assets.rst

.. index::
   single: assets
   single: static asssets

Static Assets

An :term:`asset` is any file contained within a Python :term:`package` which is not a Python source code file. For example, each of the following is an asset:

• a GIF image file contained within a Python package or contained within any subdirectory of a Python package.
• a CSS file contained within a Python package or contained within any subdirectory of a Python package.
• a JavaScript source file contained within a Python package or contained within any subdirectory of a Python package.
• A directory within a package that does not have an __init__.py in it (if it possessed an __init__.py it would be a package).
• a :term:`Chameleon` or :term:`Mako` template file contained within a Python package.

The use of assets is quite common in most web development projects. For example, when you create a :app:`Pyramid` application using one of the available scaffolds, as described in :ref:`creating_a_project`, the directory representing the application contains a Python :term:`package`. Within that Python package, there are directories full of files which are static assets. For example, there's a static directory which contains .css, .js, and .gif files. These asset files are delivered when a user visits an application URL.

Understanding Asset Specifications

Let's imagine you've created a :app:`Pyramid` application that uses a :term:`Chameleon` ZPT template via the :func:`pyramid.renderers.render_to_response` API. For example, the application might address the asset using the :term:`asset specification` myapp:templates/some_template.pt using that API within a views.py file inside a myapp package:

from pyramid.renderers import render_to_response
render_to_response('myapp:templates/some_template.pt', {}, request)

"Under the hood", when this API is called, :app:`Pyramid` attempts to make sense out of the string myapp:templates/some_template.pt provided by the developer. This string is an :term:`asset specification`. It is composed of two parts:

• The package name (myapp)
• The asset name (templates/some_template.pt), relative to the package directory.

The two parts are separated by the colon character.

:app:`Pyramid` uses the Python :term:`pkg_resources` API to resolve the package name and asset 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.

There is a second form of asset specification: a relative asset specification. Instead of using an "absolute" asset specification which includes the package name, in certain circumstances you can omit the package name from the specification. For example, you might be able to use templates/mytemplate.pt instead of myapp:templates/some_template.pt. Such asset specifications are usually relative to a "current package." The "current package" is usually the package which contains the code that uses the asset specification. :app:`Pyramid` APIs which accept relative asset specifications typically describe what the asset is relative to in their individual documentation.

.. index::
   single: add_static_view

Serving Static Assets

:app:`Pyramid` makes it possible to serve up static asset files from a directory on a filesystem to an application user's browser. Use the :meth:`pyramid.config.Configurator.add_static_view` to instruct :app:`Pyramid` to serve static assets such as JavaScript and CSS files. This mechanism makes a directory of static files available at a name relative to the application root URL, e.g. /static or as an external URL.

Note

:meth:`~pyramid.config.Configurator.add_static_view` cannot serve a single file, nor can it serve a directory of static files directly relative to the root URL of a :app:`Pyramid` application. For these features, see :ref:`advanced_static`.

Here's an example of a use of :meth:`~pyramid.config.Configurator.add_static_view` that will serve files up from the /var/www/static directory of the computer which runs the :app:`Pyramid` application as URLs beneath the /static URL prefix.

# config is an instance of pyramid.config.Configurator
config.add_static_view(name='static', path='/var/www/static')

The name prepresents a URL prefix. In order for files that live in the path directory to be served, a URL that requests one of them must begin with that prefix. In the example above, name is static, and path is /var/www/static. In English, this means that you wish to serve the files that live in /var/www/static as sub-URLs of the /static URL prefix. Therefore, the file /var/www/static/foo.css will be returned when the user visits your application's URL /static/foo.css.

A static directory named at path may contain subdirectories recursively, and any subdirectories may hold files; these will be resolved by the static view as you would expect. The Content-Type header returned by the static view for each particular type of file is dependent upon its file extension.

By default, all files made available via :meth:`~pyramid.config.Configurator.add_static_view` are accessible by completely anonymous users. Simple authorization can be required, however. To protect a set of static files using a permission, in addition to passing the required name and path arguments, also pass the permission keyword argument to :meth:`~pyramid.config.Configurator.add_static_view`. The value of the permission argument represents the :term:`permission` that the user must have relative to the current :term:`context` when the static view is invoked. A user will be required to possess this permission to view any of the files represented by path of the static view. If your static assets must be protected by a more complex authorization scheme, see :ref:`advanced_static`.

