"""
This is the Django template system.

How it works:

The Lexer.tokenize() function converts a template string (i.e., a string containing
markup with custom template tags) to tokens, which can be either plain text
(TOKEN_TEXT), variables (TOKEN_VAR) or block statements (TOKEN_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 an IfNode 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 = u'<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)
u'<html><h1>Hello</h1></html>'
>>> c = template.Context({'test':False, 'varvalue': 'Hello'})
>>> t.render(c)
u'<html></html>'
"""
import re
from inspect import getargspec
from django.conf import settings
from django.template.context import Context, RequestContext, ContextPopException
from django.utils.itercompat import is_iterable
from django.utils.functional import curry, Promise
from django.utils.text import smart_split
from django.utils.encoding import smart_unicode, force_unicode
from django.utils.translation import ugettext as _
from django.utils.safestring import SafeData, EscapeData, mark_safe, mark_for_escaping
from django.utils.html import escape

__all__ = ('Template', 'Context', 'RequestContext', 'compile_string')

TOKEN_TEXT = 0
TOKEN_VAR = 1
TOKEN_BLOCK = 2
TOKEN_COMMENT = 3

# 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 = '#}'
SINGLE_BRACE_START = '{'
SINGLE_BRACE_END = '}'

ALLOWED_VARIABLE_CHARS = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_.'

# what to report as the origin for templates that come from non-loader sources
# (e.g. strings)
UNKNOWN_SOURCE="&lt;unknown source&gt;"

# 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)))

# global dictionary of libraries that have been loaded using get_library
libraries = {}
# global list of libraries to load by default for a new parser
builtins = []

# True if TEMPLATE_STRING_IF_INVALID contains a format string (%s). None means
# uninitialised.
invalid_var_format_string = None

class TemplateSyntaxError(Exception):
    def __str__(self):
        try:
            import cStringIO as StringIO
        except ImportError:
            import StringIO
        output = StringIO.StringIO()
        output.write(Exception.__str__(self))
        # Check if we wrapped an exception and print that too.
        if hasattr(self, 'exc_info'):
            import traceback
            output.write('\n\nOriginal ')
            e = self.exc_info
            traceback.print_exception(e[0], e[1], e[2], 500, output)
        return output.getvalue()

class TemplateDoesNotExist(Exception):
    pass

class TemplateEncodingError(Exception):
    pass

class VariableDoesNotExist(Exception):

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

    def __str__(self):
        return unicode(self).encode('utf-8')

    def __unicode__(self):
        return self.msg % tuple([force_unicode(p, errors='replace') for p in self.params])

class InvalidTemplateLibrary(Exception):
    pass

class Origin(object):
    def __init__(self, name):
        self.name = name

    def reload(self):
        raise NotImplementedError

    def __str__(self):
        return self.name

class StringOrigin(Origin):
    def __init__(self, source):
        super(StringOrigin, self).__init__(UNKNOWN_SOURCE)
        self.source = source

    def reload(self):
        return self.source

class Template(object):
    def __init__(self, template_string, origin=None, name='<Unknown Template>'):
        try:
            template_string = smart_unicode(template_string)
        except UnicodeDecodeError:
            raise TemplateEncodingError("Templates can only be constructed from unicode or UTF-8 strings.")
        if settings.TEMPLATE_DEBUG and origin is None:
            origin = StringOrigin(template_string)
        self.nodelist = compile_string(template_string, origin)
        self.name = name

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

    def render(self, context):
        "Display stage -- can be called many times"
        return self.nodelist.render(context)

def compile_string(template_string, origin):
    "Compiles template_string into NodeList ready for rendering"
    if settings.TEMPLATE_DEBUG:
        from debug import DebugLexer, DebugParser
        lexer_class, parser_class = DebugLexer, DebugParser
    else:
        lexer_class, parser_class = Lexer, Parser
    lexer = lexer_class(template_string, origin)
    parser = parser_class(lexer.tokenize())
    return parser.parse()

class Token(object):
    def __init__(self, token_type, contents):
        # token_type must be TOKEN_TEXT, TOKEN_VAR, TOKEN_BLOCK or TOKEN_COMMENT.
        self.token_type, self.contents = token_type, contents

    def __str__(self):
        return '<%s token: "%s...">' % \
            ({TOKEN_TEXT: 'Text', TOKEN_VAR: 'Var', TOKEN_BLOCK: 'Block', TOKEN_COMMENT: 'Comment'}[self.token_type],
            self.contents[:20].replace('\n', ''))

