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Basic Layout

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/1.0-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 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:

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from pyramid.config import Configurator
from repoze.zodbconn.finder import PersistentApplicationFinder
from tutorial.models import appmaker

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application.
    """
    zodb_uri = settings.get('zodb_uri')
    if zodb_uri is None:
        raise ValueError("No 'zodb_uri' in application configuration.")

    finder = PersistentApplicationFinder(zodb_uri, appmaker)
    def get_root(request):
        return finder(request.environ)
    config = Configurator(root_factory=get_root, settings=settings)
    config.add_static_view('static', 'tutorial:static')
    config.scan('tutorial')
    return config.make_wsgi_app()
  1. Lines 1-3. Perform some dependency imports.
  2. Line 8. Get the ZODB configuration from the development.ini file’s [app:main] section represented by the settings dictionary passed to our app function. This will be a URI (something like file:///path/to/Data.fs).
  3. Line 12. We create a “finder” object using the PersistentApplicationFinder helper class, passing it the ZODB URI and the “appmaker” we’ve imported from models.py.
  4. Lines 13 - 14. We create a root factory which uses the finder to return a ZODB root object.
  5. Line 15. We construct a Configurator with a root factory and the settings keywords parsed by PasteDeploy. The root factory is named get_root.
  6. Line 16. 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.
  7. Line 17. 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.
  8. Line 18. 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 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:

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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.

    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 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:

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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 template 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.

The WSGI Pipeline in development.ini

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

[app:tutorial]
use = egg:tutorial
reload_templates = true
debug_authorization = false
debug_notfound = false
debug_routematch = false
debug_templates = true
default_locale_name = en
zodb_uri = file://%(here)s/Data.fs?connection_cache_size=20000

[pipeline:main]
pipeline =
    egg:WebError#evalerror
    egg:repoze.zodbconn#closer
    egg:repoze.retry#retry
    tm
    tutorial

[filter:tm]
use = egg:repoze.tm2#tm
commit_veto = repoze.tm:default_commit_veto

[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 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 egg:repoze.retry#retry middleware catches ConflictError exceptions from ZODB and retries the request up to three times (ZODB is an optimistic concurrency database that relies on application-level transaction retries when a conflict occurs).

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.

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