Here's another example that uses an :term:`asset specification` instead of an absolute path as the path argument. To convince :meth:`~pyramid.config.Configurator.add_static_view` to serve files up under the /static URL from the a/b/c/static directory of the Python package named some_package, we can use a fully qualified :term:`asset specification` as the path:

# config is an instance of pyramid.config.Configurator
config.add_static_view(name='static', path='some_package:a/b/c/static')

The path provided to :meth:`~pyramid.config.Configurator.add_static_view` may be a fully qualified :term:`asset specification` or an absolute path.

Instead of representing a URL prefix, the name argument of a call to :meth:`~pyramid.config.Configurator.add_static_view` can alternately be a URL. Each of examples we've seen so far have shown usage of the name argument as a URL prefix. However, when name is a URL, static assets can be served from an external webserver. In this mode, the name is used as the URL prefix when generating a URL using :func:`pyramid.url.static_url`.

For example, :meth:`~pyramid.config.Configurator.add_static_view` may be fed a name argument which is http://example.com/images:

# config is an instance of pyramid.config.Configurator
config.add_static_view(name='http://example.com/images',
                       path='mypackage:images')

Because :meth:`~pyramid.config.Configurator.add_static_view` is provided with a name argument that is the URL http://example.com/images, subsequent calls to :func:`~pyramid.url.static_url` with paths that start with the path argument passed to :meth:`~pyramid.config.Configurator.add_static_view` will generate a URL something like http://example.com/images/logo.png. The external webserver listening on example.com must be itself configured to respond properly to such a request. The :func:`~pyramid.url.static_url` API is discussed in more detail later in this chapter.

.. index::
   single: generating static asset urls
   single: static asset urls

Generating Static Asset URLs

When a :meth:`~pyramid.config.Configurator.add_static_view` method is used to register a static asset directory, a special helper API named :func:`pyramid.url.static_url` can be used to generate the appropriate URL for an asset that lives in one of the directories named by the static registration path attribute.

For example, let's assume you create a set of static declarations like so:

config.add_static_view(name='static1', path='mypackage:assets/1')
config.add_static_view(name='static2', path='mypackage:assets/2')

These declarations create URL-accessible directories which have URLs that begin with /static1 and /static2, respectively. The assets in the assets/1 directory of the mypackage package are consulted when a user visits a URL which begins with /static1, and the assets in the assets/2 directory of the mypackage package are consulted when a user visits a URL which begins with /static2.

You needn't generate the URLs to static assets "by hand" in such a configuration. Instead, use the :func:`~pyramid.url.static_url` API to generate them for you. For example:

from pyramid.url import static_url
from pyramid.chameleon_zpt import render_template_to_response

def my_view(request):
    css_url = static_url('mypackage:assets/1/foo.css', request)
    js_url = static_url('mypackage:assets/2/foo.js', request)
    return render_template_to_response('templates/my_template.pt',
                                       css_url = css_url,
                                       js_url = js_url)

If the request "application URL" of the running system is http://example.com, the css_url generated above would be: http://example.com/static1/foo.css. The js_url generated above would be http://example.com/static2/foo.js.

One benefit of using the :func:`~pyramid.url.static_url` function rather than constructing static URLs "by hand" is that if you need to change the name of a static URL declaration, the generated URLs will continue to resolve properly after the rename.

URLs may also be generated by :func:`~pyramid.url.static_url` to static assets that live outside the :app:`Pyramid` application. This will happen when the :meth:`~pyramid.config.Configurator.add_static_view` API associated with the path fed to :func:`~pyramid.url.static_url` is a URL instead of a view name. For example, the name argument may be http://example.com while the the path given may be mypackage:images:

config.add_static_view(name='http://example.com/images',
                       path='mypackage:images')

Under such a configuration, the URL generated by static_url for assets which begin with mypackage:images will be prefixed with http://example.com/images:

static_url('mypackage:images/logo.png', request)
# -> http://example.com/images/logo.png

Using :func:`~pyramid.url.static_url` in conjunction with a :meth:`~pyramid.configuration.Configurator.add_static_view` makes it possible to put static media on a separate webserver during production (if the name argument to :meth:`~pyramid.config.Configurator.add_static_view` is a URL), while keeping static media package-internal and served by the development webserver during development (if the name argument to :meth:`~pyramid.config.Configurator.add_static_view` is a URL prefix). To create such a circumstance, we suggest using the :attr:`pyramid.registry.Registry.settings` API in conjunction with a setting in the application .ini file named media_location. Then set the value of media_location to either a prefix or a URL depending on whether the application is being run in development or in production (use a different .ini file for production than you do for development). This is just a suggestion for a pattern; any setting name other than media_location could be used.