    def split_contents(self):
        return list(smart_split(self.contents))

class Lexer(object):
    def __init__(self, template_string, origin):
        self.template_string = template_string
        self.origin = origin

    def tokenize(self):
        "Return a list of tokens from a given template_string."
        in_tag = False
        result = []
        for bit in tag_re.split(self.template_string):
            if bit:
                result.append(self.create_token(bit, in_tag))
            in_tag = not in_tag
        return result

    def create_token(self, token_string, 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:
            if token_string.startswith(VARIABLE_TAG_START):
                token = Token(TOKEN_VAR, token_string[len(VARIABLE_TAG_START):-len(VARIABLE_TAG_END)].strip())
            elif token_string.startswith(BLOCK_TAG_START):
                token = Token(TOKEN_BLOCK, token_string[len(BLOCK_TAG_START):-len(BLOCK_TAG_END)].strip())
            elif token_string.startswith(COMMENT_TAG_START):
                token = Token(TOKEN_COMMENT, '')
        else:
            token = Token(TOKEN_TEXT, token_string)
        return token

class Parser(object):
    def __init__(self, tokens):
        self.tokens = tokens
        self.tags = {}
        self.filters = {}
        for lib in builtins:
            self.add_library(lib)

    def parse(self, parse_until=None):
        if parse_until is None: parse_until = []
        nodelist = self.create_nodelist()
        while self.tokens:
            token = self.next_token()
            if token.token_type == TOKEN_TEXT:
                self.extend_nodelist(nodelist, TextNode(token.contents), token)
            elif token.token_type == TOKEN_VAR:
                if not token.contents:
                    self.empty_variable(token)
                filter_expression = self.compile_filter(token.contents)
                var_node = self.create_variable_node(filter_expression)
                self.extend_nodelist(nodelist, var_node,token)
            elif token.token_type == TOKEN_BLOCK:
                if token.contents in parse_until:
                    # put token back on token list so calling code knows why it terminated
                    self.prepend_token(token)
                    return nodelist
                try:
                    command = token.contents.split()[0]
                except IndexError:
                    self.empty_block_tag(token)
                # execute callback function for this tag and append resulting node
                self.enter_command(command, token)
                try:
                    compile_func = self.tags[command]
                except KeyError:
                    self.invalid_block_tag(token, command)
                try:
                    compiled_result = compile_func(self, token)
                except TemplateSyntaxError, e:
                    if not self.compile_function_error(token, e):
                        raise
                self.extend_nodelist(nodelist, compiled_result, token)
                self.exit_command()
        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 == TOKEN_BLOCK and token.contents == endtag:
                return
        self.unclosed_block_tag([endtag])

    def create_variable_node(self, filter_expression):
        return VariableNode(filter_expression)

    def create_nodelist(self):
        return NodeList()

    def extend_nodelist(self, nodelist, node, token):
        if node.must_be_first and nodelist:
            try:
                if nodelist.contains_nontext:
                    raise AttributeError
            except AttributeError:
                raise TemplateSyntaxError("%r must be the first tag in the template." % node)
        if isinstance(nodelist, NodeList) and not isinstance(node, TextNode):
            nodelist.contains_nontext = True
        nodelist.append(node)

    def enter_command(self, command, token):
        pass

    def exit_command(self):
        pass

    def error(self, token, msg):
        return TemplateSyntaxError(msg)

    def empty_variable(self, token):
        raise self.error(token, "Empty variable tag")

    def empty_block_tag(self, token):
        raise self.error(token, "Empty block tag")

    def invalid_block_tag(self, token, command):
        raise self.error(token, "Invalid block tag: '%s'" % command)

    def unclosed_block_tag(self, parse_until):
        raise self.error(None, "Unclosed tags: %s " %  ', '.join(parse_until))

    def compile_function_error(self, token, e):
        pass

    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)

class TokenParser(object):
    """
    Subclass this and implement the top() method to parse a template line. When
    instantiating the parser, pass in the line from the Django template parser.

