| Authors: | Toshio Kuratomi Luke Macken |
|---|---|
| Date: | 28 May 2008 |
| For Version: | 0.3.x |
The client module allows you to easily code an application that talks to a Fedora Service. It handles the details of decoding the data sent from the Service into a python data structure and raises an Exception if an error is encountered.
.. toctree::
The :class:`~fedora.client.BaseClient` class is the basis of all your interactions with the server. It is flexible enough to be used as is for talking with a service but is really meant to be subclassed and have methods written for it that do the things you specifically need to interact with the Fedora Service you care about. Authors of a Fedora Service are encouraged to provide their own subclasses of :class:`~fedora.client.BaseClient` that make it easier for other people to use a particular Service out of the box.
If you don't want to subclass, you can use :class:`~fedora.client.BaseClient`
as a utility class to talk to any Fedora Service. There's three steps to
this. First you import the :class:`~fedora.client.BaseClient` and Exceptions
from the fedora.client module. Then you create a new
:class:`~fedora.client.BaseClient` with the URL that points to the root of the
Fedora Service you're interacting with. Finally, you retrieve data from a
method on the server. Here's some code that illustrates the process:
from fedora.client import BaseClient, AppError, ServerError
client = BaseClient('https://admin.fedoraproject.org/pkgdb')
try:
collectionData = client.send_request('/collections', auth=False)
except ServerError as e:
print('%s' % e)
except AppError as e:
print('%s: %s' % (e.name, e.message))
for collection in collectionData['collections']:
print('%s %s' % (collection['name'], collection['version'])
In our example we only provide BaseClient() with the URL fragment it uses
as the base of all requests. There are several more optional parameters that
can be helpful.
If you need to make an authenticated request you can specify the username and
password to use when you construct your :class:`~fedora.client.BaseClient`
using the username and password keyword arguments. If you do not use
these, authenticated requests will try to connect via a cookie that was saved
from previous runs of :class:`~fedora.client.BaseClient`. If that fails as
well, :class:`~fedora.client.BaseClient` will throw an Exception which you
can catch in order to prompt for a new username and password:
from fedora.client import BaseClient, AuthError
import getpass
MAX_RETRIES = 5
client = BaseClient('https://admin.fedoraproject.org/pkgdb',
username='foo', password='bar')
# Note this is simplistic. It only prompts once for another password.
# Your application may want to loop through this several times.
while (count < MAX_RETRIES):
try:
collectionData = client.send_request('/collections', auth=True)
except AuthError as e:
client.password = getpass.getpass('Retype password for %s: ' % username)
else:
# data retrieved or we had an error unrelated to username/password
break
count = count + 1
Warning
Note that although you can set the username and password as shown
above you do have to be careful in cases where your application is
multithreaded or simply processes requests for more than one user with the
same :class:`~fedora.client.BaseClient`. In those cases, you can
accidentally overwrite the username and password between two
requests. To avoid this, make sure you instantiate a separate
:class:`~fedora.client.BaseClient` for every thread of control or for
every request you handle or use :class:`~fedora.client.ProxyClient`
instead.
The useragent parameter is useful for identifying in log files that your
script is calling the server rather than another. The default value is
Fedora BaseClient/VERSION where VERSION is the version of the
:class:`~fedora.client.BaseClient` module. If you want to override this just
give another string to this:
client = BaseClient('https://admin.fedoraproject.org/pkgdb',
useragent='Package Database Client/1.0')
The debug parameter turns on a little extra output when running the
program. Set it to true if you're having trouble and want to figure out what
is happening inside of the :class:`~fedora.client.BaseClient` code.
send_request() is what does the heavy lifting of making a request of the
server, receiving the reply, and turning that into a python dictionary. The
usage is pretty straightforward.
The first argument to send_request() is method. It contains the name
of the method on the server. It also has any of the positional parameters
that the method expects (extra path information interpreted by the server for
those building non-TurboGears applications).
The auth keyword argument is a boolean. If True, the session cookie for
the user is sent to the server. If this fails, the username and
password are sent. If that fails, an Exception is raised that you can
handle in your code.
req_params contains a dictionary of additional keyword arguments for the
server method. These would be the names and values returned via a form if it
was a CGI. Note that parameters passed as extra path information should be
added to the method argument instead.
