can_render mechanism for register_output_renderer, closes #770

datasette/app.py

@@ -228,7 +228,7 @@ class Datasette:
         if config_dir and (config_dir / "config.json").exists() and not config:
             config = json.load((config_dir / "config.json").open())
         self._config = dict(DEFAULT_CONFIG, **(config or {}))
-        self.renderers = {}  # File extension -> renderer function
+        self.renderers = {}  # File extension -> (renderer, can_render) functions
         self.version_note = version_note
         self.executor = futures.ThreadPoolExecutor(
             max_workers=self.config("num_sql_threads")
@@ -574,7 +574,7 @@ class Datasette:
     def register_renderers(self):
         """ Register output renderers which output data in custom formats. """
         # Built-in renderers
-        self.renderers["json"] = json_renderer
+        self.renderers["json"] = (json_renderer, lambda: True)
 
         # Hooks
         hook_renderers = []
@@ -588,8 +588,8 @@ class Datasette:
         for renderer in hook_renderers:
             self.renderers[renderer["extension"]] = (
                 # It used to be called "callback" - remove this in Datasette 1.0
-                renderer.get("render")
+                renderer.get("render") or renderer["callback"],
-                or renderer["callback"]
+                renderer.get("can_render") or (lambda: True),
             )
 
     async def render_template(

datasette/utils/__init__.py

@@ -811,6 +811,10 @@ def call_with_supported_arguments(fn, **kwargs):
     call_with = []
     for parameter in parameters:
         if parameter not in kwargs:
-            raise TypeError("{} requires parameters {}".format(fn, tuple(parameters)))
+            raise TypeError(
+                "{} requires parameters {}, missing: {}".format(
+                    fn, tuple(parameters), set(parameters) - set(kwargs.keys())
+                )
+            )
         call_with.append(kwargs[parameter])
     return fn(*call_with)

datasette/views/base.py

@@ -389,7 +389,7 @@ class DataView(BaseView):
             # Dispatch request to the correct output format renderer
             # (CSV is not handled here due to streaming)
             result = call_with_supported_arguments(
-                self.ds.renderers[_format],
+                self.ds.renderers[_format][0],
                 datasette=self.ds,
                 columns=data.get("columns") or [],
                 rows=data.get("rows") or [],
@@ -426,10 +426,27 @@ class DataView(BaseView):
             if data.get("expandable_columns"):
                 url_labels_extra = {"_labels": "on"}
 
-            renderers = {
+            renderers = {}
-                key: path_with_format(request, key, {**url_labels_extra})
+            for key, (_, can_render) in self.ds.renderers.items():
-                for key in self.ds.renderers.keys()
+                it_can_render = call_with_supported_arguments(
-            }
+                    can_render,
+                    datasette=self.ds,
+                    columns=data.get("columns") or [],
+                    rows=data.get("rows") or [],
+                    sql=data.get("query", {}).get("sql", None),
+                    query_name=data.get("query_name"),
+                    database=database,
+                    table=data.get("table"),
+                    request=request,
+                    view_name=self.name,
+                )
+                if asyncio.iscoroutine(it_can_render):
+                    it_can_render = await it_can_render
+                if it_can_render:
+                    renderers[key] = path_with_format(
+                        request, key, {**url_labels_extra}
+                    )
+
             url_csv_args = {"_size": "max", **url_labels_extra}
             url_csv = path_with_format(request, "csv", url_csv_args)
             url_csv_path = url_csv.split("?")[0]

docs/plugins.rst

@@ -744,14 +744,17 @@ Registers a new output renderer, to output data in a custom format. The hook fun
     def register_output_renderer(datasette):
         return {
             "extension": "test",
-            "render": render_test
+            "render": render_demo,
+            "can_render": can_render_demo,  # Optional
         }
 
-This will register ``render_test`` to be called when paths with the extension ``.test`` (for example ``/database.test``, ``/database/table.test``, or ``/database/table/row.test``) are requested.
+This will register ``render_demo`` to be called when paths with the extension ``.test`` (for example ``/database.test``, ``/database/table.test``, or ``/database/table/row.test``) are requested.
 
-``render_test`` is a Python function. It can be a regular function or an ``async def render_test()`` awaitable function, depending on if it needs to make any asynchronous calls.
+``render_demo`` is a Python function. It can be a regular function or an ``async def render_demo()`` awaitable function, depending on if it needs to make any asynchronous calls.
 
