# Django 2.2.9.dev20191202104323 文档

# django.template.base 源代码

    """
    This is the Django template system.

    How it works:

    The Lexer.tokenize() method converts a template string (i.e., a string
    containing markup with custom template tags) to tokens, which can be either
    plain text (TokenType.TEXT), variables (TokenType.VAR), or block statements
    (TokenType.BLOCK).

    The Parser() class takes a list of tokens in its constructor, and its parse()
    method returns a compiled template -- which is, under the hood, a list of
    Node objects.

    Each Node is responsible for creating some sort of output -- e.g. simple text
    (TextNode), variable values in a given context (VariableNode), results of basic
    logic (IfNode), results of looping (ForNode), or anything else. The core Node
    types are TextNode, VariableNode, IfNode and ForNode, but plugin modules can
    define their own custom node types.

    Each Node has a render() method, which takes a Context and returns a string of
    the rendered node. For example, the render() method of a Variable Node returns
    the variable's value as a string. The render() method of a ForNode returns the
    rendered output of whatever was inside the loop, recursively.

    The Template class is a convenient wrapper that takes care of template
    compilation and rendering.

    Usage:

    The only thing you should ever use directly in this file is the Template class.
    Create a compiled template object with a template_string, then call render()
    with a context. In the compilation stage, the TemplateSyntaxError exception
    will be raised if the template doesn't have proper syntax.

    Sample code:

    >>> from django import template
    >>> s = '<html>{% if test %}<h1>{{ varvalue }}</h1>{% endif %}</html>'
    >>> t = template.Template(s)

    (t is now a compiled template, and its render() method can be called multiple
    times with multiple contexts)

    >>> c = template.Context({'test':True, 'varvalue': 'Hello'})
    >>> t.render(c)
    '<html><h1>Hello</h1></html>'
    >>> c = template.Context({'test':False, 'varvalue': 'Hello'})
    >>> t.render(c)
    '<html></html>'
    """

    import logging
    import re
    from enum import Enum
    from inspect import getcallargs, getfullargspec, unwrap

    from django.template.context import (  # NOQA: imported for backwards compatibility
        BaseContext, Context, ContextPopException, RequestContext,
    )
    from django.utils.formats import localize
    from django.utils.html import conditional_escape, escape
    from django.utils.safestring import SafeData, mark_safe
    from django.utils.text import (
        get_text_list, smart_split, unescape_string_literal,
    )
    from django.utils.timezone import template_localtime
    from django.utils.translation import gettext_lazy, pgettext_lazy

    from .exceptions import TemplateSyntaxError

    # template syntax constants
    FILTER_SEPARATOR = '|'
    FILTER_ARGUMENT_SEPARATOR = ':'
    VARIABLE_ATTRIBUTE_SEPARATOR = '.'
    BLOCK_TAG_START = '{%'
    BLOCK_TAG_END = '%}'
    VARIABLE_TAG_START = '{{'
    VARIABLE_TAG_END = '}}'
    COMMENT_TAG_START = '{#'
    COMMENT_TAG_END = '#}'
    TRANSLATOR_COMMENT_MARK = 'Translators'
    SINGLE_BRACE_START = '{'
    SINGLE_BRACE_END = '}'

    # what to report as the origin for templates that come from non-loader sources
    # (e.g. strings)
    UNKNOWN_SOURCE = '<unknown source>'

    # match a variable or block tag and capture the entire tag, including start/end
    # delimiters
    tag_re = (re.compile('(%s.*?%s|%s.*?%s|%s.*?%s)' %
              (re.escape(BLOCK_TAG_START), re.escape(BLOCK_TAG_END),
               re.escape(VARIABLE_TAG_START), re.escape(VARIABLE_TAG_END),
               re.escape(COMMENT_TAG_START), re.escape(COMMENT_TAG_END))))

    logger = logging.getLogger('django.template')

    class TokenType(Enum):
        TEXT = 0
        VAR = 1
        BLOCK = 2
        COMMENT = 3

    class VariableDoesNotExist(Exception):

        def __init__(self, msg, params=()):
            self.msg = msg
            self.params = params

        def __str__(self):
            return self.msg % self.params

    class Origin:
        def __init__(self, name, template_name=None, loader=None):
            self.name = name
            self.template_name = template_name
            self.loader = loader

        def __str__(self):
            return self.name

        def __eq__(self, other):
            return (
                isinstance(other, Origin) and
                self.name == other.name and
                self.loader == other.loader
            )

