render_cell(value) plugin hook, closes #352

New plugin hook for customizing the way cells values are rendered in HTML.

The first full example of this hook in use is https://github.com/simonw/datasette-json-html

datasette/app.py

@@ -2,7 +2,6 @@ import asyncio
 import click
 import collections
 import hashlib
-import importlib
 import itertools
 import os
 import sqlite3
@@ -14,7 +13,6 @@ from concurrent import futures
 from pathlib import Path
 
 from markupsafe import Markup
-import pluggy
 from jinja2 import ChoiceLoader, Environment, FileSystemLoader, PrefixLoader
 from sanic import Sanic, response
 from sanic.exceptions import InvalidUsage, NotFound
@@ -28,7 +26,6 @@ from .views.index import IndexView
 from .views.special import JsonDataView
 from .views.table import RowView, TableView
 
-from . import hookspecs
 from .utils import (
     InterruptedError,
     Results,
@@ -40,26 +37,13 @@ from .utils import (
     to_css_class
 )
 from .inspect import inspect_hash, inspect_views, inspect_tables
+from .plugins import pm
 from .version import __version__
 
-default_plugins = (
-    "datasette.publish.heroku",
-    "datasette.publish.now",
-)
-
 app_root = Path(__file__).parent.parent
 
 connections = threading.local()
 
-pm = pluggy.PluginManager("datasette")
-pm.add_hookspecs(hookspecs)
-pm.load_setuptools_entrypoints("datasette")
-
-# Load default plugins
-for plugin in default_plugins:
-    mod = importlib.import_module(plugin)
-    pm.register(mod, plugin)
-
 
 ConfigOption = collections.namedtuple(
     "ConfigOption", ("name", "default", "help")

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/plugins.py

@@ -0,0 +1,17 @@
+import importlib
+import pluggy
+from . import hookspecs
+
+default_plugins = (
+    "datasette.publish.heroku",
+    "datasette.publish.now",
+)
+
+pm = pluggy.PluginManager("datasette")
+pm.add_hookspecs(hookspecs)
+pm.load_setuptools_entrypoints("datasette")
+
+# Load default plugins
+for plugin in default_plugins:
+    mod = importlib.import_module(plugin)
+    pm.register(mod, plugin)

datasette/utils.py

@@ -16,7 +16,6 @@ import shutil
 import urllib
 import numbers
 
-
 # From https://www.sqlite.org/lang_keywords.html
 reserved_words = set((
     'abort action add after all alter analyze and as asc attach autoincrement '

datasette/views/base.py

@@ -13,6 +13,7 @@ from sanic.exceptions import NotFound
 from sanic.views import HTTPMethodView
 
 from datasette import __version__
+from datasette.plugins import pm
 from datasette.utils import (
     CustomJSONEncoder,
     InterruptedError,
@@ -493,14 +494,19 @@ class BaseView(RenderMixin):
                 display_row = []
                 for value in row:
                     display_value = value
-                    if value in ("", None):
-                        display_value = jinja2.Markup(" ")
-                    elif is_url(str(value).strip()):
-                        display_value = jinja2.Markup(
-                            '<a href="{url}">{url}</a>'.format(
-                                url=jinja2.escape(value.strip())
+                    # Let the plugins have a go
+                    plugin_value = pm.hook.render_cell(value=value)
+                    if plugin_value is not None:
+                        display_value = plugin_value
+                    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 {

datasette/views/table.py

@@ -5,6 +5,7 @@ import jinja2
 from sanic.exceptions import NotFound
 from sanic.request import RequestParameters
 
+from datasette.plugins import pm
 from datasette.utils import (
     CustomRow,
     Filters,
@@ -22,7 +23,6 @@ from datasette.utils import (
     urlsafe_components,
     value_as_boolean,
 )
-
 from .base import BaseView, DatasetteError, ureg
 
 LINK_WITH_LABEL = '<a href="/{database}/{table}/{link_id}">{label}</a>&nbsp;<em>{id}</em>'
@@ -166,7 +166,11 @@ class RowTableShared(BaseView):
                     # already shown in the link column.
                     continue
 
-                if isinstance(value, dict):
+                # First let the plugins have a go
+                plugin_display_value = pm.hook.render_cell(value=value)
+                if plugin_display_value is not None:
+                    display_value = plugin_display_value
+                elif isinstance(value, dict):
                     # It's an expanded foreign key - display link to other row
                     label = value["label"]
                     value = value["value"]