defaulttags.py

"""Default tags used by the template system, available to all templates."""

import re
import sys
import warnings
from collections import namedtuple
from collections.abc import Iterable
from datetime import datetime
from itertools import cycle as itertools_cycle
from itertools import groupby

from django.conf import settings
from django.utils import timezone
from django.utils.html import conditional_escape, escape, format_html
from django.utils.lorem_ipsum import paragraphs, words
from django.utils.safestring import mark_safe

from .base import (
    BLOCK_TAG_END,
    BLOCK_TAG_START,
    COMMENT_TAG_END,
    COMMENT_TAG_START,
    FILTER_SEPARATOR,
    SINGLE_BRACE_END,
    SINGLE_BRACE_START,
    VARIABLE_ATTRIBUTE_SEPARATOR,
    VARIABLE_TAG_END,
    VARIABLE_TAG_START,
    Node,
    NodeList,
    TemplateSyntaxError,
    VariableDoesNotExist,
    kwarg_re,
    render_value_in_context,
    token_kwargs,
)
from .context import Context
from .defaultfilters import date
from .library import Library
from .smartif import IfParser, Literal

register = Library()


class AutoEscapeControlNode(Node):
    """Implement the actions of the autoescape tag."""

    def __init__(self, setting, nodelist):
        self.setting = setting
        self.nodelist = nodelist

    def render(self, context):
        old_setting = context.autoescape
        context.autoescape = self.setting
        output = self.nodelist.render(context)
        context.autoescape = old_setting
        if self.setting:
            return mark_safe(output)
        else:
            return output


class CommentNode(Node):
    child_nodelists = ()

    def render(self, context):
        return ""


class CsrfTokenNode(Node):
    child_nodelists = ()

    def render(self, context):
        csrf_token = context.get("csrf_token")
        if csrf_token:
            if csrf_token == "NOTPROVIDED":
                return format_html("")
            else:
                return format_html(
                    '<input type="hidden" name="csrfmiddlewaretoken" value="{}">',
                    csrf_token,
                )
        else:
            # It's very probable that the token is missing because of
            # misconfiguration, so we raise a warning
            if settings.DEBUG:
                warnings.warn(
                    "A {% csrf_token %} was used in a template, but the context "
                    "did not provide the value.  This is usually caused by not "
                    "using RequestContext."
                )
            return ""


class CycleNode(Node):
    def __init__(self, cyclevars, variable_name=None, silent=False):
        self.cyclevars = cyclevars
        self.variable_name = variable_name
        self.silent = silent

    def render(self, context):
        if self not in context.render_context:
            # First time the node is rendered in template
            context.render_context[self] = itertools_cycle(self.cyclevars)
        cycle_iter = context.render_context[self]
        value = next(cycle_iter).resolve(context)
        if self.variable_name:
            context.set_upward(self.variable_name, value)
        if self.silent:
            return ""
        return render_value_in_context(value, context)

    def reset(self, context):
        """
        Reset the cycle iteration back to the beginning.
        """
        context.render_context[self] = itertools_cycle(self.cyclevars)


class DebugNode(Node):
    def render(self, context):
        if not settings.DEBUG:
            return ""

        from pprint import pformat

        output = [escape(pformat(val)) for val in context]
        output.append("\n\n")
        output.append(escape(pformat(sys.modules)))
        return "".join(output)


class FilterNode(Node):
    def __init__(self, filter_expr, nodelist):
        self.filter_expr = filter_expr
        self.nodelist = nodelist

    def render(self, context):
        output = self.nodelist.render(context)
        # Apply filters.
        with context.push(var=output):
            return self.filter_expr.resolve(context)


class FirstOfNode(Node):
    def __init__(self, variables, asvar=None):
        self.vars = variables
        self.asvar = asvar

    def render(self, context):
        first = ""
        for var in self.vars:
            value = var.resolve(context, ignore_failures=True)
            if value:
                first = render_value_in_context(value, context)
                break
        if self.asvar:
            context[self.asvar] = first
            return ""
        return first


class ForNode(Node):
    child_nodelists = ("nodelist_loop", "nodelist_empty")

    def __init__(
        self, loopvars, sequence, is_reversed, nodelist_loop, nodelist_empty=None
    ):
        self.loopvars = loopvars
        self.sequence = sequence
        self.is_reversed = is_reversed
        self.nodelist_loop = nodelist_loop
        if nodelist_empty is None:
            self.nodelist_empty = NodeList()
        else:
            self.nodelist_empty = nodelist_empty

    def __repr__(self):
        reversed_text = " reversed" if self.is_reversed else ""
        return "<%s: for %s in %s, tail_len: %d%s>" % (
            self.__class__.__name__,
            ", ".join(self.loopvars),
            self.sequence,
            len(self.nodelist_loop),
            reversed_text,
        )

    def render(self, context):
        if "forloop" in context:
            parentloop = context["forloop"]
        else:
            parentloop = {}
        with context.push():
            values = self.sequence.resolve(context, ignore_failures=True)
            if values is None:
                values = []
            if not hasattr(values, "__len__"):
                values = list(values)
            len_values = len(values)
            if len_values < 1:
                return self.nodelist_empty.render(context)
            nodelist = []
            if self.is_reversed:
                values = reversed(values)
            num_loopvars = len(self.loopvars)
            unpack = num_loopvars > 1
            # Create a forloop value in the context.  We'll update counters on each
            # iteration just below.
            loop_dict = context["forloop"] = {"parentloop": parentloop}
            for i, item in enumerate(values):
                # Shortcuts for current loop iteration number.
                loop_dict["counter0"] = i
                loop_dict["counter"] = i + 1
                # Reverse counter iteration numbers.
                loop_dict["revcounter"] = len_values - i
                loop_dict["revcounter0"] = len_values - i - 1
                # Boolean values designating first and last times through loop.
                loop_dict["first"] = i == 0
                loop_dict["last"] = i == len_values - 1