        @property
        def loader_name(self):
            if self.loader:
                return '%s.%s' % (
                    self.loader.__module__, self.loader.__class__.__name__,
                )

    class Template:
        def __init__(self, template_string, origin=None, name=None, engine=None):
            # If Template is instantiated directly rather than from an Engine and
            # exactly one Django template engine is configured, use that engine.
            # This is required to preserve backwards-compatibility for direct use
            # e.g. Template('...').render(Context({...}))
            if engine is None:
                from .engine import Engine
                engine = Engine.get_default()
            if origin is None:
                origin = Origin(UNKNOWN_SOURCE)
            self.name = name
            self.origin = origin
            self.engine = engine
            self.source = str(template_string)  # May be lazy.
            self.nodelist = self.compile_nodelist()

        def __iter__(self):
            for node in self.nodelist:
                yield from node

        def _render(self, context):
            return self.nodelist.render(context)

        def render(self, context):
            "Display stage -- can be called many times"
            with context.render_context.push_state(self):
                if context.template is None:
                    with context.bind_template(self):
                        context.template_name = self.name
                        return self._render(context)
                else:
                    return self._render(context)

        def compile_nodelist(self):
            """
            Parse and compile the template source into a nodelist. If debug
            is True and an exception occurs during parsing, the exception is
            is annotated with contextual line information where it occurred in the
            template source.
            """
            if self.engine.debug:
                lexer = DebugLexer(self.source)
            else:
                lexer = Lexer(self.source)

            tokens = lexer.tokenize()
            parser = Parser(
                tokens, self.engine.template_libraries, self.engine.template_builtins,
                self.origin,
            )

            try:
                return parser.parse()
            except Exception as e:
                if self.engine.debug:
                    e.template_debug = self.get_exception_info(e, e.token)
                raise

        def get_exception_info(self, exception, token):
            """
            Return a dictionary containing contextual line information of where
            the exception occurred in the template. The following information is
            provided:

            message
                The message of the exception raised.

            source_lines
                The lines before, after, and including the line the exception
                occurred on.

            line
                The line number the exception occurred on.

            before, during, after
                The line the exception occurred on split into three parts:
                1. The content before the token that raised the error.
                2. The token that raised the error.
                3. The content after the token that raised the error.

            total
                The number of lines in source_lines.

            top
                The line number where source_lines starts.

            bottom
                The line number where source_lines ends.

            start
                The start position of the token in the template source.

            end
                The end position of the token in the template source.
            """
            start, end = token.position
            context_lines = 10
            line = 0
            upto = 0
            source_lines = []
            before = during = after = ""
            for num, next in enumerate(linebreak_iter(self.source)):
                if start >= upto and end <= next:
                    line = num
                    before = escape(self.source[upto:start])
                    during = escape(self.source[start:end])
                    after = escape(self.source[end:next])
                source_lines.append((num, escape(self.source[upto:next])))
                upto = next
            total = len(source_lines)

            top = max(1, line - context_lines)
            bottom = min(total, line + 1 + context_lines)

            # In some rare cases exc_value.args can be empty or an invalid
            # string.
            try:
                message = str(exception.args[0])
            except (IndexError, UnicodeDecodeError):
                message = '(Could not get exception message)'

            return {
                'message': message,
                'source_lines': source_lines[top:bottom],
                'before': before,
                'during': during,
                'after': after,
                'top': top,
                'bottom': bottom,
                'total': total,
                'line': line,
                'name': self.origin.name,
                'start': start,
                'end': end,
            }

    def linebreak_iter(template_source):
        yield 0
        p = template_source.find('\n')
        while p >= 0:
            yield p + 1
            p = template_source.find('\n', p + 1)
        yield len(template_source) + 1

    class Token:
        def __init__(self, token_type, contents, position=None, lineno=None):
            """
            A token representing a string from the template.

