13. STATIC ASSETS

13.4 Overriding Assets

It can often be useful to override specific assets from “outside” a given Pyramid application. For example, you may wish to reuse an existing Pyramid application more or less unchanged. However, some specific template file owned by the application might have inappropriate HTML, or some static asset (such as a logo file or some CSS file) might not be appropriate. You could just fork the application entirely, but it’s often more convenient to just override the assets that are inappropriate and reuse the application “as is”. This is particularly true when you reuse some “core” application over and over again for some set of customers (such as a CMS application, or some bug tracking application), and you want to make arbitrary visual modifications to a particular application deployment without forking the underlying code.

To this	end,	Pyramid contains	a	feature that makes it possible to “override” one asset with
one or	more	other assets.	In	support of this feature, a Configurator API exists named

pyramid.config.Configurator.override_asset(). This API allows you to override the following kinds of assets defined in any Python package:

•Individual Chameleon templates.

•A directory containing multiple Chameleon templates.

•Individual static files served up by an instance of the pyramid.static.static_view helper class.

•A directory of static files served up by an instance of the pyramid.static.static_view helper class.

•Any other asset (or set of assets) addressed by code that uses the setuptools pkg_resources API.

13.4.1 The override_asset API

An individual call to override_asset() can override a single asset. For example:

1 config.override_asset(

2to_override=’some.package:templates/mytemplate.pt’,

3override_with=’another.package:othertemplates/anothertemplate.pt’)

The string value passed to both to_override and override_with sent to the override_asset API is called an asset specification. The colon separator in a specification separates the package name from the asset name. The colon and the following asset name are optional. If they are not specified, the override attempts to resolve every lookup into a package from the directory of another package. For example:

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13.4. OVERRIDING ASSETS

1

2

config.override_asset(to_override=’some.package’, override_with=’another.package’)

Individual subdirectories within a package can also be overridden:

1

2

config.override_asset(to_override=’some.package:templates/’, override_with=’another.package:othertemplates/’)

If you wish to override a directory with another directory, you must make sure to attach the slash to the end of both the to_override specification and the override_with specification. If you fail to attach a slash to the end of a specification that points to a directory, you will get unexpected results.

You cannot override a directory specification with a file specification, and vice versa: a startup error will occur if you try. You cannot override an asset with itself: a startup error will occur if you try.

Only individual package assets may be overridden. Overrides will not traverse through subpackages within an overridden package. This means that if you want to override assets for both some.package:templates, and some.package.views:templates, you will need to register two overrides.

The package name in a specification may start with a dot, meaning that the package is relative to the package in which the configuration construction file resides (or the package argument to the Configurator class construction). For example:

1

2

config.override_asset(to_override=’.subpackage:templates/’, override_with=’another.package:templates/’)

Multiple calls to override_asset which name a shared to_override but a different override_with specification can be “stacked” to form a search path. The first asset that exists in the search path will be used; if no asset exists in the override path, the original asset is used.

Asset	overrides	can actually	override	assets other than templates and static files.
Any	software	which uses	the	pkg_resources.get_resource_filename(),

pkg_resources.get_resource_stream() or pkg_resources.get_resource_string()

APIs will obtain an overridden file when an override is used.

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