                pop_context = False
                if unpack:
                    # If there are multiple loop variables, unpack the item into
                    # them.
                    try:
                        len_item = len(item)
                    except TypeError:  # not an iterable
                        len_item = 1
                    # Check loop variable count before unpacking
                    if num_loopvars != len_item:
                        raise ValueError(
                            "Need {} values to unpack in for loop; got {}. ".format(
                                num_loopvars, len_item
                            ),
                        )
                    unpacked_vars = dict(zip(self.loopvars, item))
                    pop_context = True
                    context.update(unpacked_vars)
                else:
                    context[self.loopvars[0]] = item

                for node in self.nodelist_loop:
                    nodelist.append(node.render_annotated(context))

                if pop_context:
                    # Pop the loop variables pushed on to the context to avoid
                    # the context ending up in an inconsistent state when other
                    # tags (e.g., include and with) push data to context.
                    context.pop()
        return mark_safe("".join(nodelist))


class IfChangedNode(Node):
    child_nodelists = ("nodelist_true", "nodelist_false")

    def __init__(self, nodelist_true, nodelist_false, *varlist):
        self.nodelist_true = nodelist_true
        self.nodelist_false = nodelist_false
        self._varlist = varlist

    def render(self, context):
        # Init state storage
        state_frame = self._get_context_stack_frame(context)
        state_frame.setdefault(self)

        nodelist_true_output = None
        if self._varlist:
            # Consider multiple parameters. This behaves like an OR evaluation
            # of the multiple variables.
            compare_to = [
                var.resolve(context, ignore_failures=True) for var in self._varlist
            ]
        else:
            # The "{% ifchanged %}" syntax (without any variables) compares
            # the rendered output.
            compare_to = nodelist_true_output = self.nodelist_true.render(context)

        if compare_to != state_frame[self]:
            state_frame[self] = compare_to
            # render true block if not already rendered
            return nodelist_true_output or self.nodelist_true.render(context)
        elif self.nodelist_false:
            return self.nodelist_false.render(context)
        return ""

    def _get_context_stack_frame(self, context):
        # The Context object behaves like a stack where each template tag can
        # create a new scope. Find the place where to store the state to detect
        # changes.
        if "forloop" in context:
            # Ifchanged is bound to the local for loop.
            # When there is a loop-in-loop, the state is bound to the inner loop,
            # so it resets when the outer loop continues.
            return context["forloop"]
        else:
            # Using ifchanged outside loops. Effectively this is a no-op
            # because the state is associated with 'self'.
            return context.render_context


class IfNode(Node):
    def __init__(self, conditions_nodelists):
        self.conditions_nodelists = conditions_nodelists

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

    def __iter__(self):
        for _, nodelist in self.conditions_nodelists:
            yield from nodelist

    @property
    def nodelist(self):
        return NodeList(self)

    def render(self, context):
        for condition, nodelist in self.conditions_nodelists:
            if condition is not None:  # if / elif clause
                try:
                    match = condition.eval(context)
                except VariableDoesNotExist:
                    match = None
            else:  # else clause
                match = True

            if match:
                return nodelist.render(context)

        return ""


class LoremNode(Node):
    def __init__(self, count, method, common):
        self.count = count
        self.method = method
        self.common = common

    def render(self, context):
        try:
            count = int(self.count.resolve(context))
        except (ValueError, TypeError):
            count = 1
        if self.method == "w":
            return words(count, common=self.common)
        else:
            paras = paragraphs(count, common=self.common)
        if self.method == "p":
            paras = ["<p>%s</p>" % p for p in paras]
        return "\n\n".join(paras)


GroupedResult = namedtuple("GroupedResult", ["grouper", "list"])


class RegroupNode(Node):
    def __init__(self, target, expression, var_name):
        self.target = target
        self.expression = expression
        self.var_name = var_name

    def resolve_expression(self, obj, context):
        # This method is called for each object in self.target. See regroup()
        # for the reason why we temporarily put the object in the context.
        context[self.var_name] = obj
        return self.expression.resolve(context, ignore_failures=True)

    def render(self, context):
        obj_list = self.target.resolve(context, ignore_failures=True)
        if obj_list is None:
            # target variable wasn't found in context; fail silently.
            context[self.var_name] = []
            return ""
        # List of dictionaries in the format:
        # {'grouper': 'key', 'list': [list of contents]}.
        context[self.var_name] = [
            GroupedResult(grouper=key, list=list(val))
            for key, val in groupby(
                obj_list, lambda obj: self.resolve_expression(obj, context)
            )
        ]
        return ""


class LoadNode(Node):
    child_nodelists = ()

    def render(self, context):
        return ""


class NowNode(Node):
    def __init__(self, format_string, asvar=None):
        self.format_string = format_string
        self.asvar = asvar

    def render(self, context):
        tzinfo = timezone.get_current_timezone() if settings.USE_TZ else None
        formatted = date(datetime.now(tz=tzinfo), self.format_string)

        if self.asvar:
            context[self.asvar] = formatted
            return ""
        else:
            return formatted


