Initial Commit

lib/mako/runtime.py

@@ -0,0 +1,878 @@
+# mako/runtime.py
+# Copyright (C) 2006-2015 the Mako authors and contributors <see AUTHORS file>
+#
+# This module is part of Mako and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+"""provides runtime services for templates, including Context,
+Namespace, and various helper functions."""
+
+from mako import exceptions, util, compat
+from mako.compat import compat_builtins
+import sys
+
+
+class Context(object):
+    """Provides runtime namespace, output buffer, and various
+    callstacks for templates.
+
+    See :ref:`runtime_toplevel` for detail on the usage of
+    :class:`.Context`.
+
+     """
+
+    def __init__(self, buffer, **data):
+        self._buffer_stack = [buffer]
+
+        self._data = data
+
+        self._kwargs = data.copy()
+        self._with_template = None
+        self._outputting_as_unicode = None
+        self.namespaces = {}
+
+        # "capture" function which proxies to the
+        # generic "capture" function
+        self._data['capture'] = compat.partial(capture, self)
+
+        # "caller" stack used by def calls with content
+        self.caller_stack = self._data['caller'] = CallerStack()
+
+    def _set_with_template(self, t):
+        self._with_template = t
+        illegal_names = t.reserved_names.intersection(self._data)
+        if illegal_names:
+            raise exceptions.NameConflictError(
+                "Reserved words passed to render(): %s" %
+                ", ".join(illegal_names))
+
+    @property
+    def lookup(self):
+        """Return the :class:`.TemplateLookup` associated
+        with this :class:`.Context`.
+
+        """
+        return self._with_template.lookup
+
+    @property
+    def kwargs(self):
+        """Return the dictionary of top level keyword arguments associated
+        with this :class:`.Context`.
+
+        This dictionary only includes the top-level arguments passed to
+        :meth:`.Template.render`.  It does not include names produced within
+        the template execution such as local variable names or special names
+        such as ``self``, ``next``, etc.
+
+        The purpose of this dictionary is primarily for the case that
+        a :class:`.Template` accepts arguments via its ``<%page>`` tag,
+        which are normally expected to be passed via :meth:`.Template.render`,
+        except the template is being called in an inheritance context,
+        using the ``body()`` method.   :attr:`.Context.kwargs` can then be
+        used to propagate these arguments to the inheriting template::
+
+            ${next.body(**context.kwargs)}
+
+        """
+        return self._kwargs.copy()
+
+    def push_caller(self, caller):
+        """Push a ``caller`` callable onto the callstack for
+        this :class:`.Context`."""
+
+
+        self.caller_stack.append(caller)
+
+    def pop_caller(self):
+        """Pop a ``caller`` callable onto the callstack for this
+        :class:`.Context`."""
+
+        del self.caller_stack[-1]
+
+    def keys(self):
+        """Return a list of all names established in this :class:`.Context`."""
+
+        return list(self._data.keys())
+
+    def __getitem__(self, key):
+        if key in self._data:
+            return self._data[key]
+        else:
+            return compat_builtins.__dict__[key]
+
+    def _push_writer(self):
+        """push a capturing buffer onto this Context and return
+        the new writer function."""
+
+        buf = util.FastEncodingBuffer()
+        self._buffer_stack.append(buf)
+        return buf.write
+
+    def _pop_buffer_and_writer(self):
+        """pop the most recent capturing buffer from this Context
+        and return the current writer after the pop.
+
+        """
+
+        buf = self._buffer_stack.pop()
+        return buf, self._buffer_stack[-1].write
+
+    def _push_buffer(self):
+        """push a capturing buffer onto this Context."""
+
+        self._push_writer()
+
+    def _pop_buffer(self):
+        """pop the most recent capturing buffer from this Context."""
+
+        return self._buffer_stack.pop()
+
+    def get(self, key, default=None):
+        """Return a value from this :class:`.Context`."""
+
+        return self._data.get(key, compat_builtins.__dict__.get(key, default))
+
+    def write(self, string):
+        """Write a string to this :class:`.Context` object's
+        underlying output buffer."""
+
+        self._buffer_stack[-1].write(string)
+
+    def writer(self):
+        """Return the current writer function."""