            token_type
                A TokenType, either .TEXT, .VAR, .BLOCK, or .COMMENT.

            contents
                The token source string.

            position
                An optional tuple containing the start and end index of the token
                in the template source. This is used for traceback information
                when debug is on.

            lineno
                The line number the token appears on in the template source.
                This is used for traceback information and gettext files.
            """
            self.token_type, self.contents = token_type, contents
            self.lineno = lineno
            self.position = position

        def __str__(self):
            token_name = self.token_type.name.capitalize()
            return ('<%s token: "%s...">' %
                    (token_name, self.contents[:20].replace('\n', '')))

        def split_contents(self):
            split = []
            bits = smart_split(self.contents)
            for bit in bits:
                # Handle translation-marked template pieces
                if bit.startswith(('_("', "_('")):
                    sentinel = bit[2] + ')'
                    trans_bit = [bit]
                    while not bit.endswith(sentinel):
                        bit = next(bits)
                        trans_bit.append(bit)
                    bit = ' '.join(trans_bit)
                split.append(bit)
            return split

    class Lexer:
        def __init__(self, template_string):
            self.template_string = template_string
            self.verbatim = False

        def tokenize(self):
            """
            Return a list of tokens from a given template_string.
            """
            in_tag = False
            lineno = 1
            result = []
            for bit in tag_re.split(self.template_string):
                if bit:
                    result.append(self.create_token(bit, None, lineno, in_tag))
                in_tag = not in_tag
                lineno += bit.count('\n')
            return result

        def create_token(self, token_string, position, lineno, in_tag):
            """
            Convert the given token string into a new Token object and return it.
            If in_tag is True, we are processing something that matched a tag,
            otherwise it should be treated as a literal string.
            """
            if in_tag and token_string.startswith(BLOCK_TAG_START):
                # The [2:-2] ranges below strip off *_TAG_START and *_TAG_END.
                # We could do len(BLOCK_TAG_START) to be more "correct", but we've
                # hard-coded the 2s here for performance. And it's not like
                # the TAG_START values are going to change anytime, anyway.
                block_content = token_string[2:-2].strip()
                if self.verbatim and block_content == self.verbatim:
                    self.verbatim = False
            if in_tag and not self.verbatim:
                if token_string.startswith(VARIABLE_TAG_START):
                    return Token(TokenType.VAR, token_string[2:-2].strip(), position, lineno)
                elif token_string.startswith(BLOCK_TAG_START):
                    if block_content[:9] in ('verbatim', 'verbatim '):
                        self.verbatim = 'end%s' % block_content
                    return Token(TokenType.BLOCK, block_content, position, lineno)
                elif token_string.startswith(COMMENT_TAG_START):
                    content = ''
                    if token_string.find(TRANSLATOR_COMMENT_MARK):
                        content = token_string[2:-2].strip()
                    return Token(TokenType.COMMENT, content, position, lineno)
            else:
                return Token(TokenType.TEXT, token_string, position, lineno)

    class DebugLexer(Lexer):
        def tokenize(self):
            """
            Split a template string into tokens and annotates each token with its
            start and end position in the source. This is slower than the default
            lexer so only use it when debug is True.
            """
            lineno = 1
            result = []
            upto = 0
            for match in tag_re.finditer(self.template_string):
                start, end = match.span()
                if start > upto:
                    token_string = self.template_string[upto:start]
                    result.append(self.create_token(token_string, (upto, start), lineno, in_tag=False))
                    lineno += token_string.count('\n')
                    upto = start
                token_string = self.template_string[start:end]
                result.append(self.create_token(token_string, (start, end), lineno, in_tag=True))
                lineno += token_string.count('\n')
                upto = end
            last_bit = self.template_string[upto:]
            if last_bit:
                result.append(self.create_token(last_bit, (upto, upto + len(last_bit)), lineno, in_tag=False))
            return result

    class Parser:
        def __init__(self, tokens, libraries=None, builtins=None, origin=None):
            self.tokens = tokens
            self.tags = {}
            self.filters = {}
            self.command_stack = []

            if libraries is None:
                libraries = {}
            if builtins is None:
                builtins = []

            self.libraries = libraries
            for builtin in builtins:
                self.add_library(builtin)
            self.origin = origin

        def parse(self, parse_until=None):
            """
            Iterate through the parser tokens and compiles each one into a node.

