docs/narr/assets.rst

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

.. _assets_chapter:

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.

.. index::
   single: asset specifications

.. _asset_specifications:

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:

.. code-block:: python
   :linenos:

   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 a 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 to what the asset is relative in their
individual documentation.

.. index::
   single: add_static_view
   pair: assets; serving

.. _static_assets_section:

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.

.. code-block:: python
   :linenos:

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

The ``name`` represents 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``:

.. code-block:: python
   :linenos:

   # 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 the 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
:meth:`pyramid.request.Request.static_url`.

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

.. code-block:: python
   :linenos:

   # 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 :meth:`~pyramid.request.Request.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 :meth:`~pyramid.request.Request.static_url` API is
discussed in more detail later in this chapter.

.. index::
   single: generating static asset urls
   single: static asset urls
   pair:   assets; generating urls

.. _generating_static_asset_urls:

Generating Static Asset URLs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When an :meth:`~pyramid.config.Configurator.add_static_view` method is used to
register a static asset directory, a special helper API named
:meth:`pyramid.request.Request.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:

.. code-block:: python
   :linenos:

   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 :meth:`~pyramid.request.Request.static_url`
API to generate them for you.  For example:

.. code-block:: python
   :linenos:

   from pyramid.renderers import render_to_response

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

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 :meth:`~pyramid.request.Request.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 :meth:`~pyramid.request.Request.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 :meth:`~pyramid.request.Request.static_url` is
a *URL* instead of a view name.  For example, the ``name`` argument may be
``http://example.com`` while the ``path`` given may be ``mypackage:images``:

.. code-block:: python
   :linenos:

   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``:

.. code-block:: python
   :linenos:

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

Using :meth:`~pyramid.request.Request.static_url` in conjunction with a
:meth:`~pyramid.config.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).

For example, we may define a :ref:`custom setting <adding_a_custom_setting>`
named ``media_location`` which we can set to an external URL in production when
our assets are hosted on a CDN.

.. code-block:: python
   :linenos:

   media_location = settings.get('media_location', 'static')

   config = Configurator(settings=settings)
   config.add_static_view(path='myapp:static', name=media_location)

Now we can optionally define the setting in our ini file:

.. code-block:: ini
   :linenos:

   # production.ini
   [app:main]
   use = egg:myapp#main

   media_location = http://static.example.com/

It is also possible to serve assets that live outside of the source by
referring to an absolute path on the filesystem. There are two ways to
accomplish this.

First, :meth:`~pyramid.config.Configurator.add_static_view` supports taking an
absolute path directly instead of an asset spec. This works as expected,
looking in the file or folder of files and serving them up at some URL within
your application or externally. Unfortunately, this technique has a drawback in
that it is not possible to use the :meth:`~pyramid.request.Request.static_url`
method to generate URLs, since it works based on an asset specification.

.. versionadded:: 1.6

The second approach, available in Pyramid 1.6+, uses the asset overriding APIs
described in the :ref:`overriding_assets_section` section. It is then possible
to configure a "dummy" package which then serves its file or folder from an
absolute path.

.. code-block:: python

   config.add_static_view(path='myapp:static_images', name='static')
   config.override_asset(to_override='myapp:static_images/',
                         override_with='/abs/path/to/images/')

From this configuration it is now possible to use
:meth:`~pyramid.request.Request.static_url` to generate URLs to the data in the
folder by doing something like
``request.static_url('myapp:static_images/foo.png')``. While it is not
necessary that the ``static_images`` file or folder actually exist in the
``myapp`` package, it is important that the ``myapp`` portion points to a valid
package. If the folder does exist, then the overriden folder is given priority,
if the file's name exists in both locations.

.. index::
   single: Cache Busting

.. _cache_busting:

Cache Busting
-------------

.. versionadded:: 1.6

In order to maximize performance of a web application, you generally want to
limit the number of times a particular client requests the same static asset.
Ideally a client would cache a particular static asset "forever", requiring it
to be sent to the client a single time.  The HTTP protocol allows you to send
headers with an HTTP response that can instruct a client to cache a particular
asset for an amount of time.  As long as the client has a copy of the asset in
its cache and that cache hasn't expired, the client will use the cached copy
rather than request a new copy from the server.  The drawback to sending cache
headers to the client for a static asset is that at some point the static asset
may change, and then you'll want the client to load a new copy of the asset.
Under normal circumstances you'd just need to wait for the client's cached copy
to expire before they get the new version of the static resource.

