [self.translate_expr_list(c.ifs) for c in n.generators]
        return DictionaryComprehension(self.visit(n.key),
                                       self.visit(n.value),
                                       targets,
                                       iters,
                                       ifs_list,
                                       [False for _ in n.generators])

    # GeneratorExp(expr elt, comprehension* generators)
    @with_line
    def visit_GeneratorExp(self, n: ast27.GeneratorExp) -> GeneratorExpr:
        targets = [self.visit(c.target) for c in n.generators]
        iters = [self.visit(c.iter) for c in n.generators]
        ifs_list = [self.translate_expr_list(c.ifs) for c in n.generators]
        return GeneratorExpr(self.visit(n.elt),
                             targets,
                             iters,
                             ifs_list,
                             [False for _ in n.generators])

    # Yield(expr? value)
    @with_line
    def visit_Yield(self, n: ast27.Yield) -> YieldExpr:
        return YieldExpr(self.visit(n.value))

    # Compare(expr left, cmpop* ops, expr* comparators)
    @with_line
    def visit_Compare(self, n: ast27.Compare) -> ComparisonExpr:
        operators = [self.from_comp_operator(o) for o in n.ops]
        operands = self.translate_expr_list([n.left] + n.comparators)
        return ComparisonExpr(operators, operands)

    # Call(expr func, expr* args, keyword* keywords)
    # keyword = (identifier? arg, expr value)
    @with_line
    def visit_Call(self, n: ast27.Call) -> CallExpr:
        arg_types = []  # type: List[ast27.expr]
        arg_kinds = []  # type: List[int]
        signature = []  # type: List[Optional[str]]

        arg_types.extend(n.args)
        arg_kinds.extend(ARG_POS for a in n.args)
        signature.extend(None for a in n.args)

        if n.starargs is not None:
            arg_types.append(n.starargs)
            arg_kinds.append(ARG_STAR)
            signature.append(None)

        arg_types.extend(k.value for k in n.keywords)
        arg_kinds.extend(ARG_NAMED for k in n.keywords)
        signature.extend(k.arg for k in n.keywords)

        if n.kwargs is not None:
            arg_types.append(n.kwargs)
            arg_kinds.append(ARG_STAR2)
            signature.append(None)

        return CallExpr(self.visit(n.func),
                        self.translate_expr_list(arg_types),
                        arg_kinds,
                        signature)

    # Num(object n) -- a number as a PyObject.
    @with_line
    def visit_Num(self, new: ast27.Num) -> Expression:
        value = new.n
        is_inverse = False
        if str(new.n).startswith('-'):  # Hackish because of complex.
            value = -new.n
            is_inverse = True

        if isinstance(value, int):
            expr = IntExpr(value)  # type: Expression
        elif isinstance(value, float):
            expr = FloatExpr(value)
        elif isinstance(value, complex):
            expr = ComplexExpr(value)
        else:
            raise RuntimeError('num not implemented for ' + str(type(new.n)))

        if is_inverse:
            expr = UnaryExpr('-', expr)

        return expr

    # Str(string s)
    @with_line
    def visit_Str(self, s: ast27.Str) -> Expression:
        # Hack: assume all string literals in Python 2 stubs are normal
        # strs (i.e. not unicode).  All stubs are parsed with the Python 3
        # parser, which causes unprefixed string literals to be interpreted
        # as unicode instead of bytes.  This hack is generally okay,
        # because mypy considers str literals to be compatible with
        # unicode.
        if isinstance(s.s, bytes):
            n = s.s
            # The following line is a bit hacky, but is the best way to maintain
            # compatibility with how mypy currently parses the contents of bytes literals.
            contents = str(n)[2:-1]
            return StrExpr(contents)
        else:
            return UnicodeExpr(s.s)

    # Ellipsis
    def visit_Ellipsis(self, n: ast27.Ellipsis) -> EllipsisExpr:
        return EllipsisExpr()

    # Attribute(expr value, identifier attr, expr_context ctx)
    @with_line
    def visit_Attribute(self, n: ast27.Attribute) -> Expression:
        if (isinstance(n.value, ast27.Call) and
                isinstance(n.value.func, ast27.Name) and
                n.value.func.id == 'super'):
            return SuperExpr(n.attr, self.visit(n.value))

        return MemberExpr(self.visit(n.value), n.attr)

    # Subscript(expr value, slice slice, expr_context ctx)
    @with_line
    def visit_Subscript(self, n: ast27.Subscript) -> IndexExpr:
        return IndexExpr(self.visit(n.value), self.visit(n.slice))

    # Name(identifier id, expr_context ctx)
    @with_line
    def visit_Name(self, n: ast27.Name) -> NameExpr:
        return NameExpr(n.id)

    # List(expr* elts, expr_context ctx)
    @with_line
    def visit_List(self, n: ast27.List) -> ListExpr:
        return ListExpr([self.visit(e) for e in n.elts])

    # Tuple(expr* elts, expr_context ctx)
    @with_line
    def visit_Tuple(self, n: ast27.Tuple) -> TupleExpr:
        return TupleExpr([self.visit(e) for e in n.elts])

    # --- slice ---

    # Slice(expr? lower, expr? upper, expr? step)
    def visit_Slice(self, n: ast27.Slice) -> SliceExpr:
        return SliceExpr(self.visit(n.lower),
                         self.visit(n.upper),
                         self.visit(n.step))

    # ExtSlice(slice* dims)
    def visit_ExtSlice(self, n: ast27.ExtSlice) -> TupleExpr:
        return TupleExpr(self.translate_expr_list(n.dims))

    # Index(expr value)
    def visit_Index(self, n: ast27.Index) -> Expression:
        return self.visit(n.value)