server/tags: add single tag retrieving

API.md

@@ -180,7 +180,27 @@ Not yet implemented.
 
 
 ## Getting tag
-Not yet implemented.
+- **Request**
+
+    `GET /tag/<name>`
+
+- **Output**
+
+    ```json5
+    {
+        "tag": <tag>
+    }
+    ```
+    ...where `<tag>` is a [tag resource](#tag).
+
+- **Errors**
+
+    - the tag does not exist
+    - privileges are too low
+
+- **Description**
+
+    Retrieves information about an existing tag.
 
 
 ## Deleting tag

server/szurubooru/api/tag_api.py

@@ -33,8 +33,12 @@ class TagListApi(BaseApi):
         return {'tag': _serialize_tag(tag)}
 
 class TagDetailApi(BaseApi):
-    def get(self, ctx):
-        raise NotImplementedError()
+    def get(self, ctx, tag_name):
+        auth.verify_privilege(ctx.user, 'tags:view')
+        tag = tags.get_by_name(ctx.session, tag_name)
+        if not tag:
+            raise tags.TagNotFoundError('Tag %r not found.' % tag_name)
+        return {'tag': _serialize_tag(tag)}
 
     def put(self, ctx, tag_name):
         tag = tags.get_by_name(ctx.session, tag_name)

server/szurubooru/tests/api/test_tag_retrieving.py

@@ -0,0 +1,58 @@
+import datetime
+import pytest
+from szurubooru import api, db, errors
+from szurubooru.util import misc, tags
+
+@pytest.fixture
+def test_ctx(
+        session, context_factory, config_injector, user_factory, tag_factory):
+    config_injector({
+        'privileges': {
+            'tags:list': 'regular_user',
+            'tags:view': 'regular_user',
+        },
+        'thumbnails': {'avatar_width': 200},
+        'ranks': ['anonymous', 'regular_user', 'mod', 'admin'],
+        'rank_names': {'regular_user': 'Peasant'},
+    })
+    ret = misc.dotdict()
+    ret.session = session
+    ret.context_factory = context_factory
+    ret.user_factory = user_factory
+    ret.tag_factory = tag_factory
+    ret.detail_api = api.TagDetailApi()
+    return ret
+
+def test_retrieving_single(test_ctx):
+    test_ctx.session.add(test_ctx.tag_factory(names=['tag']))
+    result = test_ctx.detail_api.get(
+        test_ctx.context_factory(
+            input={'query': '', 'page': 1},
+            user=test_ctx.user_factory(rank='regular_user')),
+        'tag')
+    assert result == {
+        'tag': {
+            'names': ['tag'],
+            'category': 'dummy',
+            'creationTime': datetime.datetime(1996, 1, 1),
+            'lastEditTime': None,
+            'suggestions': [],
+            'implications': [],
+        }
+    }
+
+def test_retrieving_non_existing(test_ctx):
+    with pytest.raises(tags.TagNotFoundError):
+        test_ctx.detail_api.get(
+            test_ctx.context_factory(
+                input={'query': '', 'page': 1},
+                user=test_ctx.user_factory(rank='regular_user')),
+            '-')
+
+def test_retrieving_single_without_privileges(test_ctx):
+    with pytest.raises(errors.AuthError):
+        test_ctx.detail_api.get(
+            test_ctx.context_factory(
+                input={'query': '', 'page': 1},
+                user=test_ctx.user_factory(rank='anonymous')),
+            '-')