path: root/contrib/python/Jinja2/py2/jinja2/runtime.py

# -*- coding: utf-8 -*- 
"""The runtime functions and state used by compiled templates."""
import sys 
from itertools import chain 
from types import MethodType 
 
from markupsafe import escape  # noqa: F401
from markupsafe import Markup
from markupsafe import soft_unicode
 
from ._compat import abc
from ._compat import imap
from ._compat import implements_iterator
from ._compat import implements_to_string
from ._compat import iteritems
from ._compat import PY2
from ._compat import string_types
from ._compat import text_type
from ._compat import with_metaclass
from .exceptions import TemplateNotFound  # noqa: F401
from .exceptions import TemplateRuntimeError  # noqa: F401
from .exceptions import UndefinedError
from .nodes import EvalContext
from .utils import concat
from .utils import evalcontextfunction
from .utils import internalcode
from .utils import missing
from .utils import Namespace  # noqa: F401
from .utils import object_type_repr
 
# these variables are exported to the template runtime 
exported = [
    "LoopContext",
    "TemplateReference",
    "Macro",
    "Markup",
    "TemplateRuntimeError",
    "missing",
    "concat",
    "escape",
    "markup_join",
    "unicode_join",
    "to_string",
    "identity",
    "TemplateNotFound",
    "Namespace",
    "Undefined",
]
 
#: the name of the function that is used to convert something into 
#: a string.  We can just use the text type here. 
to_string = text_type 
 
 
def identity(x):
    """Returns its argument. Useful for certain things in the
    environment.
    """
    return x
 
 
def markup_join(seq): 
    """Concatenation that escapes if necessary and converts to unicode.""" 
    buf = [] 
    iterator = imap(soft_unicode, seq) 
    for arg in iterator: 
        buf.append(arg) 
        if hasattr(arg, "__html__"):
            return Markup(u"").join(chain(buf, iterator))
    return concat(buf) 
 
 
def unicode_join(seq): 
    """Simple args to unicode conversion and concatenation.""" 
    return concat(imap(text_type, seq)) 
 
 
def new_context(
    environment,
    template_name,
    blocks,
    vars=None,
    shared=None,
    globals=None,
    locals=None,
):
    """Internal helper for context creation."""
    if vars is None: 
        vars = {} 
    if shared: 
        parent = vars 
    else: 
        parent = dict(globals or (), **vars) 
    if locals: 
        # if the parent is shared a copy should be created because 
        # we don't want to modify the dict passed 
        if shared: 
            parent = dict(parent) 
        for key, value in iteritems(locals): 
            if value is not missing: 
                parent[key] = value 
    return environment.context_class(environment, parent, template_name, blocks)
 
 
class TemplateReference(object): 
    """The `self` in templates.""" 
 
    def __init__(self, context): 
        self.__context = context 
 
    def __getitem__(self, name): 
        blocks = self.__context.blocks[name] 
        return BlockReference(name, self.__context, blocks, 0) 
 
    def __repr__(self): 
        return "<%s %r>" % (self.__class__.__name__, self.__context.name)
 
 
def _get_func(x): 
    return getattr(x, "__func__", x)
 
 
class ContextMeta(type): 
    def __new__(mcs, name, bases, d):
        rv = type.__new__(mcs, name, bases, d)
        if bases == (): 
            return rv 
 
        resolve = _get_func(rv.resolve) 
        default_resolve = _get_func(Context.resolve) 
        resolve_or_missing = _get_func(rv.resolve_or_missing) 
        default_resolve_or_missing = _get_func(Context.resolve_or_missing) 
 
        # If we have a changed resolve but no changed default or missing 
        # resolve we invert the call logic. 
        if (
            resolve is not default_resolve
            and resolve_or_missing is default_resolve_or_missing
        ):
            rv._legacy_resolve_mode = True 
        elif (
            resolve is default_resolve
            and resolve_or_missing is default_resolve_or_missing
        ):
            rv._fast_resolve_mode = True 
 
        return rv 
 
 
def resolve_or_missing(context, key, missing=missing): 
    if key in context.vars: 
        return context.vars[key] 
    if key in context.parent: 
        return context.parent[key] 
    return missing 
 
 
class Context(with_metaclass(ContextMeta)): 
    """The template context holds the variables of a template.  It stores the 
    values passed to the template and also the names the template exports. 
    Creating instances is neither supported nor useful as it's created 
    automatically at various stages of the template evaluation and should not 
    be created by hand. 
 
