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A python library for interacting with HAL+JSON APIs

Project description

REST Navigator

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REST Navigator is a python library for interacting with hypermedia apis
(`REST level
3 <>`__).
Right now, it only supports
`HAL+JSON <>`__ but it
should be general enough to extend to other formats eventually. Its
first goal is to make interacting with HAL hypermedia apis as painless
as possible, while discouraging REST anti-patterns.

To install it, simply use pip:

.. code:: bash

$ pip install restnavigator


- `How to use it <#how-to-use-it>`__

- `Links <#links>`__
- `GET requests <#get-requests>`__
- `Link relation docs <#link-relation-docs>`__
- `POST requests <#post-requests>`__
- `Errors <#errors>`__
- `Templated links <#templated-links>`__
- `Authentication <#authentication>`__

- `Additional Topics <#additional-topics>`__

- `Identity Map <#identity-map>`__
- `Iterating over a Navigator <#iterating-over-a-navigator>`__
- `Headers (Request vs. Response) <#headers-request-vs-response>`__
- `Bracket mini-language <#bracket-minilanguage>`__
- `Finding the right link <#finding-the-right-link>`__
- `Default curie <#default-curie>`__
- `Specifying an api name <#specifying-an-api-name>`__
- `Embedded documents <#embedded-documents>`__

- `Development <#development>`__

- `Testing <#testing>`__
- `Planned for the future <#planned-for-the-future>`__

.. raw:: html

<!-- end toc -->

How to use it

To begin interacting with a HAL api, you've got to create a HALNavigator
that points to the api root. Ideally, in a hypermedia API, the root URL
is the only URL that needs to be hardcoded in your application. All
other URLs are obtained from the api responses themselves (think of your
api client as 'clicking on links', rather than having the urls

As an example, we'll connect to the haltalk api.

.. code:: python

>>> from restnavigator import Navigator
>>> N = Navigator.hal('', default_curie="ht")
>>> N


Usually, with the index (normally at the api root), you're most
interested in the links. Let's look at those:

.. code:: python

>>> N.links()
{u'ht:users': HALNavigator(Haltalk.users),
u'ht:signup': HALNavigator(Haltalk.signup),
u'ht:me': TemplatedThunk(Haltalk.users.{name}),
u'ht:latest-posts': HALNavigator(Haltalk.posts.latest)}

(This may take a moment because asking for the links causes the
HALNavigator to actually request the resource from the server).

Here we can see that the links are organized by their relation type (the
key), and each key corresponds to a new HALNavigator that represents
some other resource. Relation types are extremely important in restful
apis: we need them to be able to determine what a link means in relation
to the current resource, in a way that is automatable.

GET requests

In addition, the root has some state associated with it which you can
get in two different ways:

.. code:: python

>>> N() # cached state of resource (obtained when we looked at N.links)
{u'hint_1': u'You need an account to post stuff..',
u'hint_2': u'Create one by POSTing via the ht:signup link..',
u'hint_3': u'Click the orange buttons on the right to make POST requests..',
u'hint_4': u'Click the green button to follow a link with a GET request..',
u'hint_5': u'Click the book icon to read docs for the link relation.',
u'welcome': u'Welcome to a haltalk server.'}
>>> N.fetch() # will refetch the resource from the server
{u'hint_1': u'You need an account to post stuff..',
u'hint_2': u'Create one by POSTing via the ht:signup link..',
u'hint_3': u'Click the orange buttons on the right to make POST requests..',
u'hint_4': u'Click the green button to follow a link with a GET request..',
u'hint_5': u'Click the book icon to read docs for the link relation.',
u'welcome': u'Welcome to a haltalk server.'}

Calling a HALNavigator will execute a GET request against the resource
and returns its value (which it will cache).

Link relation docs

Let's register a hal talk account. Unfortunately, we don't really know
how to do that, so let's look at the documentation. The ``ht:signup``
link looks promising, let's check that:

.. code:: python

>>> N.docsfor('ht:signup')

A browser will open to

What? Popping up a browser from a library call? Yes, that's how
rest\_navigator rolls. The way we see it: docs are for humans, and while
custom rel-types are URIs, they shouldn't automatically be dereferenced
by a program that interacts with the api. So popping up a browser serves
two purposes:

1. It allows easy access to the documentation at the time when you most
need it: when you're mucking about in the command line trying to
figure out how to interact with the api.
2. It reminds you not to try to automatically dereference the rel
documentation and parse it in your application.

If you need a more robust way to browse the api and the documentation,
`HAL Browser <>`__ is probably
your best bet.