+
+        return self._buffer_stack[-1].write
+
+    def _copy(self):
+        c = Context.__new__(Context)
+        c._buffer_stack = self._buffer_stack
+        c._data = self._data.copy()
+        c._kwargs = self._kwargs
+        c._with_template = self._with_template
+        c._outputting_as_unicode = self._outputting_as_unicode
+        c.namespaces = self.namespaces
+        c.caller_stack = self.caller_stack
+        return c
+
+    def _locals(self, d):
+        """Create a new :class:`.Context` with a copy of this
+        :class:`.Context`'s current state,
+        updated with the given dictionary.
+
+        The :attr:`.Context.kwargs` collection remains
+        unaffected.
+
+
+        """
+
+        if not d:
+            return self
+        c = self._copy()
+        c._data.update(d)
+        return c
+
+    def _clean_inheritance_tokens(self):
+        """create a new copy of this :class:`.Context`. with
+        tokens related to inheritance state removed."""
+
+        c = self._copy()
+        x = c._data
+        x.pop('self', None)
+        x.pop('parent', None)
+        x.pop('next', None)
+        return c
+
+class CallerStack(list):
+    def __init__(self):
+        self.nextcaller = None
+
+    def __nonzero__(self):
+        return self.__bool__()
+
+    def __bool__(self):
+        return len(self) and self._get_caller() and True or False
+
+    def _get_caller(self):
+        # this method can be removed once
+        # codegen MAGIC_NUMBER moves past 7
+        return self[-1]
+
+    def __getattr__(self, key):
+        return getattr(self._get_caller(), key)
+
+    def _push_frame(self):
+        frame = self.nextcaller or None
+        self.append(frame)
+        self.nextcaller = None
+        return frame
+
+    def _pop_frame(self):
+        self.nextcaller = self.pop()
+
+
+class Undefined(object):
+    """Represents an undefined value in a template.
+
+    All template modules have a constant value
+    ``UNDEFINED`` present which is an instance of this
+    object.
+
+    """
+    def __str__(self):
+        raise NameError("Undefined")
+
+    def __nonzero__(self):
+        return self.__bool__()
+
+    def __bool__(self):
+        return False
+
+UNDEFINED = Undefined()
+
+class LoopStack(object):
+    """a stack for LoopContexts that implements the context manager protocol
+    to automatically pop off the top of the stack on context exit
+    """
+
+    def __init__(self):
+        self.stack = []
+
+    def _enter(self, iterable):
+        self._push(iterable)
+        return self._top
+
+    def _exit(self):
+        self._pop()
+        return self._top
+
+    @property
+    def _top(self):
+        if self.stack:
+            return self.stack[-1]
+        else:
+            return self
+
+    def _pop(self):
+        return self.stack.pop()
+
+    def _push(self, iterable):
+        new = LoopContext(iterable)
+        if self.stack:
+            new.parent = self.stack[-1]
+        return self.stack.append(new)
+
+    def __getattr__(self, key):
+        raise exceptions.RuntimeException("No loop context is established")
+
+    def __iter__(self):
+        return iter(self._top)
+
+
+class LoopContext(object):
+    """A magic loop variable.
+    Automatically accessible in any ``% for`` block.
+
+    See the section :ref:`loop_context` for usage
+    notes.
+
+    :attr:`parent` -> :class:`.LoopContext` or ``None``
+        The parent loop, if one exists.
+    :attr:`index` -> `int`
+        The 0-based iteration count.
+    :attr:`reverse_index` -> `int`
+        The number of iterations remaining.
+    :attr:`first` -> `bool`
+        ``True`` on the first iteration, ``False`` otherwise.
+    :attr:`last` -> `bool`
+        ``True`` on the last iteration, ``False`` otherwise.
+    :attr:`even` -> `bool`
+        ``True`` when ``index`` is even.
+    :attr:`odd` -> `bool`
+        ``True`` when ``index`` is odd.
+    """
+
+    def __init__(self, iterable):
+        self._iterable = iterable
+        self.index = 0
+        self.parent = None
+
+    def __iter__(self):
+        for i in self._iterable:
+            yield i
+            self.index += 1
+
+    @util.memoized_instancemethod
+    def __len__(self):
+        return len(self._iterable)
+
+    @property
+    def reverse_index(self):
+        return len(self) - self.index - 1
+
+    @property
+    def first(self):
+        return self.index == 0
+
+    @property
+    def last(self):
+        return self.index == len(self) - 1
+
+    @property
+    def even(self):
+        return not self.odd
+
+    @property
+    def odd(self):
+        return bool(self.index % 2)
+
+    def cycle(self, *values):
+        """Cycle through values as the loop progresses.