    The context is immutable.  Modifications on :attr:`parent` **must not** 
    happen and modifications on :attr:`vars` are allowed from generated 
    template code only.  Template filters and global functions marked as 
    :func:`contextfunction`\\s get the active context passed as first argument 
    and are allowed to access the context read-only. 
 
    The template context supports read only dict operations (`get`, 
    `keys`, `values`, `items`, `iterkeys`, `itervalues`, `iteritems`, 
    `__getitem__`, `__contains__`).  Additionally there is a :meth:`resolve` 
    method that doesn't fail with a `KeyError` but returns an 
    :class:`Undefined` object for missing variables. 
    """ 

    # XXX: we want to eventually make this be a deprecation warning and 
    # remove it. 
    _legacy_resolve_mode = False 
    _fast_resolve_mode = False 
 
    def __init__(self, environment, parent, name, blocks): 
        self.parent = parent 
        self.vars = {} 
        self.environment = environment 
        self.eval_ctx = EvalContext(self.environment, name) 
        self.exported_vars = set() 
        self.name = name 
 
        # create the initial mapping of blocks.  Whenever template inheritance 
        # takes place the runtime will update this mapping with the new blocks 
        # from the template. 
        self.blocks = dict((k, [v]) for k, v in iteritems(blocks)) 
 
        # In case we detect the fast resolve mode we can set up an alias 
        # here that bypasses the legacy code logic. 
        if self._fast_resolve_mode: 
            self.resolve_or_missing = MethodType(resolve_or_missing, self) 
 
    def super(self, name, current): 
        """Render a parent block.""" 
        try: 
            blocks = self.blocks[name] 
            index = blocks.index(current) + 1 
            blocks[index] 
        except LookupError: 
            return self.environment.undefined(
                "there is no parent block called %r." % name, name="super"
            )
        return BlockReference(name, self, blocks, index) 
 
    def get(self, key, default=None): 
        """Returns an item from the template context, if it doesn't exist 
        `default` is returned. 
        """ 
        try: 
            return self[key] 
        except KeyError: 
            return default 
 
    def resolve(self, key): 
        """Looks up a variable like `__getitem__` or `get` but returns an 
        :class:`Undefined` object with the name of the name looked up. 
        """ 
        if self._legacy_resolve_mode: 
            rv = resolve_or_missing(self, key) 
        else: 
            rv = self.resolve_or_missing(key) 
        if rv is missing: 
            return self.environment.undefined(name=key) 
        return rv 
 
    def resolve_or_missing(self, key): 
        """Resolves a variable like :meth:`resolve` but returns the 
        special `missing` value if it cannot be found. 
        """ 
        if self._legacy_resolve_mode: 
            rv = self.resolve(key) 
            if isinstance(rv, Undefined): 
                rv = missing 
            return rv 
        return resolve_or_missing(self, key) 
 
    def get_exported(self): 
        """Get a new dict with the exported variables.""" 
        return dict((k, self.vars[k]) for k in self.exported_vars) 
 
    def get_all(self): 
        """Return the complete context as dict including the exported 
        variables.  For optimizations reasons this might not return an 
        actual copy so be careful with using it. 
        """ 
        if not self.vars: 
            return self.parent 
        if not self.parent: 
            return self.vars 
        return dict(self.parent, **self.vars) 
 
    @internalcode 
    def call(__self, __obj, *args, **kwargs):  # noqa: B902
        """Call the callable with the arguments and keyword arguments 
        provided but inject the active context or environment as first 
        argument if the callable is a :func:`contextfunction` or 
        :func:`environmentfunction`. 
        """ 
        if __debug__: 
            __traceback_hide__ = True  # noqa 
 