A commonly used workaround to this problem is a technique known as
:term:`cache busting`.  Cache busting schemes generally involve generating a
URL for a static asset that changes when the static asset changes.  This way
headers can be sent along with the static asset instructing the client to cache
the asset for a very long time.  When a static asset is changed, the URL used
to refer to it in a web page also changes, so the client sees it as a new
resource and requests the asset, regardless of any caching policy set for the
resource's old URL.

:app:`Pyramid` can be configured to produce cache busting URLs for static
assets using :meth:`~pyramid.config.Configurator.add_cache_buster`:

.. code-block:: python
   :linenos:

   import time
   from pyramid.static import QueryStringConstantCacheBuster

   # config is an instance of pyramid.config.Configurator
   config.add_static_view(name='static', path='mypackage:folder/static/')
   config.add_cache_buster(
       'mypackage:folder/static/',
       QueryStringConstantCacheBuster(str(int(time.time()))))

Adding the cachebuster instructs :app:`Pyramid` to add the current time for
a static asset to the query string in the asset's URL:

.. code-block:: python
   :linenos:

   js_url = request.static_url('mypackage:folder/static/js/myapp.js')
   # Returns: 'http://www.example.com/static/js/myapp.js?x=1445318121'

When the web server restarts, the time constant will change and therefore so
will its URL.

.. note::

   Cache busting is an inherently complex topic as it integrates the asset
   pipeline and the web application. It is expected and desired that
   application authors will write their own cache buster implementations
   conforming to the properties of their own asset pipelines. See
   :ref:`custom_cache_busters` for information on writing your own.

