render_cell(value) plugin hook

Still needs performance testing before I merge this into master

datasette/hookspecs.py

@@ -28,3 +28,8 @@ def extra_js_urls():
 @hookspec
 def publish_subcommand(publish):
     "Subcommands for 'datasette publish'"
+
+
+@hookspec(firstresult=True)
+def render_cell(value):
+    "Customize rendering of HTML table cell values"

datasette/views/base.py

@@ -493,14 +493,20 @@ class BaseView(RenderMixin):
                 display_row = []
                 for value in row:
                     display_value = value
-                    if value in ("", None):
+                    # Let the plugins have a go
-                        display_value = jinja2.Markup(" ")
+                    from datasette.app import pm
-                    elif is_url(str(value).strip()):
+                    plugin_value = pm.hook.render_cell(value=value)
-                        display_value = jinja2.Markup(
+                    if plugin_value is not None:
-                            '<a href="{url}">{url}</a>'.format(
+                        display_value = plugin_value
-                                url=jinja2.escape(value.strip())
+                    else:
+                        if value in ("", None):
+                            display_value = jinja2.Markup("&nbsp;")
+                        elif is_url(str(display_value).strip()):
+                            display_value = jinja2.Markup(
+                                '<a href="{url}">{url}</a>'.format(
+                                    url=jinja2.escape(value.strip())
+                                )
                             )
-                        )
                     display_row.append(display_value)
                 display_rows.append(display_row)
             return {

docs/plugins.rst

@@ -267,3 +267,52 @@ command. Datasette uses this hook internally to implement the default ``now``
 and ``heroku`` subcommands, so you can read
 `their source <https://github.com/simonw/datasette/tree/master/datasette/publish>`_
 to see examples of this hook in action.
+
+render_cell(value)
+~~~~~~~~~~~~~~~~~~
+
+Lets you customize the display of values within table cells in the HTML table view.
+
+``value`` is the value that was loaded from the database.
+
+If your hook returns ``None``, it will be ignored. Use this to indicate that your hook is not able to custom render this particular value.
+
+If the hook returns a string, that string will be rendered in the table cell.
+
+If you want to return HTML markup you can do so by returning a ``jinja2.Markup`` object.
+
+Here is an example of a custom ``render_cell()`` plugin which looks for values that are a JSON string matching the following format::
+
+    {"href": "https://www.example.com/", "label": "Name"}
+
+If the value matches that pattern, the plugin returns an HTML link element:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+    import jinja2
+    import json
+
+
+    @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;"
+        ))

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"