        # Allow callable classes to take a context 
        if hasattr(__obj, "__call__"):  # noqa: B004
            fn = __obj.__call__ 
            for fn_type in (
                "contextfunction",
                "evalcontextfunction",
                "environmentfunction",
            ):
                if hasattr(fn, fn_type): 
                    __obj = fn 
                    break 
 
        if callable(__obj):
            if getattr(__obj, "contextfunction", False) is True:
                args = (__self,) + args 
            elif getattr(__obj, "evalcontextfunction", False) is True:
                args = (__self.eval_ctx,) + args 
            elif getattr(__obj, "environmentfunction", False) is True:
                args = (__self.environment,) + args 
        try: 
            return __obj(*args, **kwargs) 
        except StopIteration: 
            return __self.environment.undefined(
                "value was undefined because "
                "a callable raised a "
                "StopIteration exception"
            )
 
    def derived(self, locals=None): 
        """Internal helper function to create a derived context.  This is 
        used in situations where the system needs a new context in the same 
        template that is independent. 
        """ 
        context = new_context(
            self.environment, self.name, {}, self.get_all(), True, None, locals
        )
        context.eval_ctx = self.eval_ctx 
        context.blocks.update((k, list(v)) for k, v in iteritems(self.blocks)) 
        return context 
 
    def _all(meth):  # noqa: B902
        def proxy(self):
            return getattr(self.get_all(), meth)()

        proxy.__doc__ = getattr(dict, meth).__doc__ 
        proxy.__name__ = meth 
        return proxy 
 
    keys = _all("keys")
    values = _all("values")
    items = _all("items")
 
    # not available on python 3 
    if PY2: 
        iterkeys = _all("iterkeys")
        itervalues = _all("itervalues")
        iteritems = _all("iteritems")
    del _all 
 
    def __contains__(self, name): 
        return name in self.vars or name in self.parent 
 
    def __getitem__(self, key): 
        """Lookup a variable or raise `KeyError` if the variable is 
        undefined. 
        """ 
        item = self.resolve_or_missing(key) 
        if item is missing: 
            raise KeyError(key) 
        return item 
 
    def __repr__(self): 
        return "<%s %s of %r>" % (
            self.__class__.__name__, 
            repr(self.get_all()), 
            self.name,
        ) 
 
 
abc.Mapping.register(Context)
 
 
class BlockReference(object): 
    """One block on a template reference.""" 
 
    def __init__(self, name, context, stack, depth): 
        self.name = name 
        self._context = context 
        self._stack = stack 
        self._depth = depth 
 
    @property 
    def super(self): 
        """Super the block.""" 
        if self._depth + 1 >= len(self._stack): 
            return self._context.environment.undefined(
                "there is no parent block called %r." % self.name, name="super"
            )
        return BlockReference(self.name, self._context, self._stack, self._depth + 1)
 
    @internalcode 
    def __call__(self): 
        rv = concat(self._stack[self._depth](self._context)) 
        if self._context.eval_ctx.autoescape: 
            rv = Markup(rv) 
        return rv 
 
 
@implements_iterator
class LoopContext:
    """A wrapper iterable for dynamic ``for`` loops, with information
    about the loop and iteration.
    """
 
    #: Current iteration of the loop, starting at 0.
    index0 = -1

    _length = None 
    _after = missing
    _current = missing
    _before = missing
    _last_changed_value = missing
 
    def __init__(self, iterable, undefined, recurse=None, depth0=0):
        """
        :param iterable: Iterable to wrap.
        :param undefined: :class:`Undefined` class to use for next and
            previous items.
        :param recurse: The function to render the loop body when the
            loop is marked recursive.
        :param depth0: Incremented when looping recursively.
        """
        self._iterable = iterable
        self._iterator = self._to_iterator(iterable)
        self._undefined = undefined 
        self._recurse = recurse 
        #: How many levels deep a recursive loop currently is, starting at 0.
        self.depth0 = depth0 
 
    @staticmethod
    def _to_iterator(iterable):
        return iter(iterable)
 
    @property
    def length(self):
        """Length of the iterable.
 