Disabling the Cache Buster
~~~~~~~~~~~~~~~~~~~~~~~~~~

It can be useful in some situations (e.g., development) to globally disable all
configured cache busters without changing calls to
:meth:`~pyramid.config.Configurator.add_cache_buster`.  To do this set the
``PYRAMID_PREVENT_CACHEBUST`` environment variable or the
``pyramid.prevent_cachebust`` configuration value to a true value.

.. _custom_cache_busters:

Customizing the Cache Buster
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Calls to :meth:`~pyramid.config.Configurator.add_cache_buster` may use
any object that implements the interface
:class:`~pyramid.interfaces.ICacheBuster`.

:app:`Pyramid` ships with a very simplistic
:class:`~pyramid.static.QueryStringConstantCacheBuster`, which adds an
arbitrary token you provide to the query string of the asset's URL. This
is almost never what you want in production as it does not allow fine-grained
busting of individual assets.

In order to implement your own cache buster, you can write your own class from
scratch which implements the :class:`~pyramid.interfaces.ICacheBuster`
interface.  Alternatively you may choose to subclass one of the existing
implementations.  One of the most likely scenarios is you'd want to change the
way the asset token is generated.  To do this just subclass
:class:`~pyramid.static.QueryStringCacheBuster` and define a
``tokenize(pathspec)`` method. Here is an example which uses Git to get
the hash of the current commit:

.. code-block:: python
   :linenos:

   import os
   import subprocess
   from pyramid.static import QueryStringCacheBuster

   class GitCacheBuster(QueryStringCacheBuster):
       """
       Assuming your code is installed as a Git checkout, as opposed to an egg
       from an egg repository like PYPI, you can use this cachebuster to get
       the current commit's SHA1 to use as the cache bust token.
       """
       def __init__(self, param='x', repo_path=None):
           super(GitCacheBuster, self).__init__(param=param)
           if repo_path is None:
               repo_path = os.path.dirname(os.path.abspath(__file__))
           self.sha1 = subprocess.check_output(
               ['git', 'rev-parse', 'HEAD'],
               cwd=repo_path).strip()

       def tokenize(self, pathspec):
           return self.sha1

A simple cache buster that modifies the path segment can be constructed as
well:

.. code-block:: python
   :linenos:

   import posixpath

   class PathConstantCacheBuster(object):
       def __init__(self, token):
           self.token = token

       def __call__(self, request, subpath, kw):
           base_subpath, ext = posixpath.splitext(subpath)
           new_subpath = base_subpath + self.token + ext
           return new_subpath, kw

The caveat with this approach is that modifying the path segment
changes the file name, and thus must match what is actually on the
filesystem in order for :meth:`~pyramid.config.Configurator.add_static_view`
to find the file. It's better to use the
:class:`~pyramid.static.ManifestCacheBuster` for these situations, as
described in the next section.

.. _path_segment_cache_busters:

Path Segments and Choosing a Cache Buster
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Many caching HTTP proxies will fail to cache a resource if the URL contains
a query string.  Therefore, in general, you should prefer a cache busting
strategy which modifies the path segment rather than methods which add a
token to the query string.

You will need to consider whether the :app:`Pyramid` application will be
serving your static assets, whether you are using an external asset pipeline
to handle rewriting urls internal to the css/javascript, and how fine-grained
do you want the cache busting tokens to be.

In many cases you will want to host the static assets on another web server
or externally on a CDN. In these cases your :app:`Pyramid` application may not
even have access to a copy of the static assets. In order to cache bust these
assets you will need some information about them.

If you are using an external asset pipeline to generate your static files you
should consider using the :class:`~pyramid.static.ManifestCacheBuster`.
This cache buster can load a standard JSON formatted file generated by your
pipeline and use it to cache bust the assets. This has many performance
advantages as :app:`Pyramid` does not need to look at the files to generate
any cache busting tokens, but still supports fine-grained per-file tokens.

Assuming an example ``manifest.json`` like:

.. code-block:: json

   {
       "css/main.css": "css/main-678b7c80.css",
       "images/background.png": "images/background-a8169106.png"
   }

The following code would set up a cachebuster:

.. code-block:: python
   :linenos:

   from pyramid.static import ManifestCacheBuster

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

   config.add_cache_buster(
       'mypackage:static/',
       ManifestCacheBuster('myapp:static/manifest.json'))

It's important to note that the cache buster only handles generating
cache-busted URLs for static assets. It does **NOT** provide any solutions for
serving those assets. For example, if you generated a URL for
``css/main-678b7c80.css`` then that URL needs to be valid either by
configuring ``add_static_view`` properly to point to the location of the files
or some other mechanism such as the files existing on your CDN or rewriting
the incoming URL to remove the cache bust tokens.

.. index::
   single: static assets view

CSS and JavaScript source and cache busting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Often one needs to refer to images and other static assets inside CSS and
JavaScript files. If cache busting is active, the final static asset URL is not
available until the static assets have been assembled. These URLs cannot be
handwritten. Below is an example of how to integrate the cache buster into
the entire stack. Remember, it is just an example and should be modified to
fit your specific tools.

* First, process the files by using a precompiler which rewrites URLs to their
  final cache-busted form. Then, you can use the
  :class:`~pyramid.static.ManifestCacheBuster` to synchronize your asset
  pipeline with :app:`Pyramid`, allowing the pipeline to have full control
  over the final URLs of your assets.

Now that you are able to generate static URLs within :app:`Pyramid`,
you'll need to handle URLs that are out of our control. To do this you may
use some of the following options to get started:

* Configure your asset pipeline to rewrite URL references inline in
  CSS and JavaScript. This is the best approach because then the files
  may be hosted by :app:`Pyramid` or an external CDN without having to
  change anything. They really are static.

* Templatize JS and CSS, and call ``request.static_url()`` inside their
  template code. While this approach may work in certain scenarios, it is not
  recommended because your static assets will not really be static and are now
  dependent on :app:`Pyramid` to be served correctly. See
  :ref:`advanced_static` for more information on this approach.

If your CSS and JavaScript assets use URLs to reference other assets it is
recommended that you implement an external asset pipeline that can rewrite the
generated static files with new URLs containing cache busting tokens. The
machinery inside :app:`Pyramid` will not help with this step as it has very
little knowledge of the asset types your application may use. The integration
into :app:`Pyramid` is simply for linking those assets into your HTML and
other dynamic content.

.. _advanced_static:

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.  Alternatively, you might like to serve a particular
static asset manually, because its download requires authentication.

Note that you cannot use the :meth:`~pyramid.request.Request.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.static.static_view` 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.static.static_view` static view cannot be made
   root-relative when you use traversal unless it's registered as a :term:`Not
   Found View`.

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.static.static_view` class inside a ``static.py`` file in your
application root as below.

.. code-block:: python
   :linenos:

   from pyramid.static import static_view
   static_view = static_view('/path/to/static/dir', use_subpath=True)

.. note::

   For better cross-system flexibility, use an :term:`asset specification` as
   the argument to :class:`~pyramid.static.static_view` 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.

.. code-block:: python
   :linenos:

   # .. 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.static.static_view` 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.

.. code-block:: python
   :linenos:

   import os
   from pyramid.response import FileResponse

   def favicon_view(request):
       here = os.path.dirname(__file__)
       icon = os.path.join(here, 'static', 'favicon.ico')
       return FileResponse(icon, request=request)

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 creates
a :class:`pyramid.response.FileResponse` using the file path as the response's
``path`` argument and the request as the response's ``request`` argument.
:class:`pyramid.response.FileResponse` will serve the file as quickly as
possible when it's used this way.  It makes sure to set the right content
length and content_type, too, based on the file extension of the file you pass.

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

.. code-block:: python
   :linenos:

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

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

.. code-block:: python
   :linenos:

   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_section:

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 template files.

- A directory containing multiple template files.

- 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
  :term:`pkg_resources` API.

.. index::
   single: override_asset

.. _override_asset:

The ``override_asset`` API
~~~~~~~~~~~~~~~~~~~~~~~~~~

