django check existence of template context variable

Question

I am writing a django template and I want to differentiate between the existence of a context variable vs it being None, empty etc. I've done my homework and it seems surprisingly hard. Specifically, this is what I'm trying to do

view 1:
...
if some_condition = True:
    context['letters'] = ['a', 'b', 'c'] # The list might also be empty or None in some cases
else
    context['numbers'] = [1, 2, 3] #This list might be empty or None in some cases

Template
...
<ul>
{% if letters %}
    {% for x in letter %}
        <li>{{x}}</li>
    {%endfor%}
{% else %}
    {%for x in numbers%}
        <li>{{x}}</li>
    {%endfor%}
</ul>

Using the {% if %} is dicey because it is fails if letters doesnt exist or the list is empty. I want to use letters even if it is empty (but defined in the context)

I have the same problem with the built-in filters default and default_if_none How can I differentiate the existence of a context variable from it being other things like None or Empty

Answer 1

I recently faced the same conundrum, and after looking into the way the {% if %} tag is structured here's what I came up with:

from django.template.base import VariableDoesNotExist
from django.template.defaulttags import IfNode
from django.template.smartif import IfParser, Literal

# Used as a value for ifdef and ifndef tags
undefined = object()

class IfDefLiteral(Literal):
    def eval(self, context):
        if not self.value in context:
            # Can't raise an exception here because Operator catches it
            return undefined

class IfDefParser(IfParser):
    def create_var(self, value):
        return IfDefLiteral(value)

class IfDefNode(IfNode):
    def __init__(self, defined=True, *args, **kwargs):
        self.defined = defined
        super(IfDefNode, self).__init__(*args, **kwargs)

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

    def render(self, context):
        for condition, nodelist in self.conditions_nodelists:

            match = undefined
            if condition is not None:           # if / elif clause
                try:
                    match = condition.eval(context)
                except VariableDoesNotExist:
                    pass

            if condition is None or (  # else clause, always render
                (self.defined and match is not undefined) or
                (match is undefined and not self.defined)):
                return nodelist.render(context)

        return ''

def _gen_ifdef(parser, token, block_tokens, defined):
    # {% if ... %}
    bits = token.split_contents()[1:]
    condition = IfDefParser(bits).parse()
    nodelist = parser.parse(block_tokens)
    conditions_nodelists = [(condition, nodelist)]
    token = parser.next_token()

    # {% elif ... %} (repeatable)
    while token.contents.startswith(block_tokens[0]):
        bits = token.split_contents()[1:]
        condition = IfDefParser(bits).parse()
        nodelist = parser.parse(block_tokens)
        conditions_nodelists.append((condition, nodelist))
        token = parser.next_token()

    # {% else %} (optional)
    if token.contents == 'else':
        nodelist = parser.parse(block_tokens[-1:])
        conditions_nodelists.append((None, nodelist))
        token = parser.next_token()

    # {% endif %}
    assert token.contents == block_tokens[-1]

    return IfDefNode(defined, conditions_nodelists)

@register.tag
def ifdef(parser, token):
    """Check if variable is defined in the context

    Unlike the {% if %} tag, this renders the block if the variable(s)
    exist within the context, not only if they are truthy. That is, variables
    with None, 0 or [] values would also render the block.
    """
    return _gen_ifdef(parser, token, ('elifdef', 'else', 'endifdef'), True)

@register.tag
def ifndef(parser, token):
    """Check if variable is *not* defined in the context

    This is the opposite of {% ifdef %}.
    """
    return _gen_ifdef(parser, token, ('elifndef', 'else', 'endifndef'), False)

Then you would use it like an {% if %} tag in your template:

{% ifdef letters or numbers %}
    {# do something with letters or numbers #}
{% else %}
    {# variables are not defined here #}
{% endifdef %}

I'm not sure if there's a simpler way to accomplish this, and while I'm not too happy with the approach, it seems to work well in my use case. Hope this helps!

Answer 2

I'm not sure if this kind of logic should be used in templates, they supposed to be simple. The way I would solve it is simply add (inside view, before "if"):

context['letters'] = False    #([]/None/False)
if some_condition = True:
    ...

now if some_condition = False, than "for" loop won't run in template, so you don't need "if" there anymore.