        If the iterable is a generator or otherwise does not have a
        size, it is eagerly evaluated to get a size.
        """
        if self._length is not None:
            return self._length
 
        try:
            self._length = len(self._iterable)
        except TypeError:
            iterable = list(self._iterator)
            self._iterator = self._to_iterator(iterable)
            self._length = len(iterable) + self.index + (self._after is not missing)
 
        return self._length
 
    def __len__(self): 
        return self.length 
 
    @property
    def depth(self):
        """How many levels deep a recursive loop currently is, starting at 1."""
        return self.depth0 + 1
 
    @property
    def index(self):
        """Current iteration of the loop, starting at 1."""
        return self.index0 + 1
 
    @property
    def revindex0(self):
        """Number of iterations from the end of the loop, ending at 0.
 
        Requires calculating :attr:`length`.
        """
        return self.length - self.index
 
    @property
    def revindex(self):
        """Number of iterations from the end of the loop, ending at 1.
 
        Requires calculating :attr:`length`.
        """
        return self.length - self.index0
 
    @property
    def first(self):
        """Whether this is the first iteration of the loop."""
        return self.index0 == 0
 
    def _peek_next(self):
        """Return the next element in the iterable, or :data:`missing`
        if the iterable is exhausted. Only peeks one item ahead, caching
        the result in :attr:`_last` for use in subsequent checks. The
        cache is reset when :meth:`__next__` is called.
        """
        if self._after is not missing:
            return self._after

        self._after = next(self._iterator, missing)
        return self._after

    @property 
    def last(self):
        """Whether this is the last iteration of the loop.
 
        Causes the iterable to advance early. See
        :func:`itertools.groupby` for issues this can cause.
        The :func:`groupby` filter avoids that issue.
        """
        return self._peek_next() is missing
 
    @property
    def previtem(self):
        """The item in the previous iteration. Undefined during the
        first iteration.
        """
        if self.first:
            return self._undefined("there is no previous item")
 
        return self._before
 
    @property
    def nextitem(self):
        """The item in the next iteration. Undefined during the last
        iteration.
 
        Causes the iterable to advance early. See
        :func:`itertools.groupby` for issues this can cause.
        The :func:`groupby` filter avoids that issue.
        """
        rv = self._peek_next()
 
        if rv is missing:
            return self._undefined("there is no next item")

        return rv

    def cycle(self, *args):
        """Return a value from the given args, cycling through based on
        the current :attr:`index0`.

        :param args: One or more values to cycle through.
        """
        if not args:
            raise TypeError("no items for cycling given")

        return args[self.index0 % len(args)]

    def changed(self, *value):
        """Return ``True`` if previously called with a different value
        (including when called for the first time).

        :param value: One or more values to compare to the last call.
        """
        if self._last_changed_value != value:
            self._last_changed_value = value
            return True

        return False

    def __iter__(self): 
        return self 
 
    def __next__(self): 
        if self._after is not missing:
            rv = self._after
            self._after = missing
        else:
            rv = next(self._iterator)
 
        self.index0 += 1
        self._before = self._current
        self._current = rv
        return rv, self
 
    @internalcode
    def __call__(self, iterable):
        """When iterating over nested data, render the body of the loop
        recursively with the given inner iterable data.

