Fedora Services

Authors:Toshio Kuratomi Luke Macken
Date:28 May 2008
For Version:0.3.x

In the loosest sense, a Fedora Service is a web application that sends data that BaseClient is able to understand. This document defines things that a web application must currently do for BaseClient to understand it.


All current Fedora Services are written in TurboGears. Examples in this document will be for that framework. However, other frameworks can be used to write a Service if they can correctly create the data that BaseClient needs.

The TurboGears framework separates an application into model, view, and controller layers. The model is typically a database and holds the raw data that the application needs. The view formats the data before output. The controller binds the other two pieces together. For the purpose of building a Service that works with BaseClient we will mostly be concerned with the controller level although there are a few advanced features that can be used to help code the model [1] and view [2] layer for particular tricky problems.

A Fedora Service differs from other web applications in that certain URLs for the web application are an API layer that the Service can send and receive JSON data from for BaseClient to interpret. This imposes certain constraints on what data can be sent and what data can be received from those URLs.

[1]Using SABase
[2]Using __json__()

Sample Controller

This is the sample controller method that we will modify throughout this document:

from turbogears.controllers import RootController
from turbogears import expose

class MyController(RootController):
    def amplify(self, data):
        return dict(data=data.upper())


In TurboGears and most web frameworks, a URL is a kind of API for accessing the data your web app provides. This data can be made available in multiple formats. TurboGears allows you to use one URL to serve multiple formats by specifying a query parameter or a special header.

Selecting JSON Output

A URL in TurboGears can serve double duty by returning multiple formats depending on how it is called. In most cases, the URL will return HTML or XHTML by default. By adding the query parameter, tg_format=json you can switch from returning the default format to returning JSON data. You need to add an @expose(allow_json=True) decorator [3] to your controller method to tell TurboGears that this controller should return JSON data:

def amplify(self, data):

allow_json=True is a shortcut for this:

@expose("json", content_type="text/javascript",
        as_format="json", accept_format="text/javascript")

That means that the controller method will use the json template (uses TurboJson to marshal the returned data to JSON) to return data of type text/javascript when either of these conditions is met: a query param of tg_format=json or an Accept: text/javascript header is sent.

BaseClient in python-fedora 0.1.x and 0.2.x use the query parameter method of selecting JSON output. BaseClient and 0.3.x use both the header method and the query parameter since the argument was made that the header method is friendlier to other web frameworks. 0.4 intends to use the header alone. If you use allow_json=True this change shouldn’t matter. If you specify the @expose("json") decorator only accept_format is set. So it is advisable to include both as_format and accept_format in your decorator. Note that this is a limitation of TurboGears <= If your application is only going to run on TurboGears > you should be able to use a plain @expose("json") [4].


Why Two Formats from a Single URL?

When designing your URLs you might wonder why you’d want to return JSON and HTML from a single controller method instead of having two separate controller methods. For instance, separating the URLs into their own namespaces might seem logical: /app/json/get_user/USERNAME as opposed to /app/user/USERNAME. Doing things with two URLs as opposed to one has both benefits and drawbacks:

Benefits of One Method Handling Multiple Formats

  • Usually less code as there’s only one controller method
  • When a user sees a page that they want to get data from, they can get it as JSON instead of screen scraping.
  • Forces the application designer to think more about the API that is being provided to the users instead of just the needs of the web page they are creating.
  • Makes it easier to see what data an application will need to implement an alternate interface since you can simply look at the template code to see what variables are being used on a particular page.

Benefits of Multiple Methods for Each Format

  • Avoids special casing for error handlers (See below)
  • Separates URLs that you intend users to grab JSON data from URLs where you only want to display HTML.
  • Allows the URLs that support JSON to concentrate on trimming the size of the data sent while URLs that only return HTML can return whole objects.
  • Organization can be better if you don’t have to include all of the pages that may only be useful for user interface elements.

Personal use has found that allowing JSON requests on one controller method works well for cases where you want the user to get data and for traditional form based user interaction. AJAX requests have been better served via dedicated methods.

Return Values

The toplevel of the return values should be a dict. This is the natural return value for TurboGears applications.


All data should be encoded in JSON before being returned. This is normally taken care of automatically by TurboGears and simplejson. If you are returning non-builtin objects you may have to define an __json__() method.


simplejson (and probably other JSON libraries) will take care of encoding Unicode strings to JSON so be sure that you are passing Unicode strings around rather than encoded byte strings.

Error Handling

In python, error conditions are handled by raising an exception. However, an exception object will not propagate automatically through a return from the server. Instead we set several special variables in the returned data to inform BaseClient of any errors.