            If parse_until is provided, parsing will stop once one of the
            specified tokens has been reached. This is formatted as a list of
            tokens, e.g. ['elif', 'else', 'endif']. If no matching token is
            reached, raise an exception with the unclosed block tag details.
            """
            if parse_until is None:
                parse_until = []
            nodelist = NodeList()
            while self.tokens:
                token = self.next_token()
                # Use the raw values here for TokenType.* for a tiny performance boost.
                if token.token_type.value == 0:  # TokenType.TEXT
                    self.extend_nodelist(nodelist, TextNode(token.contents), token)
                elif token.token_type.value == 1:  # TokenType.VAR
                    if not token.contents:
                        raise self.error(token, 'Empty variable tag on line %d' % token.lineno)
                    try:
                        filter_expression = self.compile_filter(token.contents)
                    except TemplateSyntaxError as e:
                        raise self.error(token, e)
                    var_node = VariableNode(filter_expression)
                    self.extend_nodelist(nodelist, var_node, token)
                elif token.token_type.value == 2:  # TokenType.BLOCK
                    try:
                        command = token.contents.split()[0]
                    except IndexError:
                        raise self.error(token, 'Empty block tag on line %d' % token.lineno)
                    if command in parse_until:
                        # A matching token has been reached. Return control to
                        # the caller. Put the token back on the token list so the
                        # caller knows where it terminated.
                        self.prepend_token(token)
                        return nodelist
                    # Add the token to the command stack. This is used for error
                    # messages if further parsing fails due to an unclosed block
                    # tag.
                    self.command_stack.append((command, token))
                    # Get the tag callback function from the ones registered with
                    # the parser.
                    try:
                        compile_func = self.tags[command]
                    except KeyError:
                        self.invalid_block_tag(token, command, parse_until)
                    # Compile the callback into a node object and add it to
                    # the node list.
                    try:
                        compiled_result = compile_func(self, token)
                    except Exception as e:
                        raise self.error(token, e)
                    self.extend_nodelist(nodelist, compiled_result, token)
                    # Compile success. Remove the token from the command stack.
                    self.command_stack.pop()
            if parse_until:
                self.unclosed_block_tag(parse_until)
            return nodelist

        def skip_past(self, endtag):
            while self.tokens:
                token = self.next_token()
                if token.token_type == TokenType.BLOCK and token.contents == endtag:
                    return
            self.unclosed_block_tag([endtag])

        def extend_nodelist(self, nodelist, node, token):
            # Check that non-text nodes don't appear before an extends tag.
            if node.must_be_first and nodelist.contains_nontext:
                raise self.error(
                    token, '%r must be the first tag in the template.' % node,
                )
            if isinstance(nodelist, NodeList) and not isinstance(node, TextNode):
                nodelist.contains_nontext = True
            # Set origin and token here since we can't modify the node __init__()
            # method.
            node.token = token
            node.origin = self.origin
            nodelist.append(node)

        def error(self, token, e):
            """
            Return an exception annotated with the originating token. Since the
            parser can be called recursively, check if a token is already set. This
            ensures the innermost token is highlighted if an exception occurs,
            e.g. a compile error within the body of an if statement.
            """
            if not isinstance(e, Exception):
                e = TemplateSyntaxError(e)
            if not hasattr(e, 'token'):
                e.token = token
            return e

