The starter files generated by the pyramid_zodb
template are basic, but they provide a good orientation for the high-level patterns common to most traversal
-based Pyramid
(and ZODB
based) projects.
The source code for this tutorial stage can be browsed via http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki/src/basiclayout/.
A directory on disk can be turned into a Python package
by containing an __init__.py
file. Even if empty, this marks a directory as a Python package. Our application uses __init__.py
as both a package marker, as well as to contain application configuration code.
When you run the application using the paster
command using the development.ini
generated config file, the application configuration points at an Setuptools entry point described as egg:tutorial
. In our application, because the application's setup.py
file says so, this entry point happens to be the main
function within the file named __init__.py
:
src/basiclayout/tutorial/__init__.py
- Lines 1-3. Perform some dependency imports.
- Line 8. Get the ZODB configuration from the
development.ini
file's[app:main]
section represented by thesettings
dictionary passed to ourapp
function. This will be a URI (something likefile:///path/to/Data.fs
). - Line 12. We create a "finder" object using the
PersistentApplicationFinder
helper class, passing it the ZODB URI and the "appmaker" we've imported frommodels.py
. - Lines 13 - 14. We create a
root factory
which uses the finder to return a ZODB root object. - Line 15. We construct a
Configurator
with aroot factory
and the settings keywords parsed by PasteDeploy. The root factory is namedget_root
. - Line 16. Register a 'static view' which answers requests which start with with URL path
/static
using thepyramid.config.Configurator.add_static_view method
. This statement registers a view that will serve up static assets, such as CSS and image files, for us, in this case, athttp://localhost:6543/static/
and below. The first argument is the "name"static
, which indicates that the URL path prefix of the view will be/static
. the The second argument of this tag is the "path", which is anasset specification
, so it finds the resources it should serve within thestatic
directory inside thetutorial
package. - Line 17. Perform a
scan
. A scan will findconfiguration decoration
, such as view configuration decorators (e.g.@view_config
) in the source code of thetutorial
package and will take actions based on these decorators. The argument to~pyramid.config.Configurator.scan
is the package name to scan, which istutorial
. - Line 18. Use the
pyramid.config.Configurator.make_wsgi_app
method to return aWSGI
application.
Pyramid
uses the word resource
to describe objects arranged hierarchically in a resource tree
. This tree is consulted by traversal
to map URLs to code. In this application, the resource tree represents the site structure, but it also represents the domain model
of the application, because each resource is a node stored persistently in a ZODB
database. The models.py
file is where the pyramid_zodb
Paster template put the classes that implement our resource objects, each of which happens also to be a domain model object.
Here is the source for models.py
:
src/basiclayout/tutorial/models.py
- Lines 3-4. The
MyModel
resource
class is implemented here. Instances of this class will be capable of being persisted inZODB
because the class inherits from thepersistent.mapping.PersistentMapping
class. The__parent__
and__name__
are important parts of thetraversal
protocol. By default, have these asNone
indicating that this is theroot
object. Lines 6-12.
appmaker
is used to return the application root object. It is called on every request to thePyramid
application. It also performs bootstrapping by creating an application root (inside the ZODB root object) if one does not already exist.We do so by first seeing if the database has the persistent application root. If not, we make an instance, store it, and commit the transaction. We then return the application root object.
Our paster template generated a default views.py
on our behalf. It contains a single view, which is used to render the page shown when you visit the URL http://localhost:6543/
.
Here is the source for views.py
:
src/basiclayout/tutorial/views.py
Let's try to understand the components in this module:
- Lines 1-2. Perform some dependency imports.
Line 4. Use the
pyramid.view.view_config
configuration decoration
to perform aview configuration
registration. This view configuration registration will be activated when the application is started. It will be activated by virtue of it being found as the result of ascan
(when Line 17 of__init__.py
is run).The
@view_config
decorator accepts a number of keyword arguments. We use two keyword arguments here:context
andrenderer
.The
context
argument signifies that the decorated view callable should only be run whentraversal
finds thetutorial.models.MyModel
resource
to be thecontext
of a request. In English, this means that when the URL/
is visited, becauseMyModel
is the root model, this view callable will be invoked.The
renderer
argument names anasset specification
oftutorial:templates/mytemplate.pt
. This asset specification points at aChameleon
template which lives in themytemplate.pt
file within thetemplates
directory of thetutorial
package. And indeed if you look in thetemplates
directory of this package, you'll see amytemplate.pt
template file, which renders the default home page of the generated project.Since this call to
@view_config
doesn't pass aname
argument, themy_view
function which it decorates represents the "default" view callable used when the context is of the typeMyModel
.Lines 5-6. We define a
view callable
namedmy_view
, which we decorated in the step above. This view callable is a function we write generated by thepyramid_zodb
template that is given arequest
and which returns a dictionary. Themytemplate.pt
renderer
named by the asset specification in the step above will convert this dictionary to aresponse
on our behalf.The function returns the dictionary
{'project':'tutorial'}
. This dictionary is used by the template named by themytemplate.pt
asset specification to fill in certain values on the page.
The development.ini
(in the tutorial project
directory, as opposed to the tutorial package
directory) looks like this:
src/views/development.ini
Note the existence of a [pipeline:main]
section which specifies our WSGI pipeline. This "pipeline" will be served up as our WSGI application. As far as the WSGI server is concerned the pipeline is our application. Simpler configurations don't use a pipeline: instead they expose a single WSGI application as "main". Our setup is more complicated, so we use a pipeline composed of middleware
.
The egg:WebError#evalerror
middleware is at the "top" of the pipeline. This is middleware which displays debuggable errors in the browser while you're developing (not recommended for deployment).
The egg:repoze.zodbconn#closer
middleware is in the middle of the pipeline. This is a piece of middleware which closes the ZODB connection opened by the PersistentApplicationFinder
at the end of the request.
The tm
middleware is the last piece of middleware in the pipeline. This commits a transaction near the end of the request unless there's an exception raised or the HTTP response code is an error code. The tm
refers to the [filter:tm]
section beneath the pipeline declaration, which configures the transaction manager.
The final line in the [pipeline:main]
section is tutorial
, which refers to the [app:tutorial]
section above it. The [app:tutorial]
section is the section which actually defines our application settings. The values within this section are passed as **settings
to the main
function we defined in __init__.py
when the server is started via paster serve
.