class ResetCycleNode(Node):
    def __init__(self, node):
        self.node = node

    def render(self, context):
        self.node.reset(context)
        return ""


class SpacelessNode(Node):
    def __init__(self, nodelist):
        self.nodelist = nodelist

    def render(self, context):
        from django.utils.html import strip_spaces_between_tags

        return strip_spaces_between_tags(self.nodelist.render(context).strip())


class TemplateTagNode(Node):
    mapping = {
        "openblock": BLOCK_TAG_START,
        "closeblock": BLOCK_TAG_END,
        "openvariable": VARIABLE_TAG_START,
        "closevariable": VARIABLE_TAG_END,
        "openbrace": SINGLE_BRACE_START,
        "closebrace": SINGLE_BRACE_END,
        "opencomment": COMMENT_TAG_START,
        "closecomment": COMMENT_TAG_END,
    }

    def __init__(self, tagtype):
        self.tagtype = tagtype

    def render(self, context):
        return self.mapping.get(self.tagtype, "")


class URLNode(Node):
    child_nodelists = ()

    def __init__(self, view_name, args, kwargs, asvar):
        self.view_name = view_name
        self.args = args
        self.kwargs = kwargs
        self.asvar = asvar

    def __repr__(self):
        return "<%s view_name='%s' args=%s kwargs=%s as=%s>" % (
            self.__class__.__qualname__,
            self.view_name,
            repr(self.args),
            repr(self.kwargs),
            repr(self.asvar),
        )

    def render(self, context):
        from django.urls import NoReverseMatch, reverse

        args = [arg.resolve(context) for arg in self.args]
        kwargs = {k: v.resolve(context) for k, v in self.kwargs.items()}
        view_name = self.view_name.resolve(context)
        try:
            current_app = context.request.current_app
        except AttributeError:
            try:
                current_app = context.request.resolver_match.namespace
            except AttributeError:
                current_app = None
        # Try to look up the URL. If it fails, raise NoReverseMatch unless the
        # {% url ... as var %} construct is used, in which case return nothing.
        url = ""
        try:
            url = reverse(view_name, args=args, kwargs=kwargs, current_app=current_app)
        except NoReverseMatch:
            if self.asvar is None:
                raise

        if self.asvar:
            context[self.asvar] = url
            return ""
        else:
            if context.autoescape:
                url = conditional_escape(url)
            return url


class VerbatimNode(Node):
    def __init__(self, content):
        self.content = content

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


class WidthRatioNode(Node):
    def __init__(self, val_expr, max_expr, max_width, asvar=None):
        self.val_expr = val_expr
        self.max_expr = max_expr
        self.max_width = max_width
        self.asvar = asvar

    def render(self, context):
        try:
            value = self.val_expr.resolve(context)
            max_value = self.max_expr.resolve(context)
            max_width = int(self.max_width.resolve(context))
        except VariableDoesNotExist:
            return ""
        except (ValueError, TypeError):
            raise TemplateSyntaxError("widthratio final argument must be a number")
        try:
            value = float(value)
            max_value = float(max_value)
            ratio = (value / max_value) * max_width
            result = str(round(ratio))
        except ZeroDivisionError:
            result = "0"
        except (ValueError, TypeError, OverflowError):
            result = ""

        if self.asvar:
            context[self.asvar] = result
            return ""
        else:
            return result


class WithNode(Node):
    def __init__(self, var, name, nodelist, extra_context=None):
        self.nodelist = nodelist
        # var and name are legacy attributes, being left in case they are used
        # by third-party subclasses of this Node.
        self.extra_context = extra_context or {}
        if name:
            self.extra_context[name] = var

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

    def render(self, context):
        values = {key: val.resolve(context) for key, val in self.extra_context.items()}
        with context.push(**values):
            return self.nodelist.render(context)


@register.tag
def autoescape(parser, token):
    """
    Force autoescape behavior for this block.
    """
    # token.split_contents() isn't useful here because this tag doesn't accept
    # variable as arguments.
    args = token.contents.split()
    if len(args) != 2:
        raise TemplateSyntaxError("'autoescape' tag requires exactly one argument.")
    arg = args[1]
    if arg not in ("on", "off"):
        raise TemplateSyntaxError("'autoescape' argument should be 'on' or 'off'")
    nodelist = parser.parse(("endautoescape",))
    parser.delete_first_token()
    return AutoEscapeControlNode((arg == "on"), nodelist)


@register.tag
def comment(parser, token):
    """
    Ignore everything between ``{% comment %}`` and ``{% endcomment %}``.
    """
    parser.skip_past("endcomment")
    return CommentNode()


@register.tag
def cycle(parser, token):
    """
    Cycle among the given strings each time this tag is encountered.

    Within a loop, cycles among the given strings each time through
    the loop::

        {% for o in some_list %}
            <tr class="{% cycle 'row1' 'row2' %}">
                ...
            </tr>
        {% endfor %}

    Outside of a loop, give the values a unique name the first time you call
    it, then use that name each successive time through::

            <tr class="{% cycle 'row1' 'row2' 'row3' as rowcolors %}">...</tr>
            <tr class="{% cycle rowcolors %}">...</tr>
            <tr class="{% cycle rowcolors %}">...</tr>

    You can use any number of values, separated by spaces. Commas can also
    be used to separate values; if a comma is used, the cycle values are
    interpreted as literal strings.

