render_cell(value) plugin hook

Still needs performance testing before I merge this into master

tests/fixtures.py

@@ -208,6 +208,8 @@ def extra_js_urls():
 
 PLUGIN2 = '''
 from datasette import hookimpl
+import jinja2
+import json
 
 
 @hookimpl
@@ -216,6 +218,30 @@ def extra_js_urls():
         'url': 'https://example.com/jquery.js',
         'sri': 'SRIHASH',
     }, 'https://example.com/plugin2.js']
+
+
+@hookimpl
+def render_cell(value):
+    # Render {"href": "...", "label": "..."} as link
+    stripped = value.strip()
+    if not stripped.startswith("{") and stripped.endswith("}"):
+        return None
+    try:
+        data = json.loads(value)
+    except ValueError:
+        return None
+    if set(data.keys()) != {"href", "label"}:
+        return None
+    href = data["href"]
+    if not (
+        href.startswith("/") or href.startswith("http://")
+        or href.startswith("https://")
+    ):
+        return None
+    return jinja2.Markup('<a href="{href}">{label}</a>'.format(
+        href=jinja2.escape(data["href"]),
+        label=jinja2.escape(data["label"] or "") or "&nbsp;"
+    ))
 '''
 
 TABLES = '''

tests/test_plugins.py

@@ -3,6 +3,7 @@ from .fixtures import ( # noqa
     app_client,
 )
 import pytest
+import urllib
 
 
 def test_plugins_dir_plugin(app_client):
@@ -67,3 +68,20 @@ def test_plugins_with_duplicate_js_urls(app_client):
     ) < srcs.index(
         'https://example.com/plugin2.js'
     )
+
+
+def test_plugins_render_cell(app_client):
+    sql = """
+        select '{"href": "http://example.com/", "label":"Example"}'
+    """.strip()
+    path = "/fixtures?" + urllib.parse.urlencode({
+        "sql": sql,
+    })
+    response = app_client.get(path)
+    td = Soup(
+        response.body, "html.parser"
+    ).find("table").find("tbody").find("td")
+    a = td.find("a")
+    assert a is not None, str(a)
+    assert a.attrs["href"] == "http://example.com/"
+    assert a.text == "Example"