        def invalid_block_tag(self, token, command, parse_until=None):
            if parse_until:
                raise self.error(
                    token,
                    "Invalid block tag on line %d: '%s', expected %s. Did you "
                    "forget to register or load this tag?" % (
                        token.lineno,
                        command,
                        get_text_list(["'%s'" % p for p in parse_until], 'or'),
                    ),
                )
            raise self.error(
                token,
                "Invalid block tag on line %d: '%s'. Did you forget to register "
                "or load this tag?" % (token.lineno, command)
            )

        def unclosed_block_tag(self, parse_until):
            command, token = self.command_stack.pop()
            msg = "Unclosed tag on line %d: '%s'. Looking for one of: %s." % (
                token.lineno,
                command,
                ', '.join(parse_until),
            )
            raise self.error(token, msg)

        def next_token(self):
            return self.tokens.pop(0)

        def prepend_token(self, token):
            self.tokens.insert(0, token)

        def delete_first_token(self):
            del self.tokens[0]

        def add_library(self, lib):
            self.tags.update(lib.tags)
            self.filters.update(lib.filters)

        def compile_filter(self, token):
            """
            Convenient wrapper for FilterExpression
            """
            return FilterExpression(token, self)

        def find_filter(self, filter_name):
            if filter_name in self.filters:
                return self.filters[filter_name]
            else:
                raise TemplateSyntaxError("Invalid filter: '%s'" % filter_name)

    # This only matches constant *strings* (things in quotes or marked for
    # translation). Numbers are treated as variables for implementation reasons
    # (so that they retain their type when passed to filters).
    constant_string = r"""
    (?:%(i18n_open)s%(strdq)s%(i18n_close)s|
    %(i18n_open)s%(strsq)s%(i18n_close)s|
    %(strdq)s|
    %(strsq)s)
    """ % {
        'strdq': r'"[^"\\]*(?:\\.[^"\\]*)*"',  # double-quoted string
        'strsq': r"'[^'\\]*(?:\\.[^'\\]*)*'",  # single-quoted string
        'i18n_open': re.escape("_("),
        'i18n_close': re.escape(")"),
    }
    constant_string = constant_string.replace("\n", "")

    filter_raw_string = r"""
    ^(?P<constant>%(constant)s)|
    ^(?P<var>[%(var_chars)s]+|%(num)s)|
     (?:\s*%(filter_sep)s\s*
         (?P<filter_name>\w+)
             (?:%(arg_sep)s
                 (?:
                  (?P<constant_arg>%(constant)s)|
                  (?P<var_arg>[%(var_chars)s]+|%(num)s)
                 )
             )?
     )""" % {
        'constant': constant_string,
        'num': r'[-+\.]?\d[\d\.e]*',
        'var_chars': r'\w\.',
        'filter_sep': re.escape(FILTER_SEPARATOR),
        'arg_sep': re.escape(FILTER_ARGUMENT_SEPARATOR),
    }

    filter_re = re.compile(filter_raw_string, re.VERBOSE)

    class FilterExpression:
        """
        Parse a variable token and its optional filters (all as a single string),
        and return a list of tuples of the filter name and arguments.
        Sample::

            >>> token = 'variable|default:"Default value"|date:"Y-m-d"'
            >>> p = Parser('')
            >>> fe = FilterExpression(token, p)
            >>> len(fe.filters)
            2
            >>> fe.var
            <Variable: 'variable'>
        """
        def __init__(self, token, parser):
            self.token = token
            matches = filter_re.finditer(token)
            var_obj = None
            filters = []
            upto = 0
            for match in matches:
                start = match.start()
                if upto != start:
                    raise TemplateSyntaxError("Could not parse some characters: "
                                              "%s|%s|%s" %
                                              (token[:upto], token[upto:start],
                                               token[start:]))
                if var_obj is None:
                    var, constant = match.group("var", "constant")
                    if constant:
                        try:
                            var_obj = Variable(constant).resolve({})
                        except VariableDoesNotExist:
                            var_obj = None
                    elif var is None:
                        raise TemplateSyntaxError("Could not find variable at "
                                                  "start of %s." % token)
                    else:
                        var_obj = Variable(var)
                else:
                    filter_name = match.group("filter_name")
                    args = []
                    constant_arg, var_arg = match.group("constant_arg", "var_arg")
                    if constant_arg:
                        args.append((False, Variable(constant_arg).resolve({})))
                    elif var_arg:
                        args.append((True, Variable(var_arg)))
                    filter_func = parser.find_filter(filter_name)
                    self.args_check(filter_name, filter_func, args)
                    filters.append((filter_func, args))
                upto = match.end()
            if upto != len(token):
                raise TemplateSyntaxError("Could not parse the remainder: '%s' "
                                          "from '%s'" % (token[upto:], token))