    The optional flag "silent" can be used to prevent the cycle declaration
    from returning any value::

        {% for o in some_list %}
            {% cycle 'row1' 'row2' as rowcolors silent %}
            <tr class="{{ rowcolors }}">{% include "subtemplate.html " %}</tr>
        {% endfor %}
    """
    # Note: This returns the exact same node on each {% cycle name %} call;
    # that is, the node object returned from {% cycle a b c as name %} and the
    # one returned from {% cycle name %} are the exact same object. This
    # shouldn't cause problems (heh), but if it does, now you know.
    #
    # Ugly hack warning: This stuffs the named template dict into parser so
    # that names are only unique within each template (as opposed to using
    # a global variable, which would make cycle names have to be unique across
    # *all* templates.
    #
    # It keeps the last node in the parser to be able to reset it with
    # {% resetcycle %}.

    args = token.split_contents()

    if len(args) < 2:
        raise TemplateSyntaxError("'cycle' tag requires at least two arguments")

    if len(args) == 2:
        # {% cycle foo %} case.
        name = args[1]
        if not hasattr(parser, "_named_cycle_nodes"):
            raise TemplateSyntaxError(
                "No named cycles in template. '%s' is not defined" % name
            )
        if name not in parser._named_cycle_nodes:
            raise TemplateSyntaxError("Named cycle '%s' does not exist" % name)
        return parser._named_cycle_nodes[name]

    as_form = False

    if len(args) > 4:
        # {% cycle ... as foo [silent] %} case.
        if args[-3] == "as":
            if args[-1] != "silent":
                raise TemplateSyntaxError(
                    "Only 'silent' flag is allowed after cycle's name, not '%s'."
                    % args[-1]
                )
            as_form = True
            silent = True
            args = args[:-1]
        elif args[-2] == "as":
            as_form = True
            silent = False

    if as_form:
        name = args[-1]
        values = [parser.compile_filter(arg) for arg in args[1:-2]]
        node = CycleNode(values, name, silent=silent)
        if not hasattr(parser, "_named_cycle_nodes"):
            parser._named_cycle_nodes = {}
        parser._named_cycle_nodes[name] = node
    else:
        values = [parser.compile_filter(arg) for arg in args[1:]]
        node = CycleNode(values)
    parser._last_cycle_node = node
    return node


@register.tag
def csrf_token(parser, token):
    return CsrfTokenNode()


@register.tag
def debug(parser, token):
    """
    Output a whole load of debugging information, including the current
    context and imported modules.

    Sample usage::

        <pre>
            {% debug %}
        </pre>
    """
    return DebugNode()


@register.tag("filter")
def do_filter(parser, token):
    """
    Filter the contents of the block through variable filters.

    Filters can also be piped through each other, and they can have
    arguments -- just like in variable syntax.

    Sample usage::

        {% filter force_escape|lower %}
            This text will be HTML-escaped, and will appear in lowercase.
        {% endfilter %}

    Note that the ``escape`` and ``safe`` filters are not acceptable arguments.
    Instead, use the ``autoescape`` tag to manage autoescaping for blocks of
    template code.
    """
    # token.split_contents() isn't useful here because this tag doesn't accept
    # variable as arguments.
    _, rest = token.contents.split(None, 1)
    filter_expr = parser.compile_filter("var|%s" % (rest))
    for func, unused in filter_expr.filters:
        filter_name = getattr(func, "_filter_name", None)
        if filter_name in ("escape", "safe"):
            raise TemplateSyntaxError(
                '"filter %s" is not permitted.  Use the "autoescape" tag instead.'
                % filter_name
            )
    nodelist = parser.parse(("endfilter",))
    parser.delete_first_token()
    return FilterNode(filter_expr, nodelist)


@register.tag
def firstof(parser, token):
    """
    Output the first variable passed that is not False.

    Output nothing if all the passed variables are False.

    Sample usage::

        {% firstof var1 var2 var3 as myvar %}

    This is equivalent to::

        {% if var1 %}
            {{ var1 }}
        {% elif var2 %}
            {{ var2 }}
        {% elif var3 %}
            {{ var3 }}
        {% endif %}

    but much cleaner!

    You can also use a literal string as a fallback value in case all
    passed variables are False::

        {% firstof var1 var2 var3 "fallback value" %}

    If you want to disable auto-escaping of variables you can use::

        {% autoescape off %}
            {% firstof var1 var2 var3 "<strong>fallback value</strong>" %}
        {% autoescape %}

    Or if only some variables should be escaped, you can use::

        {% firstof var1 var2|safe var3 "<strong>fallback value</strong>"|safe %}
    """
    bits = token.split_contents()[1:]
    asvar = None
    if not bits:
        raise TemplateSyntaxError("'firstof' statement requires at least one argument")

    if len(bits) >= 2 and bits[-2] == "as":
        asvar = bits[-1]
        bits = bits[:-2]
    return FirstOfNode([parser.compile_filter(bit) for bit in bits], asvar)


@register.tag("for")
def do_for(parser, token):
    """
    Loop over each item in an array.

    For example, to display a list of athletes given ``athlete_list``::

        <ul>
        {% for athlete in athlete_list %}
            <li>{{ athlete.name }}</li>
        {% endfor %}
        </ul>

    You can loop over a list in reverse by using
    ``{% for obj in list reversed %}``.

