django模板引擎有错误检查？

Question

I have a Django application which uses the Django template system to generate its (non-HTML) output, in addition the the web UI. There's a set of pages where a user can create a template for a report, adding {{ }} tags for variable substitution, and an extra templatetag library to format things nicely.

However, the current way I'm doing this is just:

t = Template(component_template)
self.output_content = t.render(component_context)

Which uses the default web-output template engine. This has string_if_invalid set to None , and dire warnings in the manual about breaking the admin pages if you change it.

So if a user gets either a typo in a variable name in a tag, it is quietly ignored and makes it into the output. If they have a mangled tag, it actually kills the web app. I'm looking for a way to validate the template at edit-time, so that the user can be warned that changes are needed.

What I'm aiming for is something like compiler output:

unknown variable 'ffsdfd' on line 33 of template
template syntax error on line 22 of template

My first thought was to create a new template Engine() and use that for this one purpose, so I could spot a distinctive default string_if_invalid but that doesn't tell me anything about the missing/incorrect variable.

engine = Engine(string_if_invalid="!!MISSING_VARIABLE!!", dirs=settings.TEMPLATES[0]['DIRS'],
                context_processors=settings.TEMPLATES[0]['OPTIONS']['context_processors'],
                app_dirs=settings.TEMPLATES[0]['APP_DIRS'])

t = Template(component_template, engine=engine)

try:
    self.output_content = t.render(component_context)
except TemplateSyntaxError:
    pass # do something useful here to collect error messages

The TemplateSyntaxError exception works, except I don't get any context information, like where the error actually is, and of course I only get the first failure. Looking in the django.template code, it looks like internally there is some sort of extended exception that has the line number and the token that caused it to choke, but it doesn't escape from the render() method.

So:

How can I provide useful error handling for errors in user-edited templates? Should I be doing this a different way altogether?

Answer 1

Here's how I solve it myself using a custom class and string_if_invalid . It gets you the variable name, but I'm sure you can tweak it further to get additional context info.

Global settings.py example, should be easily adaptable to your inline example:

class InvalidTemplateVariable(str):
    def __mod__(self,other):
        from django.template.base import TemplateSyntaxError
        raise TemplateSyntaxError("Invalid variable : '%s'" % other)

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [....],
        'APP_DIRS': True,
        'OPTIONS': {
            'string_if_invalid': InvalidTemplateVariable("%s"),
            'context_processors': [
               ....
            ],
        },
    },
]

BTW, you can get additional info on how/why this works at the following article (which I wrote) http://www.webforefront.com/django/customizedjangotemplates.html#stringifinvaliderror