How to concatenate strings in django templates?

Don't use add for strings, you should define a custom tag like this :

Create a file : <appname>\templatetags\<appname>_extras.py

from django import template

register = template.Library()

@register.filter
def addstr(arg1, arg2):
    """concatenate arg1 & arg2"""
    return str(arg1) + str(arg2)

and then use it as @Steven says

{% load <appname>_extras %}

{% with "shop/"|addstr:shop_name|addstr:"/base.html" as template %}
    {% include template %}
{% endwith %}

Reason for avoiding add:

According to the docs

This filter will first try to coerce both values to integers... Strings that can be coerced to integers will be summed, not concatenated...

If both variables happen to be integers, the result would be unexpected.

Use with:

{% with "shop/"|add:shop_name|add:"/base.html" as template %}
{% include template %}
{% endwith %}