    You can also unpack multiple values from a two-dimensional array::

        {% for key,value in dict.items %}
            {{ key }}: {{ value }}
        {% endfor %}

    The ``for`` tag can take an optional ``{% empty %}`` clause that will
    be displayed if the given array is empty or could not be found::

        <ul>
          {% for athlete in athlete_list %}
            <li>{{ athlete.name }}</li>
          {% empty %}
            <li>Sorry, no athletes in this list.</li>
          {% endfor %}
        <ul>

    The above is equivalent to -- but shorter, cleaner, and possibly faster
    than -- the following::

        <ul>
          {% if athlete_list %}
            {% for athlete in athlete_list %}
              <li>{{ athlete.name }}</li>
            {% endfor %}
          {% else %}
            <li>Sorry, no athletes in this list.</li>
          {% endif %}
        </ul>

    The for loop sets a number of variables available within the loop:

        ==========================  ================================================
        Variable                    Description
        ==========================  ================================================
        ``forloop.counter``         The current iteration of the loop (1-indexed)
        ``forloop.counter0``        The current iteration of the loop (0-indexed)
        ``forloop.revcounter``      The number of iterations from the end of the
                                    loop (1-indexed)
        ``forloop.revcounter0``     The number of iterations from the end of the
                                    loop (0-indexed)
        ``forloop.first``           True if this is the first time through the loop
        ``forloop.last``            True if this is the last time through the loop
        ``forloop.parentloop``      For nested loops, this is the loop "above" the
                                    current one
        ==========================  ================================================
    """
    bits = token.split_contents()
    if len(bits) < 4:
        raise TemplateSyntaxError(
            "'for' statements should have at least four words: %s" % token.contents
        )

    is_reversed = bits[-1] == "reversed"
    in_index = -3 if is_reversed else -2
    if bits[in_index] != "in":
        raise TemplateSyntaxError(
            "'for' statements should use the format"
            " 'for x in y': %s" % token.contents
        )

    invalid_chars = frozenset((" ", '"', "'", FILTER_SEPARATOR))
    loopvars = re.split(r" *, *", " ".join(bits[1:in_index]))
    for var in loopvars:
        if not var or not invalid_chars.isdisjoint(var):
            raise TemplateSyntaxError(
                "'for' tag received an invalid argument: %s" % token.contents
            )

    sequence = parser.compile_filter(bits[in_index + 1])
    nodelist_loop = parser.parse(
        (
            "empty",
            "endfor",
        )
    )
    token = parser.next_token()
    if token.contents == "empty":
        nodelist_empty = parser.parse(("endfor",))
        parser.delete_first_token()
    else:
        nodelist_empty = None
    return ForNode(loopvars, sequence, is_reversed, nodelist_loop, nodelist_empty)


class TemplateLiteral(Literal):
    def __init__(self, value, text):
        self.value = value
        self.text = text  # for better error messages

    def display(self):
        return self.text

    def eval(self, context):
        return self.value.resolve(context, ignore_failures=True)


class TemplateIfParser(IfParser):
    error_class = TemplateSyntaxError

    def __init__(self, parser, *args, **kwargs):
        self.template_parser = parser
        super().__init__(*args, **kwargs)

    def create_var(self, value):
        return TemplateLiteral(self.template_parser.compile_filter(value), value)


@register.tag("if")
def do_if(parser, token):
    """
    Evaluate a variable, and if that variable is "true" (i.e., exists, is not
    empty, and is not a false boolean value), output the contents of the block:

    ::

        {% if athlete_list %}
            Number of athletes: {{ athlete_list|count }}
        {% elif athlete_in_locker_room_list %}
            Athletes should be out of the locker room soon!
        {% else %}
            No athletes.
        {% endif %}

    In the above, if ``athlete_list`` is not empty, the number of athletes will
    be displayed by the ``{{ athlete_list|count }}`` variable.

    The ``if`` tag may take one or several `` {% elif %}`` clauses, as well as
    an ``{% else %}`` clause that will be displayed if all previous conditions
    fail. These clauses are optional.

    ``if`` tags may use ``or``, ``and`` or ``not`` to test a number of
    variables or to negate a given variable::

        {% if not athlete_list %}
            There are no athletes.
        {% endif %}

        {% if athlete_list or coach_list %}
            There are some athletes or some coaches.
        {% endif %}

        {% if athlete_list and coach_list %}
            Both athletes and coaches are available.
        {% endif %}

        {% if not athlete_list or coach_list %}
            There are no athletes, or there are some coaches.
        {% endif %}

        {% if athlete_list and not coach_list %}
            There are some athletes and absolutely no coaches.
        {% endif %}

    Comparison operators are also available, and the use of filters is also
    allowed, for example::

        {% if articles|length >= 5 %}...{% endif %}

    Arguments and operators _must_ have a space between them, so
    ``{% if 1>2 %}`` is not a valid if tag.

    All supported operators are: ``or``, ``and``, ``in``, ``not in``
    ``==``, ``!=``, ``>``, ``>=``, ``<`` and ``<=``.

    Operator precedence follows Python.
    """
    # {% if ... %}
    bits = token.split_contents()[1:]
    condition = TemplateIfParser(parser, bits).parse()
    nodelist = parser.parse(("elif", "else", "endif"))
    conditions_nodelists = [(condition, nodelist)]
    token = parser.next_token()

    # {% elif ... %} (repeatable)
    while token.contents.startswith("elif"):
        bits = token.split_contents()[1:]
        condition = TemplateIfParser(parser, bits).parse()
        nodelist = parser.parse(("elif", "else", "endif"))
        conditions_nodelists.append((condition, nodelist))
        token = parser.next_token()

    # {% else %} (optional)
    if token.contents == "else":
        nodelist = parser.parse(("endif",))
        conditions_nodelists.append((None, nodelist))
        token = parser.next_token()

    # {% endif %}
    if token.contents != "endif":
        raise TemplateSyntaxError(
            'Malformed template tag at line {}: "{}"'.format(
                token.lineno, token.contents
            )
        )

    return IfNode(conditions_nodelists)


@register.tag
def ifchanged(parser, token):
    """
    Check if a value has changed from the last iteration of a loop.

