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Custom context processors

When you set up data in view methods or url extra options, you do so to access the data on individual Django templates. Custom Django context processors allow you to set up data for access on all Django templates.

A Django custom context processor is structured just like a regular Python method with an HttpRequest object argument that returns a dictionary. The returning dictionary keys of the context processor represent template references and the dictionary values data objects (e.g. strings, lists, dictionaries) accessible in templates. Listing 3-13 illustrates a custom Django context processor method.

Listing 3-13. Custom Django context processor method

def onsale(request):
    # Create fixed data structures to pass to template
    # data could equally come from database queries
    # web services or social APIs
    sale_items = {'Monday':'Mocha 2x1','Tuesday':'Latte 2x1'}
    return {'SALE_ITEMS': sale_items}

As you can see in listing 3-13, the onsale method has a request argument -- representing an HttpRequest object -- and returns a dictionary. The dictionary in this case has a single key called SALE_ITEMS and a value which is a hard-coded dictionary.

However, just as you can set up any type of data in a Django view method or url option to pass to a template, a custom Django context processor method can also access data from the request argument (e.g. cookie, remote IP address) or even query a database and make this data available to all templates.

The custom context processor method can be placed inside any project file or directory. The location and naming conventions are of little importance, because Django detects context processors through the context_processors variable in OPTIONS of the TEMPLATES variable in a project's settings.py file. I'll place the context processor method in listing 3-13 in a file called processors.py in the stores app sub-directory.

Once you save the custom context processor method, you have to configure Django to locate it. Listing 3-14 shows the context_processors variable update to include the custom context processor method from listing 3-13.

Listing 3-14. Django template context processor definitions in context_processors in OPTIONS of TEMPLATES

        'OPTIONS': {
            'context_processors': [
                'coffeehouse.stores.processors.onsale',
                'django.template.context_processors.debug',
                'django.template.context_processors.request',
                'django.contrib.auth.context_processors.auth',
                'django.contrib.messages.context_processors.messages',
            ],
         }

In listing 3-14 you can see the coffeehouse.stores.processors.onsale declaration, where coffeehouse.stores represents the package.app name, processors is the file that contains the custom context processor (i.e. processors.py inside the stores app) and onsale is the actual method that contains the custom context processor logic.

Once you declare the context processors on you project's settings.py file, the custom dictionary with the SALE_ITEMS key from listing 3-13 becomes available to all Django templates.