        The loop must have the ``recursive`` marker for this to work.
        """
        if self._recurse is None:
            raise TypeError(
                "The loop must have the 'recursive' marker to be called recursively."
            )

        return self._recurse(iterable, self._recurse, depth=self.depth)

    def __repr__(self):
        return "<%s %d/%d>" % (self.__class__.__name__, self.index, self.length)


class Macro(object): 
    """Wraps a macro function.""" 
 
    def __init__(
        self,
        environment,
        func,
        name,
        arguments,
        catch_kwargs,
        catch_varargs,
        caller,
        default_autoescape=None,
    ):
        self._environment = environment 
        self._func = func 
        self._argument_count = len(arguments) 
        self.name = name 
        self.arguments = arguments 
        self.catch_kwargs = catch_kwargs 
        self.catch_varargs = catch_varargs 
        self.caller = caller 
        self.explicit_caller = "caller" in arguments
        if default_autoescape is None: 
            default_autoescape = environment.autoescape 
        self._default_autoescape = default_autoescape 
 
    @internalcode 
    @evalcontextfunction 
    def __call__(self, *args, **kwargs): 
        # This requires a bit of explanation,  In the past we used to 
        # decide largely based on compile-time information if a macro is 
        # safe or unsafe.  While there was a volatile mode it was largely 
        # unused for deciding on escaping.  This turns out to be 
        # problematic for macros because whether a macro is safe depends not
        # on the escape mode when it was defined, but rather when it was used.
        # 
        # Because however we export macros from the module system and 
        # there are historic callers that do not pass an eval context (and 
        # will continue to not pass one), we need to perform an instance 
        # check here. 
        # 
        # This is considered safe because an eval context is not a valid 
        # argument to callables otherwise anyway.  Worst case here is
        # that if no eval context is passed we fall back to the compile 
        # time autoescape flag. 
        if args and isinstance(args[0], EvalContext): 
            autoescape = args[0].autoescape 
            args = args[1:] 
        else: 
            autoescape = self._default_autoescape 
 
        # try to consume the positional arguments 
        arguments = list(args[: self._argument_count])
        off = len(arguments) 
 
        # For information why this is necessary refer to the handling 
        # of caller in the `macro_body` handler in the compiler. 
        found_caller = False 
 
        # if the number of arguments consumed is not the number of 
        # arguments expected we start filling in keyword arguments 
        # and defaults. 
        if off != self._argument_count: 
            for name in self.arguments[len(arguments) :]:
                try: 
                    value = kwargs.pop(name) 
                except KeyError: 
                    value = missing 
                if name == "caller":
                    found_caller = True 
                arguments.append(value) 
        else: 
            found_caller = self.explicit_caller 
 
        # it's important that the order of these arguments does not change 
        # if not also changed in the compiler's `function_scoping` method. 
        # the order is caller, keyword arguments, positional arguments! 
        if self.caller and not found_caller: 
            caller = kwargs.pop("caller", None)
            if caller is None: 
                caller = self._environment.undefined("No caller defined", name="caller")
            arguments.append(caller) 
 
        if self.catch_kwargs: 
            arguments.append(kwargs) 
        elif kwargs: 
            if "caller" in kwargs:
                raise TypeError(
                    "macro %r was invoked with two values for "
                    "the special caller argument.  This is "
                    "most likely a bug." % self.name
                )
            raise TypeError(
                "macro %r takes no keyword argument %r"
                % (self.name, next(iter(kwargs)))
            )
        if self.catch_varargs: 
            arguments.append(args[self._argument_count :])
        elif len(args) > self._argument_count: 
            raise TypeError(
                "macro %r takes not more than %d argument(s)"
                % (self.name, len(self.arguments))
            )
 
        return self._invoke(arguments, autoescape) 
 
    def _invoke(self, arguments, autoescape): 
        """This method is being swapped out by the async implementation.""" 
        rv = self._func(*arguments) 
        if autoescape: 
            rv = Markup(rv) 
        return rv 
 
    def __repr__(self): 
        return "<%s %s>" % (
            self.__class__.__name__, 
            self.name is None and "anonymous" or repr(self.name),
        ) 
 
 
@implements_to_string 
class Undefined(object): 
    """The default undefined type.  This undefined type can be printed and 
    iterated over, but every other access will raise an :exc:`UndefinedError`:
 
    >>> foo = Undefined(name='foo') 
    >>> str(foo) 
    '' 
    >>> not foo 
    True 
    >>> foo + 42 
    Traceback (most recent call last): 
      ... 
    jinja2.exceptions.UndefinedError: 'foo' is undefined 
    """ 
 
