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Application Configuration

Most people already understand "configuration" as settings that influence the operation of an application. For instance, it's easy to think of the values in a .ini file parsed at application startup time as "configuration". However, if you're reasonably open-minded, it's easy to think of code as configuration too. Since Pyramid, like most other web application platforms, is a framework, it calls into code that you write (as opposed to a library, which is code that exists purely for you to call). The act of plugging application code that you've written into :app:`Pyramid` is also referred to within this documentation as "configuration"; you are configuring :app:`Pyramid` to call the code that makes up your application.

There are two ways to configure a :app:`Pyramid` application: :term:`imperative configuration` and :term:`declarative configuration`. Both are described below.

Imperative Configuration

"Imperative configuration" just means configuration done by Python statements, one after the next. Here's one of the simplest :app:`Pyramid` applications, configured imperatively:

We won't talk much about what this application does yet. Just note that the "configuration' statements take place underneath the if __name__ == '__main__': stanza in the form of method calls on a :term:`Configurator` object (e.g., config.add_view(...)). These statements take place one after the other, and are executed in order, so the full power of Python, including conditionals, can be employed in this mode of configuration.

Declarative Configuration

It's sometimes painful to have all configuration done by imperative code, because often the code for a single application may live in many files. If the configuration is centralized in one place, you'll need to have at least two files open at once to see the "big picture": the file that represents the configuration, and the file that contains the implementation objects referenced by the configuration. To avoid this, :app:`Pyramid` allows you to insert :term:`configuration decoration` statements very close to code that is referred to by the declaration itself. For example:

The mere existence of configuration decoration doesn't cause any configuration registration to be performed. Before it has any effect on the configuration of a :app:`Pyramid` application, a configuration decoration within application code must be found through a process known as a :term:`scan`.

For example, the :class:`pyramid.view.view_config` decorator in the code example above adds an attribute to the hello function, making it available for a :term:`scan` to find it later.

A :term:`scan` of a :term:`module` or a :term:`package` and its subpackages for decorations happens when the :meth:`pyramid.config.Configurator.scan` method is invoked: scanning implies searching for configuration declarations in a package and its subpackages. For example:

The scanning machinery imports each module and subpackage in a package or module recursively, looking for special attributes attached to objects defined within a module. These special attributes are typically attached to code via the use of a :term:`decorator`. For example, the :class:`~pyramid.view.view_config` decorator can be attached to a function or instance method.

Once scanning is invoked, and :term:`configuration decoration` is found by the scanner, a set of calls are made to a :term:`Configurator` on your behalf. These calls replace the need to add imperative configuration statements that don't live near the code being configured.

The combination of :term:`configuration decoration` and the invocation of a :term:`scan` is collectively known as :term:`declarative configuration`.

In the example above, the scanner translates the arguments to :class:`~pyramid.view.view_config` into a call to the :meth:`pyramid.config.Configurator.add_view` method, effectively:

config.add_view(hello)

Summary

There are two ways to configure a :app:`Pyramid` application: declaratively and imperatively. You can choose the mode with which you're most comfortable; both are completely equivalent. Examples in this documentation will use both modes interchangeably.

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