Merge flask framework and other stuff from master

1 files changed, 813 insertions, 0 deletions

diff --git a/python/jinja2/runtime.py b/python/jinja2/runtime.py
new file mode 100644
index 0000000..f9d7a68
--- /dev/null
+++ b/python/jinja2/runtime.py
@@ -0,0 +1,813 @@
+# -*- coding: utf-8 -*-
+"""
+    jinja2.runtime
+    ~~~~~~~~~~~~~~
+
+    Runtime helpers.
+
+    :copyright: (c) 2017 by the Jinja Team.
+    :license: BSD.
+"""
+import sys
+
+from itertools import chain
+from types import MethodType
+
+from jinja2.nodes import EvalContext, _context_function_types
+from jinja2.utils import Markup, soft_unicode, escape, missing, concat, \
+     internalcode, object_type_repr, evalcontextfunction, Namespace
+from jinja2.exceptions import UndefinedError, TemplateRuntimeError, \
+     TemplateNotFound
+from jinja2._compat import imap, text_type, iteritems, \
+     implements_iterator, implements_to_string, string_types, PY2, \
+     with_metaclass
+
+
+# these variables are exported to the template runtime
+__all__ = ['LoopContext', 'TemplateReference', 'Macro', 'Markup',
+           'TemplateRuntimeError', 'missing', 'concat', 'escape',
+           'markup_join', 'unicode_join', 'to_string', 'identity',
+           'TemplateNotFound', 'Namespace']
+
+#: 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
+
+#: the identity function.  Useful for certain things in the environment
+identity = lambda x: x
+
+_first_iteration = object()
+_last_iteration = object()
+
+
+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 to 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__(cls, name, bases, d):
+        rv = type.__new__(cls, 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):
+        """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__'):
+            fn = __obj.__call__
+            for fn_type in ('contextfunction',
+                            'evalcontextfunction',
+                            'environmentfunction'):
+                if hasattr(fn, fn_type):
+                    __obj = fn
+                    break
+
+        if isinstance(__obj, _context_function_types):
+            if getattr(__obj, 'contextfunction', 0):
+                args = (__self,) + args
+            elif getattr(__obj, 'evalcontextfunction', 0):
+                args = (__self.eval_ctx,) + args
+            elif getattr(__obj, 'environmentfunction', 0):
+                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):
+        proxy = lambda self: 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
+        )
+
+
+# register the context as mapping if possible
+try:
+    from collections import Mapping
+    Mapping.register(Context)
+except ImportError:
+    pass
+
+
+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
+
+
+class LoopContextBase(object):
+    """A loop context for dynamic iteration."""
+
+    _before = _first_iteration
+    _current = _first_iteration
+    _after = _last_iteration
+    _length = None
+
+    def __init__(self, undefined, recurse=None, depth0=0):
+        self._undefined = undefined
+        self._recurse = recurse
+        self.index0 = -1
+        self.depth0 = depth0
+        self._last_checked_value = missing
+
+    def cycle(self, *args):
+        """Cycles among the arguments with the current loop index."""
+        if not args:
+            raise TypeError('no items for cycling given')
+        return args[self.index0 % len(args)]
+
+    def changed(self, *value):
+        """Checks whether the value has changed since the last call."""
+        if self._last_checked_value != value:
+            self._last_checked_value = value
+            return True
+        return False
+
+    first = property(lambda x: x.index0 == 0)
+    last = property(lambda x: x._after is _last_iteration)
+    index = property(lambda x: x.index0 + 1)
+    revindex = property(lambda x: x.length - x.index0)
+    revindex0 = property(lambda x: x.length - x.index)
+    depth = property(lambda x: x.depth0 + 1)
+
+    @property
+    def previtem(self):
+        if self._before is _first_iteration:
+            return self._undefined('there is no previous item')
+        return self._before
+
+    @property
+    def nextitem(self):
+        if self._after is _last_iteration:
+            return self._undefined('there is no next item')
+        return self._after
+
+    def __len__(self):
+        return self.length
+
+    @internalcode
+    def loop(self, iterable):
+        if self._recurse is None:
+            raise TypeError('Tried to call non recursive loop.  Maybe you '
+                            "forgot the 'recursive' modifier.")
+        return self._recurse(iterable, self._recurse, self.depth0 + 1)
+
+    # a nifty trick to enhance the error message if someone tried to call
+    # the the loop without or with too many arguments.
+    __call__ = loop
+    del loop
+
+    def __repr__(self):
+        return '<%s %r/%r>' % (
+            self.__class__.__name__,
+            self.index,
+            self.length
+        )
+
+
+class LoopContext(LoopContextBase):
+
+    def __init__(self, iterable, undefined, recurse=None, depth0=0):
+        LoopContextBase.__init__(self, undefined, recurse, depth0)
+        self._iterator = iter(iterable)
+
+        # try to get the length of the iterable early.  This must be done
+        # here because there are some broken iterators around where there
+        # __len__ is the number of iterations left (i'm looking at your
+        # listreverseiterator!).
+        try:
+            self._length = len(iterable)
+        except (TypeError, AttributeError):
+            self._length = None
+        self._after = self._safe_next()
+
+    @property
+    def length(self):
+        if self._length is None:
+            # if was not possible to get the length of the iterator when
+            # the loop context was created (ie: iterating over a generator)
+            # we have to convert the iterable into a sequence and use the
+            # length of that + the number of iterations so far.
+            iterable = tuple(self._iterator)
+            self._iterator = iter(iterable)
+            iterations_done = self.index0 + 2
+            self._length = len(iterable) + iterations_done
+        return self._length
+
+    def __iter__(self):
+        return LoopContextIterator(self)
+
+    def _safe_next(self):
+        try:
+            return next(self._iterator)
+        except StopIteration:
+            return _last_iteration
+
+
+@implements_iterator
+class LoopContextIterator(object):
+    """The iterator for a loop context."""
+    __slots__ = ('context',)
+
+    def __init__(self, context):
+        self.context = context
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        ctx = self.context
+        ctx.index0 += 1
+        if ctx._after is _last_iteration:
+            raise StopIteration()
+        ctx._before = ctx._current
+        ctx._current = ctx._after
+        ctx._after = ctx._safe_next()
+        return ctx._current, ctx
+
+
+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
+        # problemtic for macros because if a macro is safe or not not so
+        # much depends on the escape mode when it was defined but 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 anwyays.  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 idx, name in enumerate(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:`jinja2.exceptions.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
+
+    @internalcode
+    def _fail_with_undefined_error(self, *args, **kwargs):
+        """Regular callback function for undefined objects that raises an
+        `jinja2.exceptions.UndefinedError` on call.
+        """
+        if self._undefined_hint is None:
+            if self._undefined_obj is missing:
+                hint = '%r is undefined' % self._undefined_name
+            elif not isinstance(self._undefined_name, string_types):
+                hint = '%s has no element %r' % (
+                    object_type_repr(self._undefined_obj),
+                    self._undefined_name
+                )
+            else:
+                hint = '%r has no attribute %r' % (
+                    object_type_repr(self._undefined_obj),
+                    self._undefined_name
+                )
+        else:
+            hint = self._undefined_hint
+        raise self._undefined_exception(hint)
+
+    @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
+
+
+@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__, DebugUndefined.__slots__, StrictUndefined.__slots__