    __slots__ = (
        "_undefined_hint",
        "_undefined_obj",
        "_undefined_name",
        "_undefined_exception",
    )

    def __init__(self, hint=None, obj=missing, name=None, exc=UndefinedError): 
        self._undefined_hint = hint 
        self._undefined_obj = obj 
        self._undefined_name = name 
        self._undefined_exception = exc 
 
    @property
    def _undefined_message(self):
        """Build a message about the undefined value based on how it was
        accessed.
        """
        if self._undefined_hint:
            return self._undefined_hint

        if self._undefined_obj is missing:
            return "%r is undefined" % self._undefined_name

        if not isinstance(self._undefined_name, string_types):
            return "%s has no element %r" % (
                object_type_repr(self._undefined_obj),
                self._undefined_name,
            )

        return "%r has no attribute %r" % (
            object_type_repr(self._undefined_obj),
            self._undefined_name,
        )

    @internalcode 
    def _fail_with_undefined_error(self, *args, **kwargs): 
        """Raise an :exc:`UndefinedError` when operations are performed
        on the undefined value.
        """ 
        raise self._undefined_exception(self._undefined_message)
 
    @internalcode 
    def __getattr__(self, name): 
        if name[:2] == "__":
            raise AttributeError(name) 
        return self._fail_with_undefined_error() 
 
    __add__ = (
        __radd__
    ) = (
        __mul__
    ) = (
        __rmul__
    ) = (
        __div__
    ) = (
        __rdiv__
    ) = (
        __truediv__
    ) = (
        __rtruediv__
    ) = (
        __floordiv__
    ) = (
        __rfloordiv__
    ) = (
        __mod__
    ) = (
        __rmod__
    ) = (
        __pos__
    ) = (
        __neg__
    ) = (
        __call__
    ) = (
        __getitem__
    ) = (
        __lt__
    ) = (
        __le__
    ) = (
        __gt__
    ) = (
        __ge__
    ) = (
        __int__
    ) = (
        __float__
    ) = (
        __complex__
    ) = __pow__ = __rpow__ = __sub__ = __rsub__ = _fail_with_undefined_error
 
    def __eq__(self, other): 
        return type(self) is type(other) 
 
    def __ne__(self, other): 
        return not self.__eq__(other) 
 
    def __hash__(self): 
        return id(type(self)) 
 
    def __str__(self): 
        return u""
 
    def __len__(self): 
        return 0 
 
    def __iter__(self): 
        if 0: 
            yield None 
 
    def __nonzero__(self): 
        return False 

    __bool__ = __nonzero__ 
 
    def __repr__(self): 
        return "Undefined"
 
 
def make_logging_undefined(logger=None, base=None): 
    """Given a logger object this returns a new undefined class that will 
    log certain failures.  It will log iterations and printing.  If no 
    logger is given a default logger is created. 
 
    Example:: 
 
        logger = logging.getLogger(__name__) 
        LoggingUndefined = make_logging_undefined( 
            logger=logger, 
            base=Undefined 
        ) 
 
    .. versionadded:: 2.8 
 
    :param logger: the logger to use.  If not provided, a default logger 
                   is created. 
    :param base: the base class to add logging functionality to.  This 
                 defaults to :class:`Undefined`. 
    """ 
    if logger is None: 
        import logging 

        logger = logging.getLogger(__name__) 
        logger.addHandler(logging.StreamHandler(sys.stderr)) 
    if base is None: 
        base = Undefined 
 
    def _log_message(undef): 
        if undef._undefined_hint is None: 
            if undef._undefined_obj is missing: 
                hint = "%s is undefined" % undef._undefined_name
            elif not isinstance(undef._undefined_name, string_types): 
                hint = "%s has no element %s" % (
                    object_type_repr(undef._undefined_obj), 
                    undef._undefined_name,
                )
            else: 
                hint = "%s has no attribute %s" % (
                    object_type_repr(undef._undefined_obj), 
                    undef._undefined_name,
                )
        else: 
            hint = undef._undefined_hint 
        logger.warning("Template variable warning: %s", hint)
 
