Django - A hack to apply custom templates to tags registered using inclusion_tag, this is related to https://code.djangoproject.com/ticket/9093

using

import sys
import ctypes
import types

from django.template import TemplateSyntaxError, Token, TOKEN_BLOCK
from django.template.defaulttags import register

if sys.version_info < (3,):
    cb_func_code = 'func_code'
    cb_func_closure = 'func_closure'
else:
    cb_func_code = '__code__'
    cb_func_closure = '__closure__'

@register.tag('using')
def do_using(parser, token):
    """
    Adds custom template for template tags defined with inclusion_tag

    For example::
        {% using "my_template.html" for some_tag %}
    """
    bits = token.split_contents()
    if len(bits) < 4:
        raise TemplateSyntaxError("using takes at least 4 arguments")
    if bits[2] != 'for':
        raise TemplateSyntaxError("Invalid syntax in using tag. Expecting 'for' keyword")

    template_name = str(bits[1]).strip('"').strip("'")
    tag_name = bits[3]
    tag_content = token.contents[token.contents.find(tag_name):]

    if tag_name not in parser.tags:
        raise TemplateSyntaxError("using received an invalid tag: %s" % tag_name)
    tag_compile = parser.tags[tag_name]

    def render_injector(self, context):
        """
        This method injects a custom file_name into the render function of the bound object
        """
        func = types.MethodType(self.__class__.render, self)
        func_closure = getattr(func, cb_func_closure)

        template_idx = getattr(func, cb_func_code).co_freevars.index('file_name')
        func_var = func_closure[template_idx]

        # Prepare for c injection
        c_target = ctypes.py_object(func_var)
        c_org_value = ctypes.py_object(func_var.cell_contents)
        c_new_value = ctypes.py_object(template_name)

        # Do a very dirty HACK! to override the file_name
        ctypes.pythonapi.PyCell_Set(c_target, c_new_value)
        try:
            return func(context)
        finally:
            ctypes.pythonapi.PyCell_Set(c_target, c_org_value)

    # Compile a tag node
    tag_node = tag_compile(parser, Token(TOKEN_BLOCK, tag_content))

    # Validate render method
    if not hasattr(tag_node.render, cb_func_closure) or \
            not hasattr(tag_node.render, cb_func_code) or \
            'file_name' not in getattr(tag_node.render, cb_func_code).co_freevars:
        raise RuntimeError('using is unable to use callback invalid closure/function specified.')

    # Set the render_injector as the render method
    tag_node.render = types.MethodType(render_injector, tag_node)

    return tag_node