            self.filters = filters
            self.var = var_obj

        def resolve(self, context, ignore_failures=False):
            if isinstance(self.var, Variable):
                try:
                    obj = self.var.resolve(context)
                except VariableDoesNotExist:
                    if ignore_failures:
                        obj = None
                    else:
                        string_if_invalid = context.template.engine.string_if_invalid
                        if string_if_invalid:
                            if '%s' in string_if_invalid:
                                return string_if_invalid % self.var
                            else:
                                return string_if_invalid
                        else:
                            obj = string_if_invalid
            else:
                obj = self.var
            for func, args in self.filters:
                arg_vals = []
                for lookup, arg in args:
                    if not lookup:
                        arg_vals.append(mark_safe(arg))
                    else:
                        arg_vals.append(arg.resolve(context))
                if getattr(func, 'expects_localtime', False):
                    obj = template_localtime(obj, context.use_tz)
                if getattr(func, 'needs_autoescape', False):
                    new_obj = func(obj, autoescape=context.autoescape, *arg_vals)
                else:
                    new_obj = func(obj, *arg_vals)
                if getattr(func, 'is_safe', False) and isinstance(obj, SafeData):
                    obj = mark_safe(new_obj)
                else:
                    obj = new_obj
            return obj

        def args_check(name, func, provided):
            provided = list(provided)
            # First argument, filter input, is implied.
            plen = len(provided) + 1
            # Check to see if a decorator is providing the real function.
            func = unwrap(func)

            args, _, _, defaults, _, _, _ = getfullargspec(func)
            alen = len(args)
            dlen = len(defaults or [])
            # Not enough OR Too many
            if plen < (alen - dlen) or plen > alen:
                raise TemplateSyntaxError("%s requires %d arguments, %d provided" %
                                          (name, alen - dlen, plen))

            return True
        args_check = staticmethod(args_check)

        def __str__(self):
            return self.token

    class Variable:
        """
        A template variable, resolvable against a given context. The variable may
        be a hard-coded string (if it begins and ends with single or double quote
        marks)::

            >>> c = {'article': {'section':'News'}}
            >>> Variable('article.section').resolve(c)
            'News'
            >>> Variable('article').resolve(c)
            {'section': 'News'}
            >>> class AClass: pass
            >>> c = AClass()
            >>> c.article = AClass()
            >>> c.article.section = 'News'

        (The example assumes VARIABLE_ATTRIBUTE_SEPARATOR is '.')
        """

        def __init__(self, var):
            self.var = var
            self.literal = None
            self.lookups = None
            self.translate = False
            self.message_context = None

            if not isinstance(var, str):
                raise TypeError(
                    "Variable must be a string or number, got %s" % type(var))
            try:
                # First try to treat this variable as a number.
                #
                # Note that this could cause an OverflowError here that we're not
                # catching. Since this should only happen at compile time, that's
                # probably OK.

