pylons mako how to check if variable exist or not - pylons

Question

In django, we can do this:
views.py :
def A(request):
context = {test : 'test'}
return render_to_response('index.html', context , context_instance = RequestContext(request))
def B(request):
context = {}
return render_to_response('index.html', context , context_instance = RequestContext(request))
index.html:
{% if test %}
{{ test }}
{% endif %}
And have our template render without error, even if i use method B, where variable 'test' does not exist, but I still can put it in the template.
I want to do the same with pylons + mako, in controller :
foo.py
def A(self):
c.test = 'test'
return render('index.html')
def B(self):
return render('index.html')
index.html :
% if c.test:
${'c.test'}
% endif
In Django, I can do that, but in Pylons, I get an error, is there anyway to check wheter 'c.test' exists or not?
the error : AttributeError: 'ContextObj' object has no attribute 'test'

Answer 1

I had a similar issue where I had multiple views using the same template and needed to test if a variable was set. I looked at the docs chris referenced and found another way to solve this problem regardless of how mako.strict_undefined is set. Essentially you call the get() method on the context object. In your example you could do the following:
% if context.get('test', UNDEFINED) is not UNDEFINED:
${test}
% endif
or
${context.get('test', '')}
That will print the same as ${test} if it exists, and print an empty string if it doesn't.
Unfortunately you can't seem to use an in operator on context which would be the most intuitive.

Answer 2

From the mako Docs on Context Variables:
% if someval is UNDEFINED:
someval is: no value
% else:
someval is: ${someval}
% endif
The docs describe this as referencing variable names not in the current context. Mako will set these variables to the value UNDEFINED.
I check for variables like so:
% if not someval is UNDEFINED:
(safe to use someval)
However, if pylons/pyramid has strict_undefined=True setting, attempts to use the undefined variable results in a NameError being raised. They give a brief philisophical justification for doing it this way, instead of simply replacing un-set variables with empty strings which seems consistent with Python philosophy. Took me a while to find this, but reading that entire section on the Mako Runtime will clear up how Mako recieves, sets, and uses context variables.
Edit:
For completions sake, the documents explain the strict_undefined setting here. You can change this variable by setting it in one of your .ini files:
[app:main]
...
mako.strict_undefined = false

Answer 3

a bit late, so whenever you use a variable on your template that doesn't exist on your controller, pylons will raise an error, to disable the error, just put this in your environment.py :
config['pylons.strict_tmpl_context'] = False