An example:
import BaseClient
client = BaseClient('https://admin.fedoraproject.org/pkgdb/')
client.send_request('/package/name/python-fedora', auth=False,
req_params={'collectionVersion': '9', 'collectionName': 'Fedora'})
In this particular example, knowing how the server works, /packages/name/
defines the method that the server is going to invoke. python-fedora is a
positional parameter for the name of the package we're looking up.
auth=False means that we'll try to look at this method without having to
authenticate. The req_params sends two additional keyword arguments:
collectionName which specifies whether to filter on a single distro or
include Fedora, Fedora EPEL, Fedora OLPC, and Red Hat Linux in the output and
collectionVersion which specifies which version of the distribution to
output for.
The URL constructed by :class:`~fedora.client.BaseClient` to the server could be expressed as[#]_:
https://admin.fedoraproject.org/pkgdb/package/name/python-fedora/?collectionName=Fedora&collectionVersion=9
In previous releases of python-fedora, there would be one further query
parameter: tg_format=json. That parameter instructed the server to
return the information as JSON data instead of HTML. Although this is usually
still supported in the server, :class:`~fedora.client.BaseClient` has
deprecated this method. Servers should be configured to use an Accept
header to get this information instead. See the JSON output section of the
Fedora Service documentation for more information about the server side.
Building a client using subclassing builds on the information you've already seen inside of :class:`~fedora.client.BaseClient`. You might want to use this if you want to provide a module for third parties to access a particular Fedora Service. A subclass can provide a set of standard methods for calling the server instead of forcing the user to remember the URLs used to access the server directly.
Here's an example that turns the previous calls into the basis of a python API to the Fedora Package Database:
import getpass
import sys
from fedora.client import BaseClient, AuthError
class MyClient(BaseClient):
def __init__(self, baseURL='https://admin.fedoraproject.org/pkgdb',
username=None, password=None,
useragent='Package Database Client/1.0', debug=None):
super(BaseClient, self).__init__(baseURL, username, password,
useragent, debug)
def collection_list(self):
'''Return a list of collections.'''
return client.send_request('/collection')
def package_owners(self, package, collectionName=None,
collectionVersion=None):
'''Return a mapping of release to owner for this package.'''
pkgData = client.send_request('/packages/name/%s' % (package),
{'collectionName': collectionName,
'collectionVersion': collectionVersion})
ownerMap = {}
for listing in pkgData['packageListings']:
ownerMap['-'.join(listing['collection']['name'],
listing['collection']['version'])] = \
listing['owneruser']
return ownerMap
A few things to note:
- In our constructor we list a default
baseURLanduseragent. This is usually a good idea as we know the URL of the Fedora Service we're connecting to and we want to know that people are using our specific API. - Sometimes we'll want methods that are thin shells around the server methods
like
collection_list(). Other times we'll want to do more post processing to get specific results aspackage_owners()does. Both types of methods are valid if they fit the needs of your API. If you find yourself writing more of the latter, though, you may want to consider getting a new method implemented in the server that can return results more appropriate to your needs as it could save processing on the server and bandwidth downloading the data to get information that more closely matches what you need.
See pydoc fedora.client.fas2 for a module that implements a standard
client API for the Fedora Account System
:class:`~fedora.client.BaseClient` will throw a variety of errors that can be caught to tell you what kind of error was generated.
FedoraServiceError: | The base of all exceptions raised by :class:`~fedora.client.BaseClient`. If your code needs to catch any of the listed errors then you can catch that to do so. |
|---|---|
ServerError: | Raised if there's a problem communicating with the service. For instance, if we receive an HTML response instead of JSON. |
AuthError: | If something happens during authentication, like an invalid
usernsme or password, AuthError will be raised. You can catch this to
prompt the user for a new usernsme. |
AppError: | If there is a server side error when processing a request,
the Fedora Service can alert the client of this by setting certain
flags in the response. :class:`~fedora.client.BaseClient` will see these
flags and raise an AppError. The name of the error will be stored in
AppError's name field. The error's message will be stored in
message. |
Here's an example of the exceptions in action:
from fedora.client import ServerError, AuthError, AppError, BaseClient
import getpass
MAXRETRIES = 5
client = BaseClient('https://admin.fedoraproject.org/pkgdb')
for retry in range(0, MAXRETRIES):
try:
collectionData = client.send_request('/collections', auth=True)
except AuthError as e:
from six.moves import input
client.username = input('Username: ').strip()
client.password = getpass.getpass('Password: ')
continue
except ServerError as e:
print('Error talking to the server: %s' % e)
break
except AppError as e:
print('The server issued the following exception: %s: %s' % (
e.name, e.message))
for collection in collectionData['collections']:
print('%s %s' % (collection[0]['name'], collection[0]['version']))
Applications that use OpenId to authenticate are not able to use the standard BaseClient because the pattern of authenticating is very different. We've written a separate client object called :class:`~fedora.client.OpenIdBaseClient` to do this.