# Output Handling

Cement defines an output interface called `cement.core.output.IOutput`, as well as the default `cement.ext.ext_dummy.DummyOutputHandler` that implements the interface. This handler is part of Cement, and actually does nothing to produce output. Therefore it can be said that by default a Cement application does not handle rendering output to the console, but can if another output handler be used.

Please note that there may be other handler’s that implement the `IOutput` interface. The documentation below only references usage based on the interface and not the full capabilities of the implementation.

The following output handlers are included and maintained with Cement:

> - `cement.ext.ext_dummy.DummyOutputHandler`
> - `cement.ext.ext_json.JsonOutputHandler`
> - `cement.ext.ext_yaml.YamlOutputHandler`
> - `cement.ext.ext_genshi.GenshiOutputHandler`
> - `cement.ext.ext_handlebars.HandlebarsOutputHandler`
> - `cement.ext.ext_jinja2.Jinja2OutputHandler`
> - `cement.ext.ext_mustache.MustacheOutputHandler`
> - `cement.ext.ext_tabulate.TabulateOutputHandler`

Please reference the `cement.core.output.IOutput` interface documentation for writing your own output handler.

## Rending Output

Cement applications do not need to use an output handler by any means. Most small applications can get away with simple `print()` statements. However, anyone who has ever built a bigger application that produces a lot of output will know that this can get ugly very quickly in your code.

Using an output handler allows the developer to keep their logic clean, and offload the display of relevant data to an output handler, possibly by templates or other means (GUI?).

An output handler has a `render()` function that takes a data dictionary to produce output. Some output handlers may also accept a `template` or other parameters that define how output is rendered. This is easily accessible by the application object.

    from cement.core import foundation, output

    # Create the application
    app = foundation.CementApp('myapp')

    # Setup the application
    app.setup()

    # Run the application
    app.run()

    # Add application logic
    data = dict(foo='bar')
    app.render(data)

    # Close the application
    app.close()

The above example uses the default output handler, therefore nothing is displayed on screen. That said, if we write our own quickly we can see something happen:

    from cement.core import foundation, handler, output

    # Create a custom output handler
    class MyOutput(output.CementOutputHandler):
        class Meta:
            label = 'myoutput'

        def render(self, data):
            for key in data:
                print "%s => %s" % (key, data[key])

    app = foundation.CementApp('myapp', output_handler=MyOutputHandler)
    ...

Which looks like:

    $ python test.py
    foo => bar

## Rendering Output Via Templates

An extremely powerful feature of Cement is the ability to offload console output to a template output handler. Several are inluded with Cement but not enabled by default (listed above). The following example shows the use of the Mustache templating langugage, as well as Json output handling.

myapp.py

    from cement.core.foundation import CementApp
    from cement.core.controller import CementBaseController, expose

    class MyBaseController(CementBaseController):
        class Meta:
            label = 'base'
            description = 'MyApp Does Amazing Things'

        @expose(hide=True)
        def default(self):
            data = dict(foo='bar')
            self.app.render(data, 'default.m')

            # always return the data, some output handlers require this
            # such as Json/Yaml (which don't use templates)
            return data

    class MyApp(CementApp):
        class Meta:
            label = 'myapp'
            base_controller = MyBaseController
            extensions = ['mustache', 'json']

            # default output handler
            output_handler = 'mustache'

    with MyApp() as app:
        app.run()

/usr/lib/myapp/templates/default.m

    This is the output of the MyBaseController.default() command.

    The value of the 'foo' variable is => '{{foo}}'

And this looks like:

    $ python myapp.py

    This is the output of the MyBaseController.default() command.

    The value of the 'foo' variable is => 'bar'

Optionally, we can use the `JsonOutputHandler` via `-o json` to trigger just Json output (supressing all other output) using our return dictionary:

    $ python myapp.py -o json
    {"foo": "bar"}