+        """
+        if not values:
+            raise ValueError("You must provide values to cycle through")
+        return values[self.index % len(values)]
+
+
+class _NSAttr(object):
+    def __init__(self, parent):
+        self.__parent = parent
+    def __getattr__(self, key):
+        ns = self.__parent
+        while ns:
+            if hasattr(ns.module, key):
+                return getattr(ns.module, key)
+            else:
+                ns = ns.inherits
+        raise AttributeError(key)
+
+class Namespace(object):
+    """Provides access to collections of rendering methods, which
+      can be local, from other templates, or from imported modules.
+
+      To access a particular rendering method referenced by a
+      :class:`.Namespace`, use plain attribute access:
+
+      .. sourcecode:: mako
+
+        ${some_namespace.foo(x, y, z)}
+
+      :class:`.Namespace` also contains several built-in attributes
+      described here.
+
+      """
+
+    def __init__(self, name, context,
+                            callables=None, inherits=None,
+                            populate_self=True, calling_uri=None):
+        self.name = name
+        self.context = context
+        self.inherits = inherits
+        if callables is not None:
+            self.callables = dict([(c.__name__, c) for c in callables])
+
+    callables = ()
+
+    module = None
+    """The Python module referenced by this :class:`.Namespace`.
+
+    If the namespace references a :class:`.Template`, then
+    this module is the equivalent of ``template.module``,
+    i.e. the generated module for the template.
+
+    """
+
+    template = None
+    """The :class:`.Template` object referenced by this
+        :class:`.Namespace`, if any.
+
+    """
+
+    context = None
+    """The :class:`.Context` object for this :class:`.Namespace`.
+
+    Namespaces are often created with copies of contexts that
+    contain slightly different data, particularly in inheritance
+    scenarios. Using the :class:`.Context` off of a :class:`.Namespace` one
+    can traverse an entire chain of templates that inherit from
+    one-another.
+
+    """
+
+    filename = None
+    """The path of the filesystem file used for this
+    :class:`.Namespace`'s module or template.
+
+    If this is a pure module-based
+    :class:`.Namespace`, this evaluates to ``module.__file__``. If a
+    template-based namespace, it evaluates to the original
+    template file location.
+
+    """
+
+    uri = None
+    """The URI for this :class:`.Namespace`'s template.
+
+    I.e. whatever was sent to :meth:`.TemplateLookup.get_template()`.
+
+    This is the equivalent of :attr:`.Template.uri`.
+
+    """
+
+    _templateuri = None
+
+    @util.memoized_property
+    def attr(self):
+        """Access module level attributes by name.
+
+        This accessor allows templates to supply "scalar"
+        attributes which are particularly handy in inheritance
+        relationships.
+
+        .. seealso::
+
+            :ref:`inheritance_attr`
+
+            :ref:`namespace_attr_for_includes`
+
+        """
+        return _NSAttr(self)
+
+    def get_namespace(self, uri):
+        """Return a :class:`.Namespace` corresponding to the given ``uri``.
+
+        If the given ``uri`` is a relative URI (i.e. it does not
+        contain a leading slash ``/``), the ``uri`` is adjusted to
+        be relative to the ``uri`` of the namespace itself. This
+        method is therefore mostly useful off of the built-in
+        ``local`` namespace, described in :ref:`namespace_local`.
+
+        In
+        most cases, a template wouldn't need this function, and
+        should instead use the ``<%namespace>`` tag to load
+        namespaces. However, since all ``<%namespace>`` tags are
+        evaluated before the body of a template ever runs,
+        this method can be used to locate namespaces using
+        expressions that were generated within the body code of
+        the template, or to conditionally use a particular
+        namespace.
+
+        """
+        key = (self, uri)
+        if key in self.context.namespaces:
+            return self.context.namespaces[key]
+        else:
+            ns = TemplateNamespace(uri, self.context._copy(),
+                                templateuri=uri,
+                                calling_uri=self._templateuri)
+            self.context.namespaces[key] = ns
+            return ns
+
+    def get_template(self, uri):
+        """Return a :class:`.Template` from the given ``uri``.
+
+        The ``uri`` resolution is relative to the ``uri`` of this
+        :class:`.Namespace` object's :class:`.Template`.