                # Try to interpret values containing a period or an 'e'/'E'
                # (possibly scientific notation) as a float;  otherwise, try int.
                if '.' in var or 'e' in var.lower():
                    self.literal = float(var)
                    # "2." is invalid
                    if var.endswith('.'):
                        raise ValueError
                else:
                    self.literal = int(var)
            except ValueError:
                # A ValueError means that the variable isn't a number.
                if var.startswith('_(') and var.endswith(')'):
                    # The result of the lookup should be translated at rendering
                    # time.
                    self.translate = True
                    var = var[2:-1]
                # If it's wrapped with quotes (single or double), then
                # we're also dealing with a literal.
                try:
                    self.literal = mark_safe(unescape_string_literal(var))
                except ValueError:
                    # Otherwise we'll set self.lookups so that resolve() knows we're
                    # dealing with a bonafide variable
                    if var.find(VARIABLE_ATTRIBUTE_SEPARATOR + '_') > -1 or var[0] == '_':
                        raise TemplateSyntaxError("Variables and attributes may "
                                                  "not begin with underscores: '%s'" %
                                                  var)
                    self.lookups = tuple(var.split(VARIABLE_ATTRIBUTE_SEPARATOR))

        def resolve(self, context):
            """Resolve this variable against a given context."""
            if self.lookups is not None:
                # We're dealing with a variable that needs to be resolved
                value = self._resolve_lookup(context)
            else:
                # We're dealing with a literal, so it's already been "resolved"
                value = self.literal
            if self.translate:
                is_safe = isinstance(value, SafeData)
                msgid = value.replace('%', '%%')
                msgid = mark_safe(msgid) if is_safe else msgid
                if self.message_context:
                    return pgettext_lazy(self.message_context, msgid)
                else:
                    return gettext_lazy(msgid)
            return value

        def __repr__(self):
            return "<%s: %r>" % (self.__class__.__name__, self.var)

        def __str__(self):
            return self.var

        def _resolve_lookup(self, context):
            """
            Perform resolution of a real variable (i.e. not a literal) against the
            given context.

            As indicated by the method's name, this method is an implementation
            detail and shouldn't be called by external code. Use Variable.resolve()
            instead.
            """
            current = context
            try:  # catch-all for silent variable failures
                for bit in self.lookups:
                    try:  # dictionary lookup
                        current = current[bit]
                        # ValueError/IndexError are for numpy.array lookup on
                        # numpy < 1.9 and 1.9+ respectively
                    except (TypeError, AttributeError, KeyError, ValueError, IndexError):
                        try:  # attribute lookup
                            # Don't return class attributes if the class is the context:
                            if isinstance(current, BaseContext) and getattr(type(current), bit):
                                raise AttributeError
                            current = getattr(current, bit)
                        except (TypeError, AttributeError):
                            # Reraise if the exception was raised by a @property
                            if not isinstance(current, BaseContext) and bit in dir(current):
                                raise
                            try:  # list-index lookup
                                current = current[int(bit)]
                            except (IndexError,  # list index out of range
                                    ValueError,  # invalid literal for int()
                                    KeyError,    # current is a dict without `int(bit)` key
                                    TypeError):  # unsubscriptable object
                                raise VariableDoesNotExist("Failed lookup for key "
                                                           "[%s] in %r",
                                                           (bit, current))  # missing attribute
                    if callable(current):
                        if getattr(current, 'do_not_call_in_templates', False):
                            pass
                        elif getattr(current, 'alters_data', False):
                            current = context.template.engine.string_if_invalid
                        else:
                            try:  # method call (assuming no args required)
                                current = current()
                            except TypeError:
                                try:
                                    getcallargs(current)
                                except TypeError:  # arguments *were* required
                                    current = context.template.engine.string_if_invalid  # invalid method call
                                else:
                                    raise
            except Exception as e:
                template_name = getattr(context, 'template_name', None) or 'unknown'
                logger.debug(
                    "Exception while resolving variable '%s' in template '%s'.",
                    bit,
                    template_name,
                    exc_info=True,
                )

                if getattr(e, 'silent_variable_failure', False):
                    current = context.template.engine.string_if_invalid
                else:
                    raise

            return current

    class Node:
        # Set this to True for nodes that must be first in the template (although
        # they can be preceded by text nodes.
        must_be_first = False
        child_nodelists = ('nodelist',)
        token = None

        def render(self, context):
            """
            Return the node rendered as a string.
            """
            pass

