The use of resources is quite common in most web development projects. For example, when you create a repoze.bfg application using one of the available “paster” templates, as described in Creating the Project, the directory representing the application contains a Python package. Within that Python package, there are directories full of files which are resources. For example, there is a templates directory which contains .pt files, and a static directory which contains .css, .js, and .gif files.
Let’s imagine you’ve created a repoze.bfg application that uses a Chameleon template via the render_template_to_response API. For example, the application might address the resource named templates/some_template.pt using that API within a views.py file inside a myapp package:
“Under the hood”, when this API is called, repoze.bfg attempts to make sense out of the string templates/some_template.pt provided by the developer. To do so, it first finds the “current” package. The “current” package is the Python package in which the views.py module which contains this code lives. This would be the myapp package, according to our example so far. By resolving the current package, repoze.bfg has enough information to locate the actual template file. These are the elements it needs:
repoze.bfg uses the pkg_resources API to resolve the package name and resource name to an absolute (operating-system-specific) file name. It eventually passes this resolved absolute filesystem path to the Chameleon templating engine, which then uses it to load, parse, and execute the template file.
Package names often contain dots. For example, repoze.bfg is a package. Resource names usually look a lot like relative UNIX file paths.
It can often be useful to override specific resources “from outside” a given repoze.bfg application. For example, you may wish to reuse an existing repoze.bfg application more or less unchanged. However, some specific template file owned by the application might have inappropriate HTML, or some static resource (such as a logo file or some CSS file) might not appropriate. You could just fork the application entirely, but it’s often more convenient to just override the resources 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, repoze.bfg contains a feature that makes it possible to “override” one resource with one or more other resources. In support of this feature, a ZCML directive exists named resource. The resource directive allows you to override the following kinds of resources defined in any Python package:
Usually, overriding a resource in an existing application means performing the following steps:
Note that overriding resources is not the only way to extend or modify the behavior of an existing repoze.bfg application. A “heavier hammer” way to do the same thing is explained in Extending An Existing repoze.bfg Application. The heavier hammer way allows you to replace a view wholesale rather than resources that might be used by a view.
An individual repoze.bfg resource ZCML statement can override a single resource. For example:
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<resource to_override="some.package:templates/mytemplate.pt" override_with="another.package:othertemplates/anothertemplate.pt" />
The string value passed to both to_override and override_with attached to a resource directive is called a “specification”. The colon separator in a specification separates the package name from the resource name. The colon and the following resource 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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<resource to_override="some.package" override_with="another.package" />
Individual subdirectories within a package can also be overridden:
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<resource 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 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 a resource with itself (a startup error will occur if you try).
Only individual package resources may be overridden. Overrides will not traverse through subpackages within an overridden package. This means that if you want to override resources 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 ZCML file resides. For example:
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<resource to_override=".subpackage:templates/" override_with="another.package:templates/" />
Multiple resource statements which name a shared to_override but a different override_with specification can be “stacked” to form a search path. The first resource that exists in the search path will be used; if no resource exists in the override path, the original resource is used.
Resource overrides can actually override resources other than templates and static files. Any software which uses the pkg_resources get_resource_filename, get_resource_stream or get_resource_string APIs will obtain an overridden file when an override is used.