Async support for magic parameters

Closes #2441

datasette/views/database.py

@@ -391,7 +391,10 @@ async def post(self, request, datasette):
             or request.args.get("_json")
             or params.get("_json")
         )
-        params_for_query = MagicParameters(params, request, datasette)
+        params_for_query = MagicParameters(
+            canned_query["sql"], params, request, datasette
+        )
+        await params_for_query.execute_params()
         ok = None
         redirect_url = None
         try:
@@ -523,7 +526,8 @@ async def get(self, request, datasette):
                     validate_sql_select(sql)
                 else:
                     # Canned queries can run magic parameters
-                    params_for_query = MagicParameters(params, request, datasette)
+                    params_for_query = MagicParameters(sql, params, request, datasette)
+                    await params_for_query.execute_params()
                 results = await datasette.execute(
                     database, sql, params_for_query, truncate=True, **extra_args
                 )
@@ -792,14 +796,26 @@ async def query_actions():
 
 
 class MagicParameters(dict):
-    def __init__(self, data, request, datasette):
+    def __init__(self, sql, data, request, datasette):
         super().__init__(data)
+        self._sql = sql
         self._request = request
         self._magics = dict(
             itertools.chain.from_iterable(
                 pm.hook.register_magic_parameters(datasette=datasette)
             )
         )
+        self._prepared = {}
+
+    async def execute_params(self):
+        for key in derive_named_parameters(self._sql):
+            if key.startswith("_") and key.count("_") >= 2:
+                prefix, suffix = key[1:].split("_", 1)
+                if prefix in self._magics:
+                    result = await await_me_maybe(
+                        self._magics[prefix](suffix, self._request)
+                    )
+                    self._prepared[key] = result
 
     def __len__(self):
         # Workaround for 'Incorrect number of bindings' error
@@ -808,6 +824,9 @@ def __len__(self):
 
     def __getitem__(self, key):
         if key.startswith("_") and key.count("_") >= 2:
+            if key in self._prepared:
+                return self._prepared[key]
+            # Try the other route
             prefix, suffix = key[1:].split("_", 1)
             if prefix in self._magics:
                 try:

docs/plugin_hooks.rst

@@ -1315,7 +1315,7 @@ Magic parameters all take this format: ``_prefix_rest_of_parameter``. The prefix
 
 To register a new function, return it as a tuple of ``(string prefix, function)`` from this hook. The function you register should take two arguments: ``key`` and ``request``, where ``key`` is the ``rest_of_parameter`` portion of the parameter and ``request`` is the current :ref:`internals_request`.
 
-This example registers two new magic parameters: ``:_request_http_version`` returning the HTTP version of the current request, and ``:_uuid_new`` which returns a new UUID:
+This example registers two new magic parameters: ``:_request_http_version`` returning the HTTP version of the current request, and ``:_uuid_new`` which returns a new UUID. It also registers an `:_asynclookup_key` parameter, demonstrating that these functions can be asynchronous:
 
 .. code-block:: python
 
@@ -1337,11 +1337,15 @@ This example registers two new magic parameters: ``:_request_http_version`` retu
             raise KeyError
 
 
+    async def asynclookup(key, request):
+        return await do_something_async(key)
+
     @hookimpl
     def register_magic_parameters(datasette):
         return [
             ("request", request),
             ("uuid", uuid),
+            ("asynclookup", asynclookup),
         ]
 
 .. _plugin_hook_forbidden:

tests/plugins/my_plugin.py

@@ -360,9 +360,13 @@ def request(key, request):
         else:
             raise KeyError
 
+    async def asyncrequest(key, request):
+        return key
+
     return [
         ("request", request),
         ("uuid", uuid),
+        ("asyncrequest", asyncrequest),
     ]
 
 

tests/test_plugins.py

@@ -857,6 +857,9 @@ def test_hook_register_magic_parameters(restore_working_directory):
                         "get_uuid": {
                             "sql": "select :_uuid_new",
                         },
+                        "asyncrequest": {
+                            "sql": "select :_asyncrequest_key",
+                        },
                     }
                 }
             }
@@ -871,6 +874,10 @@ def test_hook_register_magic_parameters(restore_working_directory):
         assert 200 == response_get.status
         new_uuid = response_get.json[0][":_uuid_new"]
         assert 4 == new_uuid.count("-")
+        # And test the async one
+        response_async = client.get("/data/asyncrequest.json?_shape=array")
+        assert 200 == response_async.status
+        assert response_async.json[0][":_asyncrequest_key"] == "key"
 
 
 def test_hook_forbidden(restore_working_directory):