How do I create a jinja2 extension?

You use CallBlock

which indicates that you want your extension to act like a block. For example.

{% mytest arg1 arg2 %}
stuff
in
here
{% endmytest %}

      

        
        
        
      

    

nodes.CallBlock

expects you to pass it a list of nodes representing the body (inner statements) for your extension. You are currently passing None

- hence your fault.

Once you have analyzed your arguments, you need to move on to analyzing the block body. Fortunately, this is easy. You can simply do:

body = parser.parse_statements(['name:endmytest'], drop_needle=True)  

      

        
        
        
      

    

and then return the new node. CallBlock

gets a method to be called (in this case _mytestfunc

) that provides the logic for your extension.

body = parser.parse_statements(['name:endmytest'], drop_needle=True)  
return nodes.CallBlock(self.call_method('_mytestfunc', args),[], [], body).set_lineno(lineno)

      

        
        
        
      

    

Alternatively, if you don't want your extension to be a block tag like

{% mytest arg1 arg2 %}

      

        
        
        
      

    

you shouldn't use nodes.CallBlock

, you should just use nodes.Call

instead, which doesn't take a body parameter. So simple:

return self.call_method('_mytestfunc', args)  

      

        
        
        
      

    

self.call_method

is just a handy wrapper function that creates a call to node.

I've spent a few days writing Jinja2 extensions and it's hard. There isn't much documentation out there (other than code). The coffin GitHub project has some example extensions here .