POST requests

The docs for ``ht:signup`` explain the format of the POST request to
sign up. So let's actually sign up. Since we've set ``"ht"`` as our
default curie, we can skip typing the curie for convenience. (Note:
haltalk is a toy api for example purposes, don't ever send plaintext
passwords over an unencrypted connection in a real app!):

.. code:: python

>>> fred23 = N['signup'].create(
... {'username': 'fred23',
... 'password': 'hunter2',
... 'real_name': 'Fred 23'}
... )
>>> fred23


If the user name had already been in use, a 400 would have been returned
from the haltalk api. rest\_navigator follows the Zen of Python
guideline "Errors should never pass silently". An exception would have
been raised on a 400 or 500 status code. You can squelch this exception
and just have the post call return a ``HALNavigator`` with a 400/500
status code if you want:

.. code:: python

>>> dup_signup = N['ht:signup'].create({
... 'username': 'fred23',
... 'password': 'hunter2',
... 'real_name': 'Fred Wilson'
... }, raise_exc=False)
>>> dup_signup
OrphanHALNavigator(Haltalk.signup) # 400!
>>> dup_signup.status
(400, 'Bad Request')
>>> dup_signup.state
{u"errors": {u"username": [u"is already taken"]}}

Templated links

Now that we've signed up, lets take a look at our profile. The link for
a user's profile is a templated link, which restnavigator represents as
a ``PartialNavigator``. Similar to python's
`functools.partial <>`__,
a ``PartialNavigator`` is an object that needs a few more arguments to
give you a full navigator back. Despite its name, it can't talk to the
network by itself. Its job is to to generate new navigators for you. You
can see what variables it has by looking at its ``.variables`` attribute
(its ``__repr__`` hints at this as well):

.. code:: python

>>> N.links().keys()
['ht:latest-posts', 'ht:me', 'ht:users', 'ht:signup']
>>> N['ht:me']
>>> N['ht:me'].variables

The documentation for the ``ht:me`` rel type should tell us how the name
parameter is supposed to work, but in this case it's fairly obvious
(plug in the username). Two provide the template parameters, just call
it with keyword args:

.. code:: python

>>> partial_me = N['ht:me']
>>> partial_me.template_uri
>>> Fred = partial_me(name='fred23')
>>> Fred

Now that we have a real navigator, we can fetch the resource:

.. code:: python

>>> Fred()
{u'bio': None, u'real_name': u'Fred Wilson', u'username': u'fred23'}


In order to post something to haltalk, we need to authenticate with our
newly created account. HALNavigator allows any `authentication method
that requests
supports <>`__
(so OAuth etc). For basic auth (which haltalk uses), we can just pass a

.. code:: python

>>> N.authenticate(('fred23', 'hunter2')) # All subsequent calls are authenticated

This doesn't send anything to the server, it just sets the
authentication details that we'll use on the next request. Other
authentication methods may contact the server immediately.

Now we can put it all together to create a new post:

.. code:: python

>>> N_post = N['me'](name='fred23')['posts'].create({'content': 'My first post'})
>>> N_post
>>> N_post()
{'content': 'My first post', 'created_at': '2015-06-13T19:38:59+00:00'}

It is also possible to specify a custom requests Session object when creating
a new navigator.

For example, if you want to talk to a OAuth2 protected api, simply pass
an OAuth2 Session object that will be used for all requests
done by HALNavigator:

.. code:: python

>>> from requests_oauthlib import OAuth2Session
>>> oauth2_session = OAuth2Session(r'client_id', token='token')
>>> N = Navigator.hal('', session=oauth2_session)

Additional Topics

Identity Map

You don't need to worry about inadvertently having two different
navigators pointing to the same resource. rest\_navigator will reuse the
existing navigator instead of creating a new one

Iterating over a Navigator

If a resource has a link with the rel "next", the navigator for that
resource can be used as a python iterator. It will automatically raise a
StopIteration exception if a resource in the chain does not have a next
link. This makes moving through paged resources really simple and

.. code:: python

post_navigator = fred['ht:posts']
for post in post_navigator:
# the first post will be post_navigator itself

Headers (Request vs. Response)

HTTP response headers are available in ``N.response.headers``

Headers that will be sent on each request can be obtained through the

.. code:: python

>>> N.session.headers
# Cookies, etc

Bracket mini-language

The bracket (``[]``) operator on Navigators has a lot of power. As we
saw earlier, the main use is to get a new Navigator from a link

.. code:: python

>>> N2 = N['curie:link_rel']

But, it can also go more than one link deep, which is equivalent to
using multiple brackets in a row:

.. code:: python

>>> N3 = N['curie:first_link', 'curie:second_link']
# equivalent to:
N3 = N['curie:first_link']['curie:second_link']

And of course, if you set a default curie, you can omit it:

.. code:: python

>>> N3 = N['first_link', 'second_link']

Internally, this is completely equivalent to repeatedly applying the
bracket operator, so you can even use it to jump over intermediate
objects that aren't Navigators themselves:

.. code:: python

>>> N['some-link', 3, 'another-link']

This would use the ``some-link`` link relation, select the third link
from the list, and then follow ``another-link`` from that resource.