+
+        """
+        return _lookup_template(self.context, uri, self._templateuri)
+
+    def get_cached(self, key, **kwargs):
+        """Return a value from the :class:`.Cache` referenced by this
+        :class:`.Namespace` object's :class:`.Template`.
+
+        The advantage to this method versus direct access to the
+        :class:`.Cache` is that the configuration parameters
+        declared in ``<%page>`` take effect here, thereby calling
+        up the same configured backend as that configured
+        by ``<%page>``.
+
+        """
+
+        return self.cache.get(key, **kwargs)
+
+    @property
+    def cache(self):
+        """Return the :class:`.Cache` object referenced
+        by this :class:`.Namespace` object's
+        :class:`.Template`.
+
+        """
+        return self.template.cache
+
+    def include_file(self, uri, **kwargs):
+        """Include a file at the given ``uri``."""
+
+        _include_file(self.context, uri, self._templateuri, **kwargs)
+
+    def _populate(self, d, l):
+        for ident in l:
+            if ident == '*':
+                for (k, v) in self._get_star():
+                    d[k] = v
+            else:
+                d[ident] = getattr(self, ident)
+
+    def _get_star(self):
+        if self.callables:
+            for key in self.callables:
+                yield (key, self.callables[key])
+
+    def __getattr__(self, key):
+        if key in self.callables:
+            val = self.callables[key]
+        elif self.inherits:
+            val = getattr(self.inherits, key)
+        else:
+            raise AttributeError(
+                    "Namespace '%s' has no member '%s'" %
+                    (self.name, key))
+        setattr(self, key, val)
+        return val
+
+class TemplateNamespace(Namespace):
+    """A :class:`.Namespace` specific to a :class:`.Template` instance."""
+
+    def __init__(self, name, context, template=None, templateuri=None,
+                            callables=None, inherits=None,
+                            populate_self=True, calling_uri=None):
+        self.name = name
+        self.context = context
+        self.inherits = inherits
+        if callables is not None:
+            self.callables = dict([(c.__name__, c) for c in callables])
+
+        if templateuri is not None:
+            self.template = _lookup_template(context, templateuri,
+                                                calling_uri)
+            self._templateuri = self.template.module._template_uri
+        elif template is not None:
+            self.template = template
+            self._templateuri = template.module._template_uri
+        else:
+            raise TypeError("'template' argument is required.")
+
+        if populate_self:
+            lclcallable, lclcontext = \
+                        _populate_self_namespace(context, self.template,
+                                                    self_ns=self)
+
+    @property
+    def module(self):
+        """The Python module referenced by this :class:`.Namespace`.
+
+        If the namespace references a :class:`.Template`, then
+        this module is the equivalent of ``template.module``,
+        i.e. the generated module for the template.
+
+        """
+        return self.template.module
+
+    @property
+    def filename(self):
+        """The path of the filesystem file used for this
+        :class:`.Namespace`'s module or template.
+        """
+        return self.template.filename
+
+    @property
+    def uri(self):
+        """The URI for this :class:`.Namespace`'s template.
+
+        I.e. whatever was sent to :meth:`.TemplateLookup.get_template()`.
+
+        This is the equivalent of :attr:`.Template.uri`.
+
+        """
+        return self.template.uri
+
+    def _get_star(self):
+        if self.callables:
+            for key in self.callables:
+                yield (key, self.callables[key])
+        def get(key):
+            callable_ = self.template._get_def_callable(key)
+            return compat.partial(callable_, self.context)
+        for k in self.template.module._exports:
+            yield (k, get(k))
+
+    def __getattr__(self, key):
+        if key in self.callables:
+            val = self.callables[key]
+        elif self.template.has_def(key):
+            callable_ = self.template._get_def_callable(key)
+            val = compat.partial(callable_, self.context)
+        elif self.inherits:
+            val = getattr(self.inherits, key)
+
+        else:
+            raise AttributeError(
+                    "Namespace '%s' has no member '%s'" %
+                    (self.name, key))
+        setattr(self, key, val)
+        return val
+
+class ModuleNamespace(Namespace):
+    """A :class:`.Namespace` specific to a Python module instance."""