At present, when BaseClient receives an error it raises an exception of its own with the exception information from the server inside. Raising the same exception as the server is being investigated but may pose security risks so hasn’t yet been implemented.


All URLs which return JSON data should set the exc variable when the method fails unexpectedly (a database call failed, a place where you would normally raise an exception, or where you’d redirect to an error page if a user was viewing the HTML version of the web app). exc should be set to the name of an exception and tg_flash set to the message that would normally be given to the exception’s constructor. If the return is a success (expected values are being returned from the method or a value was updated successfully) exc may either be unset or set to None.


When viewing the HTML web app, tg_flash can be set with a message to display to the user either on the next page load or via an AJAX handler. When used in conjunction with JSON, exc=EXCEPTIONNAME, and BaseClient, tg_flash should be set to an error message that the client can use to identify what went wrong or display to the user. It’s equivalent to the message you would normally give when raising an exception.

Authentication Errors

Errors in authentication are a special case. Instead of returning an error with exc='AuthError' set, the server should return with response.status = 403. BaseClient will see the 403 and raise an AuthError.

This is the signal for the client to ask the user for new credentials (usually a new username and password).

Performing Different Actions when Returning JSON

So far we’ve run across three features of TurboGears that provide value to a web application but don’t work when returning JSON data. We provide a function that can code around this. fedora.tg.util.request_format() will return the format that the page is being returned as. Code can use this to check whether JSON output is expected and do something different based on it:

output = {'tg_flash': 'An Error Occurred'}
if fedora.tg.util.request_format() == 'json':
    output['exc'] = 'ServerError'
    output['tg_template'] = 'my.templates.error'
return output

In this example, we return an error through our “exception” mechanism if we are returning JSON and return an error page by resetting the template if not.


Redirects do not play well with JSON [5] because TurboGears is unable to turn the function returned from the redirect into a dictionary that can be turned into JSON.

Redirects are commonly used to express errors. This is actually better expressed using tg_template because that method leaves the URL intact. That allows the end user to look for spelling mistakes in their URL. If you need to use a redirect, the same recipe as above will allow you to split your code paths.

[5]Last checked in TurboGears 1.0.4


Setting what template is returned to a user by setting tg_template in the return dict (for instance, to display an error page without changing the URL) is a perfectly valid way to use TurboGears. Unfortunately, since JSON is simply another template in TurboGears you have to be sure not to interfere with the generation of JSON data. You need to check whether JSON was requested using fedora.tg.util.request_format() and only return a different template if that’s not the case. The recipe above shows how to do this.


Validators are slightly different than the issues we’ve encountered so far. Validators are used to check and convert parameters sent to a controller method so that only good data is dealt with in the controller method itself. The problem is that when a validator detects a parameter that is invalid, it performs a special internal redirect to a method that is its error_handler. We can’t intercept this redirect because it happens in the decorators before our method is invoked. So we have to deal with the aftermath of the redirect in the error_handler method:

class NotNumberValidator(turbogears.validators.FancyValidator):
    messages = {'Number': 'Numbers are not allowed'}

    def to_python(self, value, state=None):
            number = turbogears.validators.Number(value.strip())
            return value
        raise validators.Invalid(self.message('Number', state), value,

class AmplifyForm(turbogears.widgets.Form):
    template = my.templates.amplifyform
    submit_text = 'Enter word to amplify'
    fields = [

amplify_form = AmplifyForm()

class mycontroller(RootController):

    @expose(template='my.templates.errorpage', allow_json=True)
    def no_numbers(self, data):
        errors = fedora.tg.util.jsonify_validation_errors()
        if errors:
            return errors
        # Construct a dict to return the data error message as HTML via
        # the errorpage template

    @expose(template='my.templates.amplifypage', allow_json=True)
    def amplify(self, data):
        return dict(data=data.upper())

When a user hits amplify()‘s URL, the validator checks whether data is a number. If it is, it redirects to the error_handler, no_numbers(). no_numbers() will normally return HTML which is fine if we’re simply hitting amplify() from a web browser. If we’re hitting it from a BaseClient app, however, we need it to return JSON data instead. To do that we use jsonify_validation_errors() which checks whether there was a validation error and whether we need to return JSON data. If both of those are true, it returns a dictionary with the validation errors. This dictionary is appropriate for returning from the controller method in response to a JSON request.

Note When defining @error_handler() order of decorators can be important. The short story is to always make @validate() and @error_handler() the first decorators of your method. The longer version is that this is known to cause errors with the json request not being honored or skipping identity checks when the method is its own error handler.

