Add photo_client for fetching data from the PhotoAPI

Also fix an issue with dataclasses not being loaded when using python 3.6

.flake8

 [flake8]
 max-line-length=99
-exclude=.venv,venv,.tox
+exclude=.venv,venv,.tox,env

identitylib/photo_client.py

+from oauthlib.oauth2 import BackendApplicationClient
+from requests_oauthlib import OAuth2Session
+from functools import lru_cache
+from requests import auth
+
+
+class NoSuchPhotoException(Exception):
+    def __init__(self):
+        super().__init__('No photo found for query criteria')
+
+
+class PhotoClient:
+    """
+    A client which allows information to be fetched from the University Photo API.
+
+    """
+
+    def __init__(
+        self,
+        client_id: str,
+        client_secret: str,
+        base_url: str = 'https://api.apps.cam.ac.uk/photo/v1beta1/',
+        token_url: str = 'https://api.apps.cam.ac.uk/oauth/client_credential/accesstoken'
+    ):
+        self.client_id = client_id
+        self.client_secret = client_secret
+
+        self.base_url = base_url.rstrip('/')
+        self.token_url = token_url
+
+    @property
+    @lru_cache()
+    def session(self):
+        """
+        Lazy-init a OAuth2-wrapped session and fetch a token before it's used.
+
+        """
+
+        client = BackendApplicationClient(client_id=self.client_id)
+        session = OAuth2Session(client=client)
+
+        session.fetch_token(
+            token_url=self.token_url,
+            auth=auth.HTTPBasicAuth(self.client_id, self.client_secret)
+        )
+        return session
+
+    def get_transient_image_url(self, photo_id: str):
+        """
+        Returns a transient image url which can be used to by unauthenticated clients
+        to fetch the url of the given photo id.
+
+        """
+        response = self.session.get(
+            f'{self.base_url}/photos/{photo_id}/content', allow_redirects=False
+        )
+        if response.status_code == 404:
+            raise NoSuchPhotoException()
+        response.raise_for_status()
+
+        if not response.headers.get('Location'):
+            raise RuntimeError('Photo API did not return a transient image url within redirect')
+
+        return response.headers['Location']

requirements-tests.txt

 coverage
 pytest
-pytest-cov
\ No newline at end of file
+pytest-cov
+requests_mock

requirements.txt

-dataclasses
\ No newline at end of file
+dataclasses
+
+# for the API clients
+requests
+requests_oauthlib

tests/test_identifiers.py

 from unittest import TestCase
 
-from identitylib.identifiers import Identifier, IdentifierSchemes
+from identitylib.identifiers import Identifier, IdentifierSchemes, IdentifierScheme
 
 
 class IdentifiersTestCase(TestCase):
@@ -75,3 +75,14 @@ class IdentifiersTestCase(TestCase):
             'Invalid identifier scheme barcode.identifiers.lib.cam.ac.uk'
         ):
             Identifier.from_string('VBE@barcode.identifiers.lib.cam.ac.uk')
+
+    def test_identifier_schemes_can_be_compared(self):
+        self.assertTrue(
+            IdentifierSchemes.PHOTO == IdentifierScheme(
+                'photo.v1.photo.university.identifiers.cam.ac.uk',
+                'Photo Identifier',
+                {
+                    'deprecated': 'photo_id.photo.identifiers.uis.cam.ac.uk',
+                }
+            )
+        )

tests/test_photo_client.py

+from unittest import TestCase
+from requests import Session, HTTPError
+from requests_mock import Mocker
+from base64 import b64encode
+
+from identitylib import photo_client
+
+
+class PhotoClientTestCase(TestCase):
+
+    @Mocker()
+    def setUp(self, request_mock: Mocker):
+        """
+        Setup the photo client here in order to intercept the initial auth request.
+
+        """
+        auth_request = request_mock.post(
+            'https://api.apps.cam.ac.uk/oauth/client_credential/accesstoken',
+            json={
+                'token_type': 'Bearer',
+                'access_token': 'fake-token',
+            }
+        )
+
+        self.client = photo_client.PhotoClient('mock-client-id', 'mock-client-key')
+        self.assertIsInstance(self.client.session, Session)
+
+        self.assertEqual(len(auth_request.request_history), 1)
+        self.assertEqual(
+            auth_request.request_history[0].headers['Authorization'],
+            f'Basic {b64encode(b"mock-client-id:mock-client-key").decode("utf-8")}'
+        )
+
+    @Mocker()
+    def test_can_fetch_transient_photo_url(self, request_mock: Mocker):
+        request_mock.get(
+            f'{self.client.base_url}/photos/test-photo-uuid-123/content',
+            status_code=302,
+            headers={
+                'Location': 'https://storage.google.com/test-photo-uuid-123?expiresIn=10'
+            }
+        )
+
+        photo_url = self.client.get_transient_image_url('test-photo-uuid-123')
+        self.assertEqual(photo_url, 'https://storage.google.com/test-photo-uuid-123?expiresIn=10')
+
+    @Mocker()
+    def test_will_throw_if_no_photo_for_transient_url_query(self, request_mock: Mocker):
+        request_mock.get(
+            f'{self.client.base_url}/photos/no-photo-available/content', status_code=404,
+        )
+
+        with self.assertRaises(photo_client.NoSuchPhotoException):
+            self.client.get_transient_image_url('no-photo-available')
+
+    @Mocker()
+    def test_will_throw_if_unexpected_response_for_transient_url(self, request_mock: Mocker):
+        request_mock.get(
+            f'{self.client.base_url}/photos/test-photo-uuid/content', status_code=200,
+            json={'success': True}
+        )
+
+        with self.assertRaisesRegex(
+            RuntimeError, 'Photo API did not return a transient image url within redirect'
+        ):
+            self.client.get_transient_image_url('test-photo-uuid')
+
+    @Mocker()
+    def test_will_throw_if_photo_api_fails_for_transient_url_query(self, request_mock: Mocker):
+        request_mock.get(
+            f'{self.client.base_url}/photos/something-will-break/content', status_code=500,
+            json={'success': True}
+        )
+
+        with self.assertRaises(HTTPError):
+            self.client.get_transient_image_url('something-will-break')