+
+    def __init__(self, name, context, module,
+                            callables=None, inherits=None,
+                            populate_self=True, calling_uri=None):
+        self.name = name
+        self.context = context
+        self.inherits = inherits
+        if callables is not None:
+            self.callables = dict([(c.__name__, c) for c in callables])
+
+        mod = __import__(module)
+        for token in module.split('.')[1:]:
+            mod = getattr(mod, token)
+        self.module = mod
+
+    @property
+    def filename(self):
+        """The path of the filesystem file used for this
+        :class:`.Namespace`'s module or template.
+        """
+        return self.module.__file__
+
+    def _get_star(self):
+        if self.callables:
+            for key in self.callables:
+                yield (key, self.callables[key])
+        for key in dir(self.module):
+            if key[0] != '_':
+                callable_ = getattr(self.module, key)
+                if compat.callable(callable_):
+                    yield key, compat.partial(callable_, self.context)
+
+
+    def __getattr__(self, key):
+        if key in self.callables:
+            val = self.callables[key]
+        elif hasattr(self.module, key):
+            callable_ = getattr(self.module, key)
+            val = compat.partial(callable_, self.context)
+        elif self.inherits:
+            val = getattr(self.inherits, key)
+        else:
+            raise AttributeError(
+                    "Namespace '%s' has no member '%s'" %
+                    (self.name, key))
+        setattr(self, key, val)
+        return val
+
+def supports_caller(func):
+    """Apply a caller_stack compatibility decorator to a plain
+    Python function.
+
+    See the example in :ref:`namespaces_python_modules`.
+
+    """
+
+    def wrap_stackframe(context, *args, **kwargs):
+        context.caller_stack._push_frame()
+        try:
+            return func(context, *args, **kwargs)
+        finally:
+            context.caller_stack._pop_frame()
+    return wrap_stackframe
+
+def capture(context, callable_, *args, **kwargs):
+    """Execute the given template def, capturing the output into
+    a buffer.
+
+    See the example in :ref:`namespaces_python_modules`.
+
+    """
+
+    if not compat.callable(callable_):
+        raise exceptions.RuntimeException(
+                        "capture() function expects a callable as "
+                        "its argument (i.e. capture(func, *args, **kwargs))"
+                        )
+    context._push_buffer()
+    try:
+        callable_(*args, **kwargs)
+    finally:
+        buf = context._pop_buffer()
+    return buf.getvalue()
+
+def _decorate_toplevel(fn):
+    def decorate_render(render_fn):
+        def go(context, *args, **kw):
+            def y(*args, **kw):
+                return render_fn(context, *args, **kw)
+            try:
+                y.__name__ = render_fn.__name__[7:]
+            except TypeError:
+                # < Python 2.4
+                pass
+            return fn(y)(context, *args, **kw)
+        return go
+    return decorate_render
+
+def _decorate_inline(context, fn):
+    def decorate_render(render_fn):
+        dec = fn(render_fn)
+        def go(*args, **kw):
+            return dec(context, *args, **kw)
+        return go
+    return decorate_render
+
+def _include_file(context, uri, calling_uri, **kwargs):
+    """locate the template from the given uri and include it in
+    the current output."""
+
+    template = _lookup_template(context, uri, calling_uri)
+    (callable_, ctx) = _populate_self_namespace(
+                                context._clean_inheritance_tokens(),
+                                template)
+    callable_(ctx, **_kwargs_for_include(callable_, context._data, **kwargs))
+
+def _inherit_from(context, uri, calling_uri):
+    """called by the _inherit method in template modules to set
+    up the inheritance chain at the start of a template's
+    execution."""
+
+    if uri is None:
+        return None
+    template = _lookup_template(context, uri, calling_uri)
+    self_ns = context['self']
+    ih = self_ns
+    while ih.inherits is not None:
+        ih = ih.inherits
+    lclcontext = context._locals({'next': ih})
+    ih.inherits = TemplateNamespace("self:%s" % template.uri,
+                                lclcontext,
+                                template=template,
+                                populate_self=False)
+    context._data['parent'] = lclcontext._data['local'] = ih.inherits
+    callable_ = getattr(template.module, '_mako_inherit', None)
+    if callable_ is not None:
+        ret = callable_(template, lclcontext)
+        if ret:
+            return ret
+
+    gen_ns = getattr(template.module, '_mako_generate_namespaces', None)
+    if gen_ns is not None:
+        gen_ns(context)
+    return (template.callable_, lclcontext)
+
+def _lookup_template(context, uri, relativeto):
+    lookup = context._with_template.lookup
+    if lookup is None:
+        raise exceptions.TemplateLookupException(
+                            "Template '%s' has no TemplateLookup associated" %
+                            context._with_template.uri)
+    uri = lookup.adjust_uri(uri, relativeto)
+    try:
+        return lookup.get_template(uri)
+    except exceptions.TopLevelLookupException:
+        raise exceptions.TemplateLookupException(str(compat.exception_as()))
+
+def _populate_self_namespace(context, template, self_ns=None):
+    if self_ns is None:
+        self_ns = TemplateNamespace('self:%s' % template.uri,
+                                context, template=template,
+                                populate_self=False)
+    context._data['self'] = context._data['local'] = self_ns
+    if hasattr(template.module, '_mako_inherit'):
+        ret = template.module._mako_inherit(template, context)
+        if ret:
+            return ret
+    return (template.callable_, context)
+
+def _render(template, callable_, args, data, as_unicode=False):
+    """create a Context and return the string
+    output of the given template and template callable."""