    The parser's "tagname" instance-variable stores the name of the tag that
    the filter was called with.
    """
    def __init__(self, subject):
        self.subject = subject
        self.pointer = 0
        self.backout = []
        self.tagname = self.tag()

    def top(self):
        "Overload this method to do the actual parsing and return the result."
        raise NotImplementedError()

    def more(self):
        "Returns True if there is more stuff in the tag."
        return self.pointer < len(self.subject)

    def back(self):
        "Undoes the last microparser. Use this for lookahead and backtracking."
        if not len(self.backout):
            raise TemplateSyntaxError("back called without some previous parsing")
        self.pointer = self.backout.pop()

    def tag(self):
        "A microparser that just returns the next tag from the line."
        subject = self.subject
        i = self.pointer
        if i >= len(subject):
            raise TemplateSyntaxError("expected another tag, found end of string: %s" % subject)
        p = i
        while i < len(subject) and subject[i] not in (' ', '\t'):
            i += 1
        s = subject[p:i]
        while i < len(subject) and subject[i] in (' ', '\t'):
            i += 1
        self.backout.append(self.pointer)
        self.pointer = i
        return s

    def value(self):
        "A microparser that parses for a value: some string constant or variable name."
        subject = self.subject
        i = self.pointer
        if i >= len(subject):
            raise TemplateSyntaxError("Searching for value. Expected another value but found end of string: %s" % subject)
        if subject[i] in ('"', "'"):
            p = i
            i += 1
            while i < len(subject) and subject[i] != subject[p]:
                i += 1
            if i >= len(subject):
                raise TemplateSyntaxError("Searching for value. Unexpected end of string in column %d: %s" % (i, subject))
            i += 1
            res = subject[p:i]
            while i < len(subject) and subject[i] in (' ', '\t'):
                i += 1
            self.backout.append(self.pointer)
            self.pointer = i
            return res
        else:
            p = i
            while i < len(subject) and subject[i] not in (' ', '\t'):
                if subject[i] in ('"', "'"):
                    c = subject[i]
                    i += 1
                    while i < len(subject) and subject[i] != c:
                        i += 1
                    if i >= len(subject):
                        raise TemplateSyntaxError("Searching for value. Unexpected end of string in column %d: %s" % (i, subject))
                i += 1
            s = subject[p:i]
            while i < len(subject) and subject[i] in (' ', '\t'):
                i += 1
            self.backout.append(self.pointer)
            self.pointer = i
            return s

filter_raw_string = r"""
^%(i18n_open)s"(?P<i18n_constant>%(str)s)"%(i18n_close)s|
^"(?P<constant>%(str)s)"|
^(?P<var>[%(var_chars)s]+)|
 (?:%(filter_sep)s
     (?P<filter_name>\w+)
         (?:%(arg_sep)s
             (?:
              %(i18n_open)s"(?P<i18n_arg>%(str)s)"%(i18n_close)s|
              "(?P<constant_arg>%(str)s)"|
              (?P<var_arg>[%(var_chars)s]+)
             )
         )?
 )""" % {
    'str': r"""[^"\\]*(?:\\.[^"\\]*)*""",
    'var_chars': "\w\." ,
    'filter_sep': re.escape(FILTER_SEPARATOR),
    'arg_sep': re.escape(FILTER_ARGUMENT_SEPARATOR),
    'i18n_open' : re.escape("_("),
    'i18n_close' : re.escape(")"),
  }

filter_raw_string = filter_raw_string.replace("\n", "").replace(" ", "")
filter_re = re.compile(filter_raw_string, re.UNICODE)

class FilterExpression(object):
    """
    Parses 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'

    This class should never be instantiated outside of the
    get_filters_from_token helper function.
    """
    def __init__(self, token, parser):
        self.token = token
        matches = filter_re.finditer(token)
        var = 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 == None:
                var, constant, i18n_constant = match.group("var", "constant", "i18n_constant")
                if i18n_constant:
                    var = '"%s"' %  _(i18n_constant.replace(r'\"', '"'))
                elif constant:
                    var = '"%s"' % constant.replace(r'\"', '"')
                upto = match.end()
                if var == None:
                    raise TemplateSyntaxError("Could not find variable at start of %s" % token)
                elif var.find(VARIABLE_ATTRIBUTE_SEPARATOR + '_') > -1 or var[0] == '_':
                    raise TemplateSyntaxError("Variables and attributes may not begin with underscores: '%s'" % var)
            else:
                filter_name = match.group("filter_name")
                args = []
                constant_arg, i18n_arg, var_arg = match.group("constant_arg", "i18n_arg", "var_arg")
                if i18n_arg:
                    args.append((False, _(i18n_arg.replace(r'\"', '"'))))
                elif constant_arg is not None:
                    args.append((False, constant_arg.replace(r'\"', '"')))
                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 = Variable(var)