.. index::
   single: static assets view

Advanced: Serving Static Assets Using a View Callable

For more flexibility, static assets can be served by a :term:`view callable` which you register manually. For example, if you're using :term:`URL dispatch`, you may want static assets to only be available as a fallback if no previous route matches. Alternately, you might like to serve a particular static asset manually, because its download requires authentication.

Note that you cannot use the :func:`~pyramid.url.static_url` API to generate URLs against assets made accessible by registering a custom static view.

Root-Relative Custom Static View (URL Dispatch Only)

The :class:`pyramid.view.static` helper class generates a Pyramid view callable. This view callable can serve static assets from a directory. An instance of this class is actually used by the :meth:`~pyramid.config.Configurator.add_static_view` configuration method, so its behavior is almost exactly the same once it's configured.

Warning

The following example will not work for applications that use :term:`traversal`, it will only work if you use :term:`URL dispatch` exclusively. The root-relative route we'll be registering will always be matched before traversal takes place, subverting any views registered via add_view (at least those without a route_name). A :class:`~pyramid.view.static` static view cannot be made root-relative when you use traversal.

To serve files within a directory located on your filesystem at /path/to/static/dir as the result of a "catchall" route hanging from the root that exists at the end of your routing table, create an instance of the :class:`~pyramid.view.static` class inside a static.py file in your application root as below.

from pyramid.view import static
static_view = static('/path/to/static/dir')

Note

For better cross-system flexibility, use an :term:`asset specification` as the argument to :class:`~pyramid.view.static` instead of a physical absolute filesystem path, e.g. mypackage:static instead of /path/to/mypackage/static.

Subsequently, you may wire the files that are served by this view up to be accessible as /<filename> using a configuration method in your application's startup code.

# .. every other add_route declaration should come
# before this one, as it will, by default, catch all requests

config.add_route('catchall_static', '/*subpath')
config.add_view('myapp.static.static_view', route_name='catchall_static')

The special name *subpath above is used by the :class:`~pyramid.view.static` view callable to signify the path of the file relative to the directory you're serving.

Registering A View Callable to Serve a "Static" Asset

You can register a simple view callable to serve a single static asset. To do so, do things "by hand". First define the view callable.

import os
from webob import Response

def favicon_view(request):
    here = os.path.dirname(__file__)
    icon = open(os.path.join(here, 'static', 'favicon.ico'))
    return Response(content_type='image/x-icon', app_iter=icon)

The above bit of code within favicon_view computes "here", which is a path relative to the Python file in which the function is defined. It then uses the Python open function to obtain a file handle to a file within "here" named static, and returns a response using the open the file handle as the response's app_iter. It makes sure to set the right content_type too.

You might register such a view via configuration as a view callable that should be called as the result of a traversal:

config.add_view('myapp.views.favicon_view', name='favicon.ico')

Or you might register it to be the view callable for a particular route:

config.add_route('favicon', '/favicon.ico')
config.add_view('myapp.views.favicon_view', route_name='favicon')

Because this is a simple view callable, it can be protected with a :term:`permission` or can be configured to respond under different circumstances using :term:`view predicate` arguments.

.. index::
   pair: overriding; assets

Overriding Assets

It can often be useful to override specific assets from "outside" a given :app:`Pyramid` application. For example, you may wish to reuse an existing :app:`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, :app:`Pyramid` contains a feature that makes it possible to "override" one asset with one or more other assets. In support of this feature, a :term:`Configurator` API exists named :meth:`pyramid.config.Configurator.override_asset`. This API allows you to override the following kinds of assets defined in any Python package:

• Individual :term:`Chameleon` templates.
• A directory containing multiple Chameleon templates.
• Individual static files served up by an instance of the pyramid.view.static helper class.
• A directory of static files served up by an instance of the pyramid.view.static helper class.
• Any other asset (or set of assets) addressed by code that uses the setuptools :term:`pkg_resources` API.

Note

The :term:`ZCML` directive named asset serves the same purpose as the :meth:`~pyramid.config.Configurator.override_asset` method.

.. index::
   single: override_asset

The override_asset API

An individual call to :meth:`~pyramid.config.Configurator.override_asset` can override a single asset. For example:

config.override_asset(
         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 sent to the override_asset API is called an :term:`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:

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

Individual subdirectories within a package can also be overridden:

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 :class:`~pyramid.config.Configurator` class construction). For example:

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 :func:`pkg_resources.get_resource_filename`, :func:`pkg_resources.get_resource_stream` or :func:`pkg_resources.get_resource_string` APIs will obtain an overridden file when an override is used.