template tag 'add' requires 2 arguments, 1 provided

Question

I have used add before but getting this strange error this time:

django.template.exceptions.TemplateSyntaxError: add requires 2 arguments, 1 provided

with the following code:

{% render_field form.full_name type="hidden" class="form-control" \
   value=user.first_name|add:"something" %}

However I don't get the error if I move add to with:

{% with fullname=user.first_name|add:"something"%}
   {% render_field form.full_name type="hidden" class="form-control" \
      value=fullname %}
{% endwith %}

Answer 1

So it looks like you are using the render_field template tag from https://github.com/jazzband/django-widget-tweaks/

The render_field is implemented as an Advanced Custom Tag that is implemented by the package maintainer, that sadly does not support filters in it's argument list.

A simple_tag or inclusion_tag on the other hand; which is defined by Django; does support other filters within its argument list.

What I mean to say is that this will work if it were a simple_tag or inclusion_tag {% my_tag 123 "abcd" book.title warning=message|lower profile=user.profile %}

However, if my_tag was registered as an advanced custom tag; supporting filters within the arguments is up to the implementation.

As I see from the implementation of the render_field code; it is an advanced custom tag and filters support appears to be broken:

@register.tag
def render_field(parser, token):
    """
    Render a form field using given attribute-value pairs

    Takes form field as first argument and list of attribute-value pairs for
    all other arguments.  Attribute-value pairs should be in the form of
    attribute=value or attribute="a value" for assignment and attribute+=value
    or attribute+="value" for appending.
    """
    error_msg = '%r tag requires a form field followed by a list of attributes and values in the form attr="value"' % token.split_contents()[0]
    try:
        bits = token.split_contents()
        tag_name = bits[0]
        form_field = bits[1]
        attr_list = bits[2:]
    except ValueError:
        raise TemplateSyntaxError(error_msg)

    form_field = parser.compile_filter(form_field)

    set_attrs = []
    append_attrs = []
    for pair in attr_list:
        match = ATTRIBUTE_RE.match(pair)
        if not match:
            raise TemplateSyntaxError(error_msg + ": %s" % pair)
        dct = match.groupdict()
        attr, sign, value = \
            dct['attr'], dct['sign'], parser.compile_filter(dct['value'])
        if sign == "=":
            set_attrs.append((attr, value))
        else:
            append_attrs.append((attr, value))

    return FieldAttributeNode(form_field, set_attrs, append_attrs)

Specifically; the pairs of key=value are stored in attr_list in the code and run against the ATTRIBUTE_RE regex matching

Then the filter is run using parser.compile_filter(dct['value']), however dct['value'] is 'user.first_name|add:"' -> the text "something" is lost here. My guess is that the ATTRIBUTE_RE regex needs to be improved to support this.

>>> ATTRIBUTE_RE.match('value=user.first_name|add:"something"')
<_sre.SRE_Match object at 0x7f8617f5a160>
>>> match.groupdict()
{'attr': 'value', 'value': 'user.first_name|add:"', 'sign': '='}

As you can see, the value key is broken and missing the full text you supplied; which breaks the add filter downstream.

This is exactly the use-case that the with tag is meant to solve; as it abstracts the filtering process to a level above; and you can pass the new variable to your custom render_field tag nicely.

References:

How to use a template filter on a custom template tag?

https://docs.djangoproject.com/en/2.0/howto/custom-template-tags/#advanced-custom-template-tags