+
+    if as_unicode:
+        buf = util.FastEncodingBuffer(as_unicode=True)
+    elif template.bytestring_passthrough:
+        buf = compat.StringIO()
+    else:
+        buf = util.FastEncodingBuffer(
+                        as_unicode=as_unicode,
+                        encoding=template.output_encoding,
+                        errors=template.encoding_errors)
+    context = Context(buf, **data)
+    context._outputting_as_unicode = as_unicode
+    context._set_with_template(template)
+
+    _render_context(template, callable_, context, *args,
+                            **_kwargs_for_callable(callable_, data))
+    return context._pop_buffer().getvalue()
+
+def _kwargs_for_callable(callable_, data):
+    argspec = compat.inspect_func_args(callable_)
+    # for normal pages, **pageargs is usually present
+    if argspec[2]:
+        return data
+
+    # for rendering defs from the top level, figure out the args
+    namedargs = argspec[0] + [v for v in argspec[1:3] if v is not None]
+    kwargs = {}
+    for arg in namedargs:
+        if arg != 'context' and arg in data and arg not in kwargs:
+            kwargs[arg] = data[arg]
+    return kwargs
+
+def _kwargs_for_include(callable_, data, **kwargs):
+    argspec = compat.inspect_func_args(callable_)
+    namedargs = argspec[0] + [v for v in argspec[1:3] if v is not None]
+    for arg in namedargs:
+        if arg != 'context' and arg in data and arg not in kwargs:
+            kwargs[arg] = data[arg]
+    return kwargs
+
+def _render_context(tmpl, callable_, context, *args, **kwargs):
+    import mako.template as template
+    # create polymorphic 'self' namespace for this
+    # template with possibly updated context
+    if not isinstance(tmpl, template.DefTemplate):
+        # if main render method, call from the base of the inheritance stack
+        (inherit, lclcontext) = _populate_self_namespace(context, tmpl)
+        _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
+    else:
+        # otherwise, call the actual rendering method specified
+        (inherit, lclcontext) = _populate_self_namespace(context, tmpl.parent)
+        _exec_template(callable_, context, args=args, kwargs=kwargs)
+
+def _exec_template(callable_, context, args=None, kwargs=None):
+    """execute a rendering callable given the callable, a
+    Context, and optional explicit arguments
+
+    the contextual Template will be located if it exists, and
+    the error handling options specified on that Template will
+    be interpreted here.
+    """
+    template = context._with_template
+    if template is not None and \
+            (template.format_exceptions or template.error_handler):
+        try:
+            callable_(context, *args, **kwargs)
+        except Exception:
+            _render_error(template, context, compat.exception_as())
+        except:
+            e = sys.exc_info()[0]
+            _render_error(template, context, e)
+    else:
+        callable_(context, *args, **kwargs)
+
+def _render_error(template, context, error):
+    if template.error_handler:
+        result = template.error_handler(context, error)
+        if not result:
+            compat.reraise(*sys.exc_info())
+    else:
+        error_template = exceptions.html_error_template()
+        if context._outputting_as_unicode:
+            context._buffer_stack[:] = [
+                                    util.FastEncodingBuffer(as_unicode=True)]
+        else:
+            context._buffer_stack[:] = [util.FastEncodingBuffer(
+                                            error_template.output_encoding,
+                                            error_template.encoding_errors)]
+
+        context._set_with_template(error_template)
+        error_template.render_context(context, error=error)