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Continuing and breaking from loops in Django templates

Author:
gsakkis
Posted:
July 4, 2010
Language:
Python
Version:
1.2
Score:
0 (after 4 ratings)

This snippet makes Django templates support break and continue in loops. It is actually more powerful than the respective Python statements as it allows breaking and continuing from an outer loop, not just the innermost.

break and continue are implemented as template filters, with the input value being the loop variable. For example, to break from the current for loop use forloop|break, and to continue from the next outer loop use forloop.parentloop|continue.

The implementation monkeypatches Django (specifically Nodelist and ForNode) and has been tested on v1.2 with Python 2.6.

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from django import template

register = template.Library()


@register.filter('break')
def break_(loop):
    '''Breaks from a loop.

    The 'break' filter is used within a loop and takes as input a loop variable,
    e.g. 'forloop' in case of a for loop. For example, to display the items
    from list ``items`` up to the first item that is equal to ``end``::

        <ul>
        {% for item in items %}
            {% if item == 'end' %}
                {{ forloop|break }}
            {% endif %}
            <li>{{ item }}</li>
        {% endfor %}
        </ul>

    Breaking from nested loops is also supported by passing the appropriate loop
    variable, e.g. ``forloop.parentloop|break``.
    '''
    raise StopLoopException(loop, False)


@register.filter('continue')
def continue_(loop):
    '''Continues a loop by jumping to its beginning.

    The 'continue' filter is used within a loop and takes as input a loop
    variable, e.g. 'forloop' in case of a for loop. It can also be used (and is
    mostly useful) for nested loops by passing the appropriate loop variable,
    e.g. ``forloop.parentloop|continue``. For example::

        {% for key,values in mapping.iteritems %}<br/>
            {% for value in values %}
                {{ key }}: {{ value }}<br/>
                {% if value|divisibleby:3  %}
                    {{ value }} is divisible by 3<br/>
                    {{ forloop.parentloop|continue }}
                {% endif %}
            {% endfor %}
            {{ key }}: No value divisible by 3<br/>
        {% endfor %}
    '''
    raise StopLoopException(loop, True)


# monkeypatch NodeList to handle break/continue
def render(self, context):
    return template.mark_safe(''.join(map(template.force_unicode,
                                          _render_nodelist_items(self,context))))
template.NodeList.render = render


# monkeypatch ForNode to handle break/continue
def render(self, context):
    try:
        values = self.sequence.resolve(context, True)
    except template.VariableDoesNotExist:
        values = []
    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)
    if self.is_reversed:
        values = reversed(values)
    unpack = len(self.loopvars) > 1
    # push a forloop value onto the context
    loop = BoundedLoop('forloop', context, self.nodelist_loop, len_values)
    for value in values:
        if unpack:
            # if there are multiple loop variables, unpack the value into them
            context.update(dict(zip(self.loopvars, value)))
        else:
            context[self.loopvars[0]] = value
        status = loop.next()
        if unpack and status is loop.PASS:
            # The loop variables were pushed on to the context so pop them
            # off again. This is necessary because the tag lets the length
            # of loopvars differ to the length of each set of items and we
            # don't want to leave any vars from the previous loop on the
            # context. If status is not PASS, all the additional dicts,
            # including the one with the loop variables, have already been
            # popped off in loop.next() so we don't have to pop it here
            context.pop()
        if status is loop.BREAK:
            break
    return loop.render(close=True)
template.defaulttags.ForNode.render = render


class StopLoopException(Exception):
    def __init__(self, loop, continue_, nodelist=None):
        if not isinstance(loop, Loop):
            raise TypeError('Loop instance expected, %s given' % loop.__class__.__name__)
        super(StopLoopException, self).__init__(loop, continue_, nodelist)
        self.loop, self.continue_, self.nodelist = self.args


class Loop(dict):
    '''Base class of loop variables passed in the context (e.g. 'forloop').

    A loop instance holds and keeps up to date the attributes exposed in the
    context. This class exposes ``counter``, ``counter0``, ``first`` and
    ``parentloop``; its :class:`BoundedLoop` subclass adds ``revcounter``,
    ``revcounter0`` and ``last``.