Expected Methods

Certain controller methods are necessary in order for BaseClient to properly talk to your service. TurboGears can quickstart an application template for you that sets most of these variables correctly:

$ tg-admin quickstart -i -s -p my my
# edit my/my/controllers.py


You need to have a login() method in your application’s root. This method allows BaseClient to authenticate against your Service:

+    @expose(allow_json=True)
     def login(self, forward_url=None, previous_url=None, *args, **kw):

         if not identity.current.anonymous \
             and identity.was_login_attempted() \
             and not identity.get_identity_errors():
+            # User is logged in
+            if 'json' == fedora.tg.util.request_format():
+                return dict(user=identity.current.user)
+            if not forward_url:
+                forward_url = turbogears.url('/')
             raise redirect(forward_url)

For non-TurboGears Implementors

If you are implementing a server in a non-TurboGears framework, note that one of the ways to reach the login() method is through special parameters parsed by the TurboGears framework. BaseClient uses these parameters instead of invoking the login() method directly as it saves a round trip when authenticating to the server. It will be necessary for you to implement handling of these parameters (passed via POST) on your application as well.

The parameters are: user_name, password, and login. When these three parameters are sent to the server, the server authenticates the user and records their information before deciding what information to return to them from the URL.


The logout() method is similar to login(). It also needs to be modified to allow people to connect to it via JSON:

-    @expose()
+    @expose(allow_json=True)
     def logout(self):
+        if 'json' in fedora.tg.util.request_format():
+            return dict()
         raise redirect("/")

Using SABase

fedora.tg.json contains several functions that help to convert SQLAlchemy objects into JSON. For the most part, these do their work behind the scenes. The SABase object, however, is one that you might need to take an active role in using.

When you return an SQLAlchemy object in a controller to a template, the template is able to access any of the relations mapped to it. So, instead of having to construct a list of people records from a table and the the list of groups that each of them are in you can pass in the list of people and let your template reference the relation properties to get the groups. This is extremely convenient for templates but has a negative effect when returning JSON. Namely, the default methods for marshaling SQLAlchemy objects to JSON only return the attributes of the object, not the relations that are linked to it. So you can easily run into a situation where someone querying the JSON data for a page will not have all the information that a template has access to.

SABase fixes this by allowing you to specify relations that your SQLAlchemy backed objects should marshal as JSON data.

Further information on SABase can be found in the API documentation:

pydoc fedora.tg.json


SABase is a base class that you can use when defining objects in your project’s model. So the first step is defining the classes in your model to inherit from SABase:

from fedora.tg.json import SABase
from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey
from turbogears.database import metadata, mapper

class Person(SABase):
PersonTable = Table('person', metadata
    Column('name', String, primary_key=True),

class Address(SABase):
AddressTable = Table (
    Column('id', Integer, primary_key=True),
    Column('street', string),
    Column('person_id', Integer, ForeignKey('person.name')

mapper(PersonTable, Person)
mapper(AddressTable, Address, properties = {
    person: relation(Person, backref = 'addresses'),

The next step is to tell SABase which properties should be copied (this allows you to omit large trees of objects when you only need the data from a few of them):

def my_info(self):
    person = Person.query.filter_by(name='Myself').one()
    person.jsonProps = {'Person': ['addresses']}
    return dict(myself=person}

Now, when someone requests JSON data from my_info, they should get back a record for person that includes a property addresses. Addresses will be a list of address records associated with the person.

How it Works

SABase adds a special __json__() method to the class. By default, this method returns a dict with all of the attributes that are backed by fields in the database.

Adding entries to jsonProps adds the values for those properties to the returned dict as well. If you need to override the __json__() method in your class you probably want to call SABase’s __json__() unless you know that neither you nor any future subclasses will need it.

Using __json__()

Sometimes you need to return an object that isn’t a basic python type (list, tuple, dict, number. string, etc). When that occurs, simplejson won’t know how to marshal the data into JSON until you write own method to transform the values. If this method is named __json__(), TurboGears will automatically perform the conversion when you return the object.


class MyObject(object):
    def _init__(self, number):
        self.someNumber = number
        self.cached = None

    def _calc_data(self):
        if not self.cached:
            self.cached = self.someNumber * 2
        return self.cached

    twiceData = property(_calc_data)

    def __json__(self):
        return {'someNumber': self.someNumber, 'twiceData': self.twiceData}

In this class, you have a variable and a property. If you were to return it from a controller method without defining the __json__() method, TurboGears would give you an error that it was unable to adapt the object to JSON. The JSON method transforms the object into a dict with sensibly named values for the variable and property so that simplejson is able to marshal the data to JSON. Note that you will often have to choose between space (more data takes more bandwidth to deliver to the end user) and completeness (you need to return enough data so the client isn’t looking for another method that can complete its needs) when returning data.