How to check if django template variable is defined?

I have a django template which is used from many views. The template has a block for messages used to notify user of anything that should take their attention. Whether a message is sent or not depends on the views. Some views may send a message variable to the template while others may not.

view_1:
    message = "This is an important message"
    render_to_response("my_template.html", 
                       {'message':message, 'foo':foo, 'bar':bar},
                       context_instance = RequestContext(request))

view_2:
    message = "This is an important message"
    render_to_response("my_template.html", 
                       {'foo':foo, 'bar':bar},
                       context_instance = RequestContext(request))

In the template, I check for the message variable and include the block as below:

base_template.html:
    ....
    {% block main_body %}
         {% block messages %}
         {% endblock %}
         {% block content %}
         {% endblock %}
    {% endblock %}
    ....

 my_template.html:
     {% extends base_template.html %}
     ....
     {% if message %}
          {% block messages %}
              <div class='imp_msg'>{{ message }} </div>
          {% endblock %}
     {% endif %}
     ...

Problem is that even if view_2 does not pass a message, the final html is rendered with <div class='imp_msg'></div> -- basically an empty div.

Since that CSS is designed to give a light_red background to messages, what I see is an empty light_red bar on the top of the page.

I also tried: {% ifnotequal message None %}, {% ifnotequal message '' %}, tried setting the message to None or '' explicitly, but does not seem to help.

Would appreciate some help!

You need to switch your {% block %} and your {% if %}

{% block messages %}
    {% if message %}<div class='imp_message'>{{ message }}</div>{% endif %}
{% endblock %}

To check, in an if statement, you need to compare the value to None, like this:

{% if some_missing_var is None %}
   // code here if some_missing_var exists
{% else %}
   // code here if some_missing_var does not exist
{% endif %}

In other cases (from the docs):

Generally, if a variable doesn’t exist, the template system inserts the value of the engine’s string_if_invalid configuration option, which is set to '' (the empty string) by default.

I tried some of the other answers, and they didn't work until I read the docs on how invalid variables are handled and the above was made clear.

link to docs that describe handling invalid variables

If you don't want to litter your logs with KeyError when there is no variable in the template context I recommend to use templatetags filters.

In myapp/templatetags/filters.py I add:

@register.simple_tag(takes_context=True)
def var_exists(context, name):
    dicts = context.dicts  # array of dicts
    if dicts:
        for d in dicts:
            if name in d:
                return True
    return False

In html template:

{% load filters %}
...
{% var_exists 'project' as project_exists %}
{% if project_exists %}
  ...
{% endif}