    def resolve(self, context, ignore_failures=False):
        try:
            obj = self.var.resolve(context)
        except VariableDoesNotExist:
            if ignore_failures:
                obj = None
            else:
                if settings.TEMPLATE_STRING_IF_INVALID:
                    global invalid_var_format_string
                    if invalid_var_format_string is None:
                        invalid_var_format_string = '%s' in settings.TEMPLATE_STRING_IF_INVALID
                    if invalid_var_format_string:
                        return settings.TEMPLATE_STRING_IF_INVALID % self.var
                    return settings.TEMPLATE_STRING_IF_INVALID
                else:
                    obj = settings.TEMPLATE_STRING_IF_INVALID
        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, '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)
            elif isinstance(obj, EscapeData):
                obj = mark_for_escaping(new_obj)
            else:
                obj = new_obj
        return obj

    def args_check(name, func, provided):
        provided = list(provided)
        plen = len(provided)
        # Check to see if a decorator is providing the real function.
        func = getattr(func, '_decorated_function', func)
        args, varargs, varkw, defaults = getargspec(func)
        # First argument is filter input.
        args.pop(0)
        if defaults:
            nondefs = args[:-len(defaults)]
        else:
            nondefs = args
        # Args without defaults must be provided.
        try:
            for arg in nondefs:
                provided.pop(0)
        except IndexError:
            # Not enough
            raise TemplateSyntaxError("%s requires %d arguments, %d provided" % (name, len(nondefs), plen))

        # Defaults can be overridden.
        defaults = defaults and list(defaults) or []
        try:
            for parg in provided:
                defaults.pop(0)
        except IndexError:
            # Too many.
            raise TemplateSyntaxError("%s requires %d arguments, %d provided" % (name, len(nondefs), plen))

        return True
    args_check = staticmethod(args_check)

    def __str__(self):
        return self.token

def resolve_variable(path, context):
    """
    Returns the resolved variable, which may contain attribute syntax, within
    the given context.

    Deprecated; use the Variable class instead.
    """
    return Variable(path).resolve(context)

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

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

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

        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.
            self.literal = float(var)

            # So it's a float... is it an int? If the original value contained a
            # dot or an "e" then it was a float, not an int.
            if '.' not in var and 'e' not in var.lower():
                self.literal = int(self.literal)

            # "2." is invalid
            if var.endswith('.'):
                raise ValueError

        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.
            if var[0] in "\"'" and var[0] == var[-1]:
                self.literal = mark_safe(var[1:-1])
            else:
                # Otherwise we'll set self.lookups so that resolve() knows we're
                # dealing with a bonafide variable
                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:
            return _(value)
        return value

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

    def __str__(self):
        return self.var

    def _resolve_lookup(self, context):
        """
        Performs 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
        for bit in self.lookups:
            try: # dictionary lookup
                current = current[bit]
            except (TypeError, AttributeError, KeyError):
                try: # attribute lookup
                    current = getattr(current, bit)
                    if callable(current):
                        if getattr(current, 'alters_data', False):
                            current = settings.TEMPLATE_STRING_IF_INVALID
                        else:
                            try: # method call (assuming no args required)
                                current = current()
                            except TypeError: # arguments *were* required
                                # GOTCHA: This will also catch any TypeError
                                # raised in the function itself.
                                current = settings.TEMPLATE_STRING_IF_INVALID # invalid method call
                            except Exception, e:
                                if getattr(e, 'silent_variable_failure', False):
                                    current = settings.TEMPLATE_STRING_IF_INVALID
                                else:
                                    raise
                except (TypeError, AttributeError):
                    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
                except Exception, e:
                    if getattr(e, 'silent_variable_failure', False):
                        current = settings.TEMPLATE_STRING_IF_INVALID
                    else:
                        raise

        return current

class Node(object):
    # 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

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

    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)
        if hasattr(self, 'nodelist'):
            nodes.extend(self.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):
                bits.append(self.render_node(node, context))
            else:
                bits.append(node)
        return mark_safe(''.join([force_unicode(b) for b in 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

    def render_node(self, node, context):
        return node.render(context)

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

    def __repr__(self):
        return "<Text Node: '%s'>" % self.s[:25]