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

.. code-block:: python
   :linenos:

   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:

.. code-block:: python
   :linenos:

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

Individual subdirectories within a package can also be overridden:

.. code-block:: python
   :linenos:

   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:

.. code-block:: python
   :linenos:

   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.

.. versionadded:: 1.6
  As of Pyramid 1.6, it is also possible to override an asset by supplying an
  absolute path to a file or directory. This may be useful if the assets are
  not distributed as part of a Python package.

Cache Busting and Asset Overrides
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Overriding static assets that are being hosted using
:meth:`pyramid.config.Configurator.add_static_view` can affect your cache
busting strategy when using any cache busters that are asset-aware such as
:class:`pyramid.static.ManifestCacheBuster`. What sets asset-aware cache
busters apart is that they have logic tied to specific assets. For example,
a manifest is only generated for a specific set of pre-defined assets. Now,
imagine you have overridden an asset defined in this manifest with a new,
unknown version. By default, the cache buster will be invoked for an asset
it has never seen before and will likely end up returning a cache busting
token for the original asset rather than the asset that will actually end up
being served! In order to get around this issue, it's possible to attach a
different :class:`pyramid.interfaces.ICacheBuster` implementation to the
new assets. This would cause the original assets to be served by their
manifest, and the new assets served by their own cache buster. To do this,
:meth:`pyramid.config.Configurator.add_cache_buster` supports an ``explicit``
option. For example:

.. code-block:: python
   :linenos:

   from pyramid.static import ManifestCacheBuster

   # define a static view for myapp:static assets
   config.add_static_view('static', 'myapp:static')

   # setup a cache buster for your app based on the myapp:static assets
   my_cb = ManifestCacheBuster('myapp:static/manifest.json')
   config.add_cache_buster('myapp:static', my_cb)

   # override an asset
   config.override_asset(
       to_override='myapp:static/background.png',
       override_with='theme:static/background.png')

   # override the cache buster for theme:static assets
   theme_cb = ManifestCacheBuster('theme:static/manifest.json')
   config.add_cache_buster('theme:static', theme_cb, explicit=True)

In the above example there is a default cache buster, ``my_cb``, for all
assets served from the ``myapp:static`` folder. This would also affect
``theme:static/background.png`` when generating URLs via
``request.static_url('myapp:static/background.png')``.

The ``theme_cb`` is defined explicitly for any assets loaded from the
``theme:static`` folder. Explicit cache busters have priority and thus
``theme_cb`` would be invoked for
``request.static_url('myapp:static/background.png')``, but ``my_cb`` would
be used for any other assets like
``request.static_url('myapp:static/favicon.ico')``.