    The ``{% ifchanged %}`` block tag is used within a loop. It has two
    possible uses.

    1. Check its own rendered contents against its previous state and only
       displays the content if it has changed. For example, this displays a
       list of days, only displaying the month if it changes::

            <h1>Archive for {{ year }}</h1>

            {% for date in days %}
                {% ifchanged %}<h3>{{ date|date:"F" }}</h3>{% endifchanged %}
                <a href="{{ date|date:"M/d"|lower }}/">{{ date|date:"j" }}</a>
            {% endfor %}

    2. If given one or more variables, check whether any variable has changed.
       For example, the following shows the date every time it changes, while
       showing the hour if either the hour or the date has changed::

            {% for date in days %}
                {% ifchanged date.date %} {{ date.date }} {% endifchanged %}
                {% ifchanged date.hour date.date %}
                    {{ date.hour }}
                {% endifchanged %}
            {% endfor %}
    """
    bits = token.split_contents()
    nodelist_true = parser.parse(("else", "endifchanged"))
    token = parser.next_token()
    if token.contents == "else":
        nodelist_false = parser.parse(("endifchanged",))
        parser.delete_first_token()
    else:
        nodelist_false = NodeList()
    values = [parser.compile_filter(bit) for bit in bits[1:]]
    return IfChangedNode(nodelist_true, nodelist_false, *values)


def find_library(parser, name):
    try:
        return parser.libraries[name]
    except KeyError:
        raise TemplateSyntaxError(
            "'%s' is not a registered tag library. Must be one of:\n%s"
            % (
                name,
                "\n".join(sorted(parser.libraries)),
            ),
        )


def load_from_library(library, label, names):
    """
    Return a subset of tags and filters from a library.
    """
    subset = Library()
    for name in names:
        found = False
        if name in library.tags:
            found = True
            subset.tags[name] = library.tags[name]
        if name in library.filters:
            found = True
            subset.filters[name] = library.filters[name]
        if found is False:
            raise TemplateSyntaxError(
                "'%s' is not a valid tag or filter in tag library '%s'"
                % (
                    name,
                    label,
                ),
            )
    return subset


@register.tag
def load(parser, token):
    """
    Load a custom template tag library into the parser.

    For example, to load the template tags in
    ``django/templatetags/news/photos.py``::

        {% load news.photos %}

    Can also be used to load an individual tag/filter from
    a library::

        {% load byline from news %}
    """
    # token.split_contents() isn't useful here because this tag doesn't accept
    # variable as arguments.
    bits = token.contents.split()
    if len(bits) >= 4 and bits[-2] == "from":
        # from syntax is used; load individual tags from the library
        name = bits[-1]
        lib = find_library(parser, name)
        subset = load_from_library(lib, name, bits[1:-2])
        parser.add_library(subset)
    else:
        # one or more libraries are specified; load and add them to the parser
        for name in bits[1:]:
            lib = find_library(parser, name)
            parser.add_library(lib)
    return LoadNode()


@register.tag
def lorem(parser, token):
    """
    Create random Latin text useful for providing test data in templates.

    Usage format::

        {% lorem [count] [method] [random] %}

    ``count`` is a number (or variable) containing the number of paragraphs or
    words to generate (default is 1).

    ``method`` is either ``w`` for words, ``p`` for HTML paragraphs, ``b`` for
    plain-text paragraph blocks (default is ``b``).

    ``random`` is the word ``random``, which if given, does not use the common
    paragraph (starting "Lorem ipsum dolor sit amet, consectetuer...").

    Examples:

    * ``{% lorem %}`` outputs the common "lorem ipsum" paragraph
    * ``{% lorem 3 p %}`` outputs the common "lorem ipsum" paragraph
      and two random paragraphs each wrapped in HTML ``<p>`` tags
    * ``{% lorem 2 w random %}`` outputs two random latin words
    """
    bits = list(token.split_contents())
    tagname = bits[0]
    # Random bit
    common = bits[-1] != "random"
    if not common:
        bits.pop()
    # Method bit
    if bits[-1] in ("w", "p", "b"):
        method = bits.pop()
    else:
        method = "b"
    # Count bit
    if len(bits) > 1:
        count = bits.pop()
    else:
        count = "1"
    count = parser.compile_filter(count)
    if len(bits) != 1:
        raise TemplateSyntaxError("Incorrect format for %r tag" % tagname)
    return LoremNode(count, method, common)


@register.tag
def now(parser, token):
    """
    Display the date, formatted according to the given string.

    Use the same format as PHP's ``date()`` function; see https://php.net/date
    for all the possible values.

    Sample usage::

        It is {% now "jS F Y H:i" %}
    """
    bits = token.split_contents()
    asvar = None
    if len(bits) == 4 and bits[-2] == "as":
        asvar = bits[-1]
        bits = bits[:-2]
    if len(bits) != 2:
        raise TemplateSyntaxError("'now' statement takes one argument")
    format_string = bits[1][1:-1]
    return NowNode(format_string, asvar)


@register.simple_tag(takes_context=True)
def query_string(context, query_dict=None, **kwargs):
    """
    Add, remove, and change parameters of a ``QueryDict`` and return the result
    as a query string. If the ``query_dict`` argument is not provided, default
    to ``request.GET``.