-When a request is received, the callback function is called with zero or more of the following arguments. Datasette will inspect your callback function and pass arguments that match its function signature.
+``can_render_demo`` is a Python function (or ``async def`` function) which acepts the same arguments as ``render_demo`` but just returns ``True`` or ``False``. It lets Datasette know if the current SQL query can be represented by the plugin - and hence influnce if a link to this output format is displayed in the user interface. If you omit the ``"can_render"`` key from the dictionary every query will be treated as being supported by the plugin.
+
+When a request is received, the ``"render"`` callback function is called with zero or more of the following arguments. Datasette will inspect your callback function and pass arguments that match its function signature.
 
 ``datasette`` - :ref:`internals_datasette`
     For accessing plugin configuration and executing queries.
@@ -798,7 +801,7 @@ A simple example of an output renderer callback function:
 
 .. code-block:: python
 
-    def render_test():
+    def render_demo():
         return {
             "body": "Hello World"
         }
@@ -807,7 +810,7 @@ Here is a more complex example:
 
 .. code-block:: python
 
-    async def render_test(datasette, columns, rows):
+    async def render_demo(datasette, columns, rows):
         db = next(iter(datasette.databases.values()))
         result = await db.execute("select sqlite_version()")
         first_row = " | ".join(columns)
@@ -821,6 +824,13 @@ Here is a more complex example:
             "headers": {"x-sqlite-version": result.first()[0]},
         }
 
+And here is an example ``can_render`` function which returns ``True`` only if the query results contain the columns ``atom_id``, ``atom_title`` and ``atom_updated``:
+
+.. code-block:: python
+
+    def can_render_demo(columns):
+        return {"atom_id", "atom_title", "atom_updated"}.issubset(columns)
+
 Examples: `datasette-atom <https://github.com/simonw/datasette-atom>`_, `datasette-ics <https://github.com/simonw/datasette-ics>`_
 
 .. _plugin_register_facet_classes:

tests/plugins/register_output_renderer.py

@@ -2,6 +2,26 @@ from datasette import hookimpl
 import json
 
 
+async def can_render(
+    datasette, columns, rows, sql, query_name, database, table, request, view_name
+):
+    # We stash this on datasette so the calling unit test can see it
+    datasette._can_render_saw = {
+        "datasette": datasette,
+        "columns": columns,
+        "rows": rows,
+        "sql": sql,
+        "query_name": query_name,
+        "database": database,
+        "table": table,
+        "request": request,
+        "view_name": view_name,
+    }
+    if request.args.get("_no_can_render"):
+        return False
+    return True
+
+
 async def render_test_all_parameters(
     datasette, columns, rows, sql, query_name, database, table, request, view_name, data
 ):
@@ -39,6 +59,10 @@ def render_test_no_parameters():
 @hookimpl
 def register_output_renderer(datasette):
     return [
-        {"extension": "testall", "render": render_test_all_parameters},
+        {
+            "extension": "testall",
+            "render": render_test_all_parameters,
+            "can_render": can_render,
+        },
         {"extension": "testnone", "callback": render_test_no_parameters},
     ]

tests/test_plugins.py

@@ -9,7 +9,7 @@ from .fixtures import (
 from datasette.app import Datasette
 from datasette import cli
 from datasette.plugins import get_plugins, DEFAULT_PLUGINS, pm
-from datasette.utils import sqlite3
+from datasette.utils import sqlite3, CustomRow
 from jinja2.environment import Template
 import base64
 import json
@@ -411,6 +411,41 @@ def test_register_output_renderer_custom_headers(app_client):
     assert "2" == response.headers["x-gosh"]
 
 
+def test_register_output_renderer_can_render(app_client):
+    response = app_client.get("/fixtures/facetable?_no_can_render=1")
+    assert response.status == 200
+    links = (
+        Soup(response.body, "html.parser")
+        .find("p", {"class": "export-links"})
+        .findAll("a")
+    )
+    actual = [l["href"].split("/")[-1] for l in links]
+    # Should not be present because we sent ?_no_can_render=1
+    assert "facetable.testall?_labels=on" not in actual
+    # Check that it was passed the values we expected
+    assert hasattr(app_client.ds, "_can_render_saw")
+    assert {
+        "datasette": app_client.ds,
+        "columns": [
+            "pk",
+            "created",
+            "planet_int",
+            "on_earth",
+            "state",
+            "city_id",
+            "neighborhood",
+            "tags",
+            "complex_array",
+            "distinct_some_null",
+        ],
+        "sql": "select pk, created, planet_int, on_earth, state, city_id, neighborhood, tags, complex_array, distinct_some_null from facetable order by pk limit 51",
+        "query_name": None,
+        "database": "fixtures",
+        "table": "facetable",
+        "view_name": "table",
+    }.items() <= app_client.ds._can_render_saw.items()
+
+
 @pytest.mark.asyncio
 async def test_prepare_jinja2_environment(app_client):
     template = app_client.ds.jinja_env.from_string(