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

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 = force_unicode(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 ''
        if (context.autoescape and not isinstance(output, SafeData)) or isinstance(output, EscapeData):
            return force_unicode(escape(output))
        else:
            return force_unicode(output)

def generic_tag_compiler(params, defaults, name, node_class, parser, token):
    "Returns a template.Node subclass."
    bits = token.split_contents()[1:]
    bmax = len(params)
    def_len = defaults and len(defaults) or 0
    bmin = bmax - def_len
    if(len(bits) < bmin or len(bits) > bmax):
        if bmin == bmax:
            message = "%s takes %s arguments" % (name, bmin)
        else:
            message = "%s takes between %s and %s arguments" % (name, bmin, bmax)
        raise TemplateSyntaxError(message)
    return node_class(bits)

class Library(object):
    def __init__(self):
        self.filters = {}
        self.tags = {}

    def tag(self, name=None, compile_function=None):
        if name == None and compile_function == None:
            # @register.tag()
            return self.tag_function
        elif name != None and compile_function == None:
            if(callable(name)):
                # @register.tag
                return self.tag_function(name)
            else:
                # @register.tag('somename') or @register.tag(name='somename')
                def dec(func):
                    return self.tag(name, func)
                return dec
        elif name != None and compile_function != None:
            # register.tag('somename', somefunc)
            self.tags[name] = compile_function
            return compile_function
        else:
            raise InvalidTemplateLibrary("Unsupported arguments to Library.tag: (%r, %r)", (name, compile_function))

    def tag_function(self,func):
        self.tags[getattr(func, "_decorated_function", func).__name__] = func
        return func

    def filter(self, name=None, filter_func=None):
        if name == None and filter_func == None:
            # @register.filter()
            return self.filter_function
        elif filter_func == None:
            if(callable(name)):
                # @register.filter
                return self.filter_function(name)
            else:
                # @register.filter('somename') or @register.filter(name='somename')
                def dec(func):
                    return self.filter(name, func)
                return dec
        elif name != None and filter_func != None:
            # register.filter('somename', somefunc)
            self.filters[name] = filter_func
            return filter_func
        else:
            raise InvalidTemplateLibrary("Unsupported arguments to Library.filter: (%r, %r)", (name, filter_func))

    def filter_function(self, func):
        self.filters[getattr(func, "_decorated_function", func).__name__] = func
        return func

    def simple_tag(self,func):
        params, xx, xxx, defaults = getargspec(func)

        class SimpleNode(Node):
            def __init__(self, vars_to_resolve):
                self.vars_to_resolve = map(Variable, vars_to_resolve)

            def render(self, context):
                resolved_vars = [var.resolve(context) for var in self.vars_to_resolve]
                return func(*resolved_vars)

        compile_func = curry(generic_tag_compiler, params, defaults, getattr(func, "_decorated_function", func).__name__, SimpleNode)
        compile_func.__doc__ = func.__doc__
        self.tag(getattr(func, "_decorated_function", func).__name__, compile_func)
        return func

    def inclusion_tag(self, file_name, context_class=Context, takes_context=False):
        def dec(func):
            params, xx, xxx, defaults = getargspec(func)
            if takes_context:
                if params[0] == 'context':
                    params = params[1:]
                else:
                    raise TemplateSyntaxError("Any tag function decorated with takes_context=True must have a first argument of 'context'")

            class InclusionNode(Node):
                def __init__(self, vars_to_resolve):
                    self.vars_to_resolve = map(Variable, vars_to_resolve)

                def render(self, context):
                    resolved_vars = [var.resolve(context) for var in self.vars_to_resolve]
                    if takes_context:
                        args = [context] + resolved_vars
                    else:
                        args = resolved_vars

                    dict = func(*args)

                    if not getattr(self, 'nodelist', False):
                        from django.template.loader import get_template, select_template
                        if not isinstance(file_name, basestring) and is_iterable(file_name):
                            t = select_template(file_name)
                        else:
                            t = get_template(file_name)
                        self.nodelist = t.nodelist
                    return self.nodelist.render(context_class(dict,
                            autoescape=context.autoescape))

            compile_func = curry(generic_tag_compiler, params, defaults, getattr(func, "_decorated_function", func).__name__, InclusionNode)
            compile_func.__doc__ = func.__doc__
            self.tag(getattr(func, "_decorated_function", func).__name__, compile_func)
            return func
        return dec

def get_library(module_name):
    lib = libraries.get(module_name, None)
    if not lib:
        try:
            mod = __import__(module_name, {}, {}, [''])
        except ImportError, e:
            raise InvalidTemplateLibrary("Could not load template library from %s, %s" % (module_name, e))
        try:
            lib = mod.register
            libraries[module_name] = lib
        except AttributeError:
            raise InvalidTemplateLibrary("Template library %s does not have a variable named 'register'" % module_name)
    return lib

def add_to_builtins(module_name):
    builtins.append(get_library(module_name))

add_to_builtins('django.template.defaulttags')
add_to_builtins('django.template.defaultfilters')