    For example::

        {% query_string foo=3 %}

    To remove a key::

        {% query_string foo=None %}

    To use with pagination::

        {% query_string page=page_obj.next_page_number %}

    A custom ``QueryDict`` can also be used::

        {% query_string my_query_dict foo=3 %}
    """
    if query_dict is None:
        query_dict = context.request.GET
    query_dict = query_dict.copy()
    for key, value in kwargs.items():
        if value is None:
            if key in query_dict:
                del query_dict[key]
        elif isinstance(value, Iterable) and not isinstance(value, str):
            query_dict.setlist(key, value)
        else:
            query_dict[key] = value
    if not query_dict:
        return ""
    query_string = query_dict.urlencode()
    return f"?{query_string}"


@register.tag
def regroup(parser, token):
    """
    Regroup a list of alike objects by a common attribute.

    This complex tag is best illustrated by use of an example: say that
    ``musicians`` is a list of ``Musician`` objects that have ``name`` and
    ``instrument`` attributes, and you'd like to display a list that
    looks like:

        * Guitar:
            * Django Reinhardt
            * Emily Remler
        * Piano:
            * Lovie Austin
            * Bud Powell
        * Trumpet:
            * Duke Ellington

    The following snippet of template code would accomplish this dubious task::

        {% regroup musicians by instrument as grouped %}
        <ul>
        {% for group in grouped %}
            <li>{{ group.grouper }}
            <ul>
                {% for musician in group.list %}
                <li>{{ musician.name }}</li>
                {% endfor %}
            </ul>
        {% endfor %}
        </ul>

    As you can see, ``{% regroup %}`` populates a variable with a list of
    objects with ``grouper`` and ``list`` attributes. ``grouper`` contains the
    item that was grouped by; ``list`` contains the list of objects that share
    that ``grouper``. In this case, ``grouper`` would be ``Guitar``, ``Piano``
    and ``Trumpet``, and ``list`` is the list of musicians who play this
    instrument.

    Note that ``{% regroup %}`` does not work when the list to be grouped is not
    sorted by the key you are grouping by! This means that if your list of
    musicians was not sorted by instrument, you'd need to make sure it is sorted
    before using it, i.e.::

        {% regroup musicians|dictsort:"instrument" by instrument as grouped %}
    """
    bits = token.split_contents()
    if len(bits) != 6:
        raise TemplateSyntaxError("'regroup' tag takes five arguments")
    target = parser.compile_filter(bits[1])
    if bits[2] != "by":
        raise TemplateSyntaxError("second argument to 'regroup' tag must be 'by'")
    if bits[4] != "as":
        raise TemplateSyntaxError("next-to-last argument to 'regroup' tag must be 'as'")
    var_name = bits[5]
    # RegroupNode will take each item in 'target', put it in the context under
    # 'var_name', evaluate 'var_name'.'expression' in the current context, and
    # group by the resulting value. After all items are processed, it will
    # save the final result in the context under 'var_name', thus clearing the
    # temporary values. This hack is necessary because the template engine
    # doesn't provide a context-aware equivalent of Python's getattr.
    expression = parser.compile_filter(
        var_name + VARIABLE_ATTRIBUTE_SEPARATOR + bits[3]
    )
    return RegroupNode(target, expression, var_name)


@register.tag
def resetcycle(parser, token):
    """
    Reset a cycle tag.

    If an argument is given, reset the last rendered cycle tag whose name
    matches the argument, else reset the last rendered cycle tag (named or
    unnamed).
    """
    args = token.split_contents()

    if len(args) > 2:
        raise TemplateSyntaxError("%r tag accepts at most one argument." % args[0])

    if len(args) == 2:
        name = args[1]
        try:
            return ResetCycleNode(parser._named_cycle_nodes[name])
        except (AttributeError, KeyError):
            raise TemplateSyntaxError("Named cycle '%s' does not exist." % name)
    try:
        return ResetCycleNode(parser._last_cycle_node)
    except AttributeError:
        raise TemplateSyntaxError("No cycles in template.")


@register.tag
def spaceless(parser, token):
    """
    Remove whitespace between HTML tags, including tab and newline characters.

    Example usage::

        {% spaceless %}
            <p>
                <a href="foo/">Foo</a>
            </p>
        {% endspaceless %}

    This example returns this HTML::

        <p><a href="foo/">Foo</a></p>

    Only space between *tags* is normalized -- not space between tags and text.
    In this example, the space around ``Hello`` isn't stripped::

        {% spaceless %}
            <strong>
                Hello
            </strong>
        {% endspaceless %}
    """
    nodelist = parser.parse(("endspaceless",))
    parser.delete_first_token()
    return SpacelessNode(nodelist)


@register.tag
def templatetag(parser, token):
    """
    Output one of the bits used to compose template tags.

    Since the template system has no concept of "escaping", to display one of
    the bits used in template tags, you must use the ``{% templatetag %}`` tag.