        def render_annotated(self, context):
            """
            Render the node. If debug is True and an exception occurs during
            rendering, the exception is annotated with contextual line information
            where it occurred in the template. For internal usage this method is
            preferred over using the render method directly.
            """
            try:
                return self.render(context)
            except Exception as e:
                if context.template.engine.debug and not hasattr(e, 'template_debug'):
                    e.template_debug = context.render_context.template.get_exception_info(e, self.token)
                raise

        def __iter__(self):
            yield self

        def get_nodes_by_type(self, nodetype):
            """
            Return a list of all nodes (within this node and its nodelist)
            of the given type
            """
            nodes = []
            if isinstance(self, nodetype):
                nodes.append(self)
            for attr in self.child_nodelists:
                nodelist = getattr(self, attr, None)
                if nodelist:
                    nodes.extend(nodelist.get_nodes_by_type(nodetype))
            return nodes

    class NodeList(list):
        # Set to True the first time a non-TextNode is inserted by
        # extend_nodelist().
        contains_nontext = False

        def render(self, context):
            bits = []
            for node in self:
                if isinstance(node, Node):
                    bit = node.render_annotated(context)
                else:
                    bit = node
                bits.append(str(bit))
            return mark_safe(''.join(bits))

        def get_nodes_by_type(self, nodetype):
            "Return a list of all nodes of the given type"
            nodes = []
            for node in self:
                nodes.extend(node.get_nodes_by_type(nodetype))
            return nodes

    class TextNode(Node):
        def __init__(self, s):
            self.s = s

        def __repr__(self):
            return "<%s: %r>" % (self.__class__.__name__, self.s[:25])

        def render(self, context):
            return self.s

    def render_value_in_context(value, context):
        """
        Convert any value to a string to become part of a rendered template. This
        means escaping, if required, and conversion to a string. If value is a
        string, it's expected to already be translated.
        """
        value = template_localtime(value, use_tz=context.use_tz)
        value = localize(value, use_l10n=context.use_l10n)
        if context.autoescape:
            if not issubclass(type(value), str):
                value = str(value)
            return conditional_escape(value)
        else:
            return str(value)

    class VariableNode(Node):
        def __init__(self, filter_expression):
            self.filter_expression = filter_expression

        def __repr__(self):
            return "<Variable Node: %s>" % self.filter_expression

        def render(self, context):
            try:
                output = self.filter_expression.resolve(context)
            except UnicodeDecodeError:
                # Unicode conversion can fail sometimes for reasons out of our
                # control (e.g. exception rendering). In that case, we fail
                # quietly.
                return ''
            return render_value_in_context(output, context)

    # Regex for token keyword arguments
    kwarg_re = re.compile(r"(?:(\w+)=)?(.+)")

    def token_kwargs(bits, parser, support_legacy=False):
        """
        Parse token keyword arguments and return a dictionary of the arguments
        retrieved from the ``bits`` token list.

        `bits` is a list containing the remainder of the token (split by spaces)
        that is to be checked for arguments. Valid arguments are removed from this
        list.

        `support_legacy` - if True, the legacy format ``1 as foo`` is accepted.
        Otherwise, only the standard ``foo=1`` format is allowed.

        There is no requirement for all remaining token ``bits`` to be keyword
        arguments, so return the dictionary as soon as an invalid argument format
        is reached.
        """
        if not bits:
            return {}
        match = kwarg_re.match(bits[0])
        kwarg_format = match and match.group(1)
        if not kwarg_format:
            if not support_legacy:
                return {}
            if len(bits) < 3 or bits[1] != 'as':
                return {}

        kwargs = {}
        while bits:
            if kwarg_format:
                match = kwarg_re.match(bits[0])
                if not match or not match.group(1):
                    return kwargs
                key, value = match.groups()
                del bits[:1]
            else:
                if len(bits) < 3 or bits[1] != 'as':
                    return kwargs
                key, value = bits[2], bits[0]
                del bits[:3]
            kwargs[key] = parser.compile_filter(value)
            if bits and not kwarg_format:
                if bits[0] != 'and':
                    return kwargs
                del bits[:1]
        return kwargs