Basic Layout

The starter files generated by the pyramid_zodb scaffold 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/1.2-branch/docs/tutorials/wiki/src/basiclayout/.

App Startup with __init__.py

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 a 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:

from pyramid.config import Configurator
from pyramid_zodbconn import get_connection
from tutorial.models import appmaker

def root_factory(request):
    conn = get_connection(request)
    return appmaker(conn.root())

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application.
    """
    config = Configurator(root_factory=root_factory, settings=settings)
    config.add_static_view('static', 'tutorial:static', cache_max_age=3600)
    config.scan('tutorial')
    return config.make_wsgi_app()

1. Lines 1-3. Perform some dependency imports.
2. Lines 5-7 Define a root factory for our Pyramid application.
3. Line 12. We construct a Configurator with a root factory and the settings keywords parsed by PasteDeploy. The root factory is named get_root.
4. Line 13. Register a ‘static view’ which answers requests which start with with URL path /static using the pyramid.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, at http://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 an asset specification, so it finds the resources it should serve within the static directory inside the tutorial package.
5. Line 14. Perform a scan. A scan will find configuration decoration, such as view configuration decorators (e.g. @view_config) in the source code of the tutorial package and will take actions based on these decorators. The argument to scan() is the package name to scan, which is tutorial.
6. Line 15. Use the pyramid.config.Configurator.make_wsgi_app() method to return a WSGI application.

Resources and Models with models.py

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 scaffold 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:

from persistent.mapping import PersistentMapping

class MyModel(PersistentMapping):
    __parent__ = __name__ = None

def appmaker(zodb_root):
    if not 'app_root' in zodb_root:
        app_root = MyModel()
        zodb_root['app_root'] = app_root
        import transaction
        transaction.commit()
    return zodb_root['app_root']

1. Lines 3-4. The MyModel resource class is implemented here. Instances of this class will be capable of being persisted in ZODB because the class inherits from the persistent.mapping.PersistentMapping class. The __parent__ and __name__ are important parts of the traversal protocol. By default, have these as None indicating that this is the root object.

2. Lines 6-12. appmaker is used to return the application root object. It is called on every request to the Pyramid application. It also performs bootstrapping by creating an application root (inside the ZODB root object) if one does not already exist. It is used by the “root_factory” we’ve defined in our __init__.py.

   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.

Views With views.py

Our scaffold 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:

from pyramid.view import view_config
from tutorial.models import MyModel

@view_config(context=MyModel,
             renderer='tutorial:templates/mytemplate.pt')
def my_view(request):
    return {'project':'tutorial'}

Let’s try to understand the components in this module:

1. Lines 1-2. Perform some dependency imports.

2. Line 4. Use the pyramid.view.view_config() configuration decoration to perform a view 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 a scan (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 and renderer.

   The context argument signifies that the decorated view callable should only be run when traversal finds the tutorial.models.MyModel resource to be the context of a request. In English, this means that when the URL / is visited, because MyModel is the root model, this view callable will be invoked.

   The renderer argument names an asset specification of tutorial:templates/mytemplate.pt. This asset specification points at a Chameleon template which lives in the mytemplate.pt file within the templates directory of the tutorial package. And indeed if you look in the templates directory of this package, you’ll see a mytemplate.pt template file, which renders the default home page of the generated project.

   Since this call to @view_config doesn’t pass a name argument, the my_view function which it decorates represents the “default” view callable used when the context is of the type MyModel.

3. Lines 5-6. We define a view callable named my_view, which we decorated in the step above. This view callable is a function we write generated by the pyramid_zodb scaffold that is given a request and which returns a dictionary. The mytemplate.pt renderer named by the asset specification in the step above will convert this dictionary to a response on our behalf.

   The function returns the dictionary {'project':'tutorial'}. This dictionary is used by the template named by the mytemplate.pt asset specification to fill in certain values on the page.

Configuration in development.ini

The development.ini (in the tutorial project directory, as opposed to the tutorial package directory) looks like this:

[app:main]
use = egg:tutorial
pyramid.reload_templates = true
pyramid.debug_authorization = false
pyramid.debug_notfound = false
pyramid.debug_routematch = false
pyramid.debug_templates = true
pyramid.default_locale_name = en
pyramid.includes = pyramid_debugtoolbar
                   pyramid_zodbconn
                   pyramid_tm
tm.attempts = 3
zodbconn.uri = file://%(here)s/Data.fs?connection_cache_size=20000

[server:main]
use = egg:Paste#http
host = 0.0.0.0
port = 6543

# Begin logging configuration

[loggers]
keys = root

[handlers]
keys = console

[formatters]
keys = generic

[logger_root]
level = INFO
handlers = console

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[formatter_generic]
format = %(asctime)s %(levelname)-5.5s [%(name)s] %(message)s

# End logging configuration

Note the existence of an [app:main] section which specifies our WSGI application. Our ZODB database settings are specified as the zodbconn.uri setting within this section. This value, and the other 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.