    Additionally, a loop instance renders the items of the nodelist that comprise
    the loop and accumulates the rendered strings on every call to :meth:`next`.
    :meth:`next` also handles continuing or breaking from the loop and informs
    the caller accordingly.
    '''

    PASS = object()
    BREAK = object()
    CONTINUE = object()

    def __init__(self, name, context, nodelist):
        self._name = name
        self._context = context
        self._nodelist = nodelist
        self._rendered_nodelist = template.NodeList()
        self['parentloop'] = context.get(name)
        context.push()
        context[name] = self

    def render(self, close=False):
        '''Renders the accumulated nodelist for this loop.

        As a convenience, if ``close`` is true, the loop is also :meth:`close`d.
        '''
        if close:
            self.close()
        return self._rendered_nodelist.render(self._context)
    render.alters_data = True

    def next(self):
        '''Updates this loop for one iteration step.

        :returns: The status of the loop after this step: :attr:`CONTINUE` if a
            ``continue`` targeting this loop was encountered, :attr:`BREAK` for
            a break, or :attr:`PASS` otherwise.
        :raises StopLoopException: If a ``break`` or ``continue`` for a loop
            other than this one (presumably an ancestor) was encountered.
        '''
        if self._nodelist is None:
            raise RuntimeError('This loop is inactive')
        try: # update the exposed attributes
            counter = self['counter']
            self.update(counter0=counter, counter=counter+1, first=False)
        except KeyError:
            # initialize the exposed attributes the first time this is called
            self.update(counter0=0, counter=1, first=True)
        try:
            _render_nodelist_items(self._nodelist, self._context, self._rendered_nodelist)
            status = self.PASS
        except StopLoopException, ex:
            # if this is not the target loop, keep bubbling up the exception
            if ex.loop is not self:
                raise
            # pop context until (but excluding) the dict that contains this loop
            self._pop_context_until_self(inclusive=False)
            status = ex.continue_ and self.CONTINUE or self.BREAK
        return status
    next.alters_data = True

    def close(self):
        '''Mark this loop as closed.

        After a loop is closed, subsequent calls to :meth:`next` are not allowed.
        This should be called when the loop is "done" to remove any loop-specific
        context entries.
        '''
        if self._nodelist:
            self._pop_context_until_self(inclusive=True)
            self._nodelist = None
    close.alters_data = True

    def _pop_context_until_self(self, inclusive):
        name = self._name
        dicts = self._context.dicts
        while len(dicts) > 1:
            if dicts[-1].get(name) is self:
                if inclusive:
                    del dicts[-1]
                break
            del dicts[-1]


class BoundedLoop(Loop):
    '''A :class:`Loop` of known length.

    ``BoundedLoop`` instances expose ``revcounter``, ``revcounter0`` and ``last``,
    in addition to the attributes exposed by ``Loop`` itself.
    '''

    def __init__(self, name, context, nodelist, length):
        if length < 1:
            raise ValueError('Length must be at least 1')
        self._length = length
        super(BoundedLoop, self).__init__(name, context, nodelist)

    def next(self):
        try: # update the exposed attributes
            revcounter0 = self['revcounter0']
            if revcounter0 <= 0:
                raise RuntimeError('Attempted to call `next()` more than %d times' % self._length)
            self.update(revcounter0=revcounter0-1, revcounter=revcounter0, last=revcounter0==1)
        except KeyError:
            # initialize the exposed attributes the first time this is called
            length = self._length
            self.update(revcounter0=length-1, revcounter=length, last=length==1)
        return super(BoundedLoop, self).next()
    next.alters_data = True


def _render_nodelist_items(nodelist, context, result=None):
    if result is None:
        result = []
    for node in nodelist:
        if not isinstance(node, template.Node):
            result.append(node)
        else:
            try:
                result.append(nodelist.render_node(node, context))
            except Exception, ex:
                # get the wrapped exception if settings.DEBUG is True
                if hasattr(ex, 'exc_info'):
                    ex = ex.exc_info[1]
                # let every exception other than StopLoopException propagate
                if not isinstance(ex, StopLoopException):
                    raise
                # reraise the StopLoopException with the updated nodelist
                if ex.nodelist:
                    result.extend(ex.nodelist)
                ex.nodelist = result
                raise ex
    return result

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Comments

namwodahs (on June 23, 2011):

Thanks for posting this! Very useful.

To get it working with Django 1.3, I had to change your render method to:

# monkeypatch NodeList to handle break/continue
def render(self, context):
    from django.utils.safestring import mark_safe
    from django.utils.encoding import force_unicode

    return mark_safe(''.join(map(force_unicode, _render_nodelist_items(self,context))))
template.NodeList.render = render

#

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