    The argument tells which template bit to output:

        ==================  =======
        Argument            Outputs
        ==================  =======
        ``openblock``       ``{%``
        ``closeblock``      ``%}``
        ``openvariable``    ``{{``
        ``closevariable``   ``}}``
        ``openbrace``       ``{``
        ``closebrace``      ``}``
        ``opencomment``     ``{#``
        ``closecomment``    ``#}``
        ==================  =======
    """
    # token.split_contents() isn't useful here because this tag doesn't accept
    # variable as arguments.
    bits = token.contents.split()
    if len(bits) != 2:
        raise TemplateSyntaxError("'templatetag' statement takes one argument")
    tag = bits[1]
    if tag not in TemplateTagNode.mapping:
        raise TemplateSyntaxError(
            "Invalid templatetag argument: '%s'."
            " Must be one of: %s" % (tag, list(TemplateTagNode.mapping))
        )
    return TemplateTagNode(tag)


@register.tag
def url(parser, token):
    r"""
    Return an absolute URL matching the given view with its parameters.

    This is a way to define links that aren't tied to a particular URL
    configuration::

        {% url "url_name" arg1 arg2 %}

        or

        {% url "url_name" name1=value1 name2=value2 %}

    The first argument is a URL pattern name. Other arguments are
    space-separated values that will be filled in place of positional and
    keyword arguments in the URL. Don't mix positional and keyword arguments.
    All arguments for the URL must be present.

    For example, if you have a view ``app_name.views.client_details`` taking
    the client's id and the corresponding line in a URLconf looks like this::

        path('client/<int:id>/', views.client_details, name='client-detail-view')

    and this app's URLconf is included into the project's URLconf under some
    path::

        path('clients/', include('app_name.urls'))

    then in a template you can create a link for a certain client like this::

        {% url "client-detail-view" client.id %}

    The URL will look like ``/clients/client/123/``.

    The first argument may also be the name of a template variable that will be
    evaluated to obtain the view name or the URL name, e.g.::

        {% with url_name="client-detail-view" %}
        {% url url_name client.id %}
        {% endwith %}
    """
    bits = token.split_contents()
    if len(bits) < 2:
        raise TemplateSyntaxError(
            "'%s' takes at least one argument, a URL pattern name." % bits[0]
        )
    viewname = parser.compile_filter(bits[1])
    args = []
    kwargs = {}
    asvar = None
    bits = bits[2:]
    if len(bits) >= 2 and bits[-2] == "as":
        asvar = bits[-1]
        bits = bits[:-2]

    for bit in bits:
        match = kwarg_re.match(bit)
        if not match:
            raise TemplateSyntaxError("Malformed arguments to url tag")
        name, value = match.groups()
        if name:
            kwargs[name] = parser.compile_filter(value)
        else:
            args.append(parser.compile_filter(value))

    return URLNode(viewname, args, kwargs, asvar)


@register.tag
def verbatim(parser, token):
    """
    Stop the template engine from rendering the contents of this block tag.

    Usage::

        {% verbatim %}
            {% don't process this %}
        {% endverbatim %}

    You can also designate a specific closing tag block (allowing the
    unrendered use of ``{% endverbatim %}``)::

        {% verbatim myblock %}
            ...
        {% endverbatim myblock %}
    """
    nodelist = parser.parse(("endverbatim",))
    parser.delete_first_token()
    return VerbatimNode(nodelist.render(Context()))


@register.tag
def widthratio(parser, token):
    """
    For creating bar charts and such. Calculate the ratio of a given value to a
    maximum value, and then apply that ratio to a constant.

    For example::

        <img src="bar.png" alt="Bar"
             height="10" width="{% widthratio this_value max_value max_width %}">

    If ``this_value`` is 175, ``max_value`` is 200, and ``max_width`` is 100,
    the image in the above example will be 88 pixels wide
    (because 175/200 = .875; .875 * 100 = 87.5 which is rounded up to 88).

    In some cases you might want to capture the result of widthratio in a
    variable. It can be useful for instance in a blocktranslate like this::

        {% widthratio this_value max_value max_width as width %}
        {% blocktranslate %}The width is: {{ width }}{% endblocktranslate %}
    """
    bits = token.split_contents()
    if len(bits) == 4:
        tag, this_value_expr, max_value_expr, max_width = bits
        asvar = None
    elif len(bits) == 6:
        tag, this_value_expr, max_value_expr, max_width, as_, asvar = bits
        if as_ != "as":
            raise TemplateSyntaxError(
                "Invalid syntax in widthratio tag. Expecting 'as' keyword"
            )
    else:
        raise TemplateSyntaxError("widthratio takes at least three arguments")

    return WidthRatioNode(
        parser.compile_filter(this_value_expr),
        parser.compile_filter(max_value_expr),
        parser.compile_filter(max_width),
        asvar=asvar,
    )


@register.tag("with")
def do_with(parser, token):
    """
    Add one or more values to the context (inside of this block) for caching
    and easy access.

    For example::

        {% with total=person.some_sql_method %}
            {{ total }} object{{ total|pluralize }}
        {% endwith %}

    Multiple values can be added to the context::

        {% with foo=1 bar=2 %}
            ...
        {% endwith %}

    The legacy format of ``{% with person.some_sql_method as total %}`` is
    still accepted.
    """
    bits = token.split_contents()
    remaining_bits = bits[1:]
    extra_context = token_kwargs(remaining_bits, parser, support_legacy=True)
    if not extra_context:
        raise TemplateSyntaxError(
            "%r expected at least one variable assignment" % bits[0]
        )
    if remaining_bits:
        raise TemplateSyntaxError(
            "%r received an invalid token: %r" % (bits[0], remaining_bits[0])
        )
    nodelist = parser.parse(("endwith",))
    parser.delete_first_token()
    return WithNode(None, None, nodelist, extra_context=extra_context)