    class LoggingUndefined(base): 
        def _fail_with_undefined_error(self, *args, **kwargs): 
            try: 
                return base._fail_with_undefined_error(self, *args, **kwargs) 
            except self._undefined_exception as e: 
                logger.error("Template variable error: %s", str(e))
                raise e 
 
        def __str__(self): 
            rv = base.__str__(self) 
            _log_message(self) 
            return rv 
 
        def __iter__(self): 
            rv = base.__iter__(self) 
            _log_message(self) 
            return rv 
 
        if PY2: 

            def __nonzero__(self): 
                rv = base.__nonzero__(self) 
                _log_message(self) 
                return rv 
 
            def __unicode__(self): 
                rv = base.__unicode__(self) 
                _log_message(self) 
                return rv 

        else: 

            def __bool__(self): 
                rv = base.__bool__(self) 
                _log_message(self) 
                return rv 
 
    return LoggingUndefined 
 
 
# No @implements_to_string decorator here because __str__
# is not overwritten from Undefined in this class.
# This would cause a recursion error in Python 2.
class ChainableUndefined(Undefined):
    """An undefined that is chainable, where both ``__getattr__`` and
    ``__getitem__`` return itself rather than raising an
    :exc:`UndefinedError`.

    >>> foo = ChainableUndefined(name='foo')
    >>> str(foo.bar['baz'])
    ''
    >>> foo.bar['baz'] + 42
    Traceback (most recent call last):
      ...
    jinja2.exceptions.UndefinedError: 'foo' is undefined

    .. versionadded:: 2.11.0
    """

    __slots__ = ()

    def __html__(self):
        return self.__str__()

    def __getattr__(self, _):
        return self

    __getitem__ = __getattr__


@implements_to_string 
class DebugUndefined(Undefined): 
    """An undefined that returns the debug info when printed. 
 
    >>> foo = DebugUndefined(name='foo') 
    >>> str(foo) 
    '{{ foo }}' 
    >>> not foo 
    True 
    >>> foo + 42 
    Traceback (most recent call last): 
      ... 
    jinja2.exceptions.UndefinedError: 'foo' is undefined 
    """ 

    __slots__ = () 
 
    def __str__(self): 
        if self._undefined_hint is None: 
            if self._undefined_obj is missing: 
                return u"{{ %s }}" % self._undefined_name
            return "{{ no such element: %s[%r] }}" % (
                object_type_repr(self._undefined_obj), 
                self._undefined_name,
            ) 
        return u"{{ undefined value printed: %s }}" % self._undefined_hint
 
 
@implements_to_string 
class StrictUndefined(Undefined): 
    """An undefined that barks on print and iteration as well as boolean 
    tests and all kinds of comparisons.  In other words: you can do nothing 
    with it except checking if it's defined using the `defined` test. 
 
    >>> foo = StrictUndefined(name='foo') 
    >>> str(foo) 
    Traceback (most recent call last): 
      ... 
    jinja2.exceptions.UndefinedError: 'foo' is undefined 
    >>> not foo 
    Traceback (most recent call last): 
      ... 
    jinja2.exceptions.UndefinedError: 'foo' is undefined 
    >>> foo + 42 
    Traceback (most recent call last): 
      ... 
    jinja2.exceptions.UndefinedError: 'foo' is undefined 
    """ 

    __slots__ = () 
    __iter__ = (
        __str__
    ) = (
        __len__
    ) = (
        __nonzero__
    ) = __eq__ = __ne__ = __bool__ = __hash__ = Undefined._fail_with_undefined_error
 
 
# remove remaining slots attributes, after the metaclass did the magic they 
# are unneeded and irritating as they contain wrong data for the subclasses. 
del (
    Undefined.__slots__,
    ChainableUndefined.__slots__,
    DebugUndefined.__slots__,
    StrictUndefined.__slots__,
)