Finding the right link

Normally, you can chain together brackets to jump from one resource to
another in one go:

.. code:: python

>>> N['ht:widget']['ht:gadget']

This will return a Navigator for the ``ht:widget`` link relation and
then immediately fetch the resource and return a Navigator for the
``ht:gadget`` link relation. This works great if you have only one link
per relation, but HAL allows multiple links per relation. Say for
instance we have some links like the following:

.. code:: javascript{

"ht:some_rel: [
"href": "/api/widget/1",
"name": "widget1",
"profile": "widget"
"href": "/api/widget/2",
"name": "widget2",
"profile": "widget"
"href": "/api/gadget/1",
"name": "gadget1",
"profile": "gadget"

When we go to get the ``ht:some_rel``, we'll get multiple results:

.. code:: python

>>> N['ht:some_rel']

How do we know which one is the one we want? The `HAL
spec <>`__
says links with the same rel can be disambiguated by the ``name`` link

.. code:: python

>>> N.links['ht:some_rel'].get_by('name', 'gadget1')
>>> N.links['ht:some_rel'].named('gadget1') # same as previous

We could also use other properties to slice and dice the list:

.. code:: python

>>> N.links['ht:some_rel'].get_by('profile', 'gadget')
>>> N.links['ht:some_rel'].getall_by('profile', 'widget')
[HALNavigator(api.widget[1]), HALNavigator(api.widget[2])]

This works for any property on links, not just the standard HAL

Default curie

You may specify a default curie when creating your Navigator:

.. code:: python

>>> N = HALNavigator('', curie='ht')

Now, when you follow links, you may leave off the default curie if you

.. code:: python

>>> N.links
{'ht:users': [HALNavigator(Haltalk.users)],
'ht:signup': [HALNavigator(Haltalk.signup)],
'ht:me': [HALNavigator(Haltalk.users.{name})],
'ht:latest-posts': [HALNavigator(Haltalk.posts.latest)]
>>> N['ht:users']
>>> N['users']

The only exception is where the key being supplied is a `IANA registered
relation <>`__,
and there is a conflict (hint: this should be quite rare):

.. code:: python

>>> N.links
{'ht:next': HALNavigator(Haltalk.unregistered),
'next': HALNavigator(Haltalk.registered)}
>>> N['next']

Specifying an api name

Sometimes the automatic api naming guesses poorly. If you'd like to
override the default name, you can specify it when creating the

.. code:: python

>>> N = Navigator.hal('', apiname='MySpecialAPI')

Embedded documents

In rest\_navigator, embedded documents are treated transparently. This
means that in many cases you don't need to worry about whether a
document is embedded or whether it's just linked.

As an example, assume we have a resource like the following:

.. code:: json

"_links": {
"xx:yams": {
"href": "/yams"
"_embedded": {
"xx:pickles": {
"_links": {
"self": {"href": "/pickles"}
"state": "A pickle"

From here, you would access both the ``yams`` and the ``pickles``
resource with normal bracket syntax:

.. code:: python

>>> Yams = N['xx:yams']
>>> Pickles = N['xx:pickles']

The only difference here is that ``Yams`` hasn't been fetched yet, while
``Pickles`` is considered "resolved" already because we got it as an
embedded document.


>>> Yams.resolved
>>> Yams.state # None
>>> Pickles.resolved
>>> Pickles.state
{'state': 'A pickle'}

If an embedded document has a self link, you can treat it just like you
would any other resource. So if you want to refresh the resource, it's
as easy as:

.. code:: python

>>> Pickles.fetch()

This will fetch the current state of the resource from the uri in its
self link, even if you've never directly requested that uri before. If
an embedded resource doesn't have a self link, it will be an
``OrphanNavigator`` with the parent set to the resource it was embedded

Of course, if you need to directly distinguish between linked resources
and embedded resources, there is an out:

.. code:: python

>>> N.embedded()
{'xx:pickles': HALNavigator(api.pickles)
>>> N.links()
{'xx:yams': HALNavigator(api.yams)

However, when using the ``in`` operator, it will look in both for a key
you're interested in:

.. code:: python

>>> 'yams' in N # default curie is taken into account!
>>> 'xx:yams in N
>>> 'xx:pickles' in N



To run tests, first install the `pytest
framework <>`__:


$ pip install -U pytest

To run tests, execute following from the root of the source directory:


$ py.test

Planned for the future

- Ability to add hooks for different types, rels and profiles. If a
link has one of these properties, it will call your hook when doing a
server call.
- Since HAL doesn't specify what content type POSTs, PUTs, and PATCHes
need to have, you can specify the hooks based on what the server will
accept. This can trigger off either the rel type of the link, or rest
navigator can do content negotiation over HTTP with the server
directly to see what content types that resource will accept.


Thanks very much to rest navigator's contributors:

- `dudycooly <>`__
- `bubenkoff <>`__
- `bbsgfalconer <>`__

.. |Build Status| image::
.. |Coverage Status| image::
.. |Documentation Status| image::
:alt: Documentation Status
.. |Pypi Status| image::





- Embedded support
- Ability to specify default curies
- Resources with no URL are now represented by a special Navigator type called OrphanNavigators
- IP addresses can be used in the url (@dudycooly)
- All tests pass in python 2.6 -> 3.4 (@bubenkoff), and travis now runs tox to ensure they stay that way
- Support the DELETE, and PATCH methods
- posts allow an empty body (@bbsgfalconer)
- Much improved content negotiation (@bbsgfalconer)
- There was also a major refactoring that changed how Navigators are created and internally cleaned up a
lot of really messy code.

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