Extending django-classy-tags

You can extend django-classy-tags by writing your own subclasses of classytags.arguments.Argument which behave to your needs. If that does not cover your needs, you may also subclass classytags.core.Options and set a custom argument parser, which should subclass classytags.parser.Parser.

Creating a custom argument class

The most important method in this class for customization is classytags.arguments.Argument.parse(), so let’s have a closer look at it. It takes exactly four arguments, which are as follows:

  • parser: An instance of django.template.Parser.
  • token: The current token as a string.
  • tagname: The name of the tag being handled.
  • kwargs: The dictionary of already parsed arguments.

The parse method must return a boolean value:

  • If your method returns True, it means it has successfully handled the provided token. Your method has to add content to kwargs itself. The parser does not do that! When you return True, the next token will also try to get parsed by this argument’s parse method.
  • If your method returns False, it means it has not handled this token and the next argument class in the stack should be used to handle this token. Usually you would return False when your argument’s name is already in kwargs. Obviously this only applies to single-value arguments.

So let’s look at the standard classytags.arguments.Argument.parse():

def parse(self, parser, token, tagname, kwargs):
    """
    Parse a token.
    """
    if self.name in kwargs:
        return False
    else:
        kwargs[self.name] = self.parse_token(parser, token)
        return True

First it checks if the name is already in kwargs. If so, return False and let the next argument handle this token. Otherwise do some checking if we should resolve this token or not and add it to kwargs. Finally return True.

You might notice the classytags.arguments.Argument.parse_token() method used there. This method is responsible for turning an token into a template variable, a filter expression or any other object which allows to be resolved against a context. The one in classytags.arguments.Argument looks like this:

def parse_token(self, parser, token):
    if self.resolve:
        return parser.compile_filter(token)
    else:
        return TemplateConstant(token)

Cleaning arguments

If all you want to do is clean arguments or enforce a certain type, you can just change the classytags.arguments.Argument.value_class of your subclass of classytags.arguments.Argument to a subclass of classytags.values.StringValue which implements a clean method in which you can check the type and/or cast a type on the value. For further information on value classes, see classytags.values.

Custom argument parser

The argument parser was written with extensibility in mind. All important steps are split into individual methods which can be overwritten. For information about those methods, please refer to the reference about classytags.parser.Parser.

To use a custom parser, provide it as the parser_class keyword argument to classytags.core.Options.

Note

Each time your tag gets parsed, a new instance of the parser class gets created. This makes it safe to use self.

Example

Let’s make an argument which, when resolved, returns a template.

First we need a helper class which, after resolving loads the template specified by the value:

from django.template.loader import get_template

class TemplateResolver(object):
    def __init__(self, real):
        self.real = real

    def resolve(self, context):
        value = self.real.resolve(context)
        return get_template(value)

Now for the real argument:

from classytags.arguments import Argument

class TemplateArgument(Argument):
    def parse_token(self, parser, token):
        real = super(TemplateArgument, self).parse_token(parser, token)
        return TemplateResolver(real)