New plugin hook: register_output_renderer hook (#441)

Thanks @russss!

* Add register_output_renderer hook

This changeset refactors out the JSON renderer and then adds a hook and
dispatcher system to allow custom output renderers to be registered.

The CSV output renderer is untouched because supporting streaming
renderers through this system would be significantly more complex, and
probably not worthwhile.

We can't simply allow hooks to be called at request time because we need
a list of supported file extensions when the request is being routed in
order to resolve ambiguous database/table names. So, renderers need to
be registered at startup.

I've tried to make this API independent of Sanic's request/response
objects so that this can remain stable during the switch to ASGI. I'm
using dictionaries to keep it simple and to make adding additional
options in the future easy.

Fixes #440

datasette/renderer.py

@@ -0,0 +1,96 @@
+import json
+from datasette.utils import (
+    value_as_boolean,
+    remove_infinites,
+    CustomJSONEncoder,
+    path_from_row_pks,
+)
+
+
+def convert_specific_columns_to_json(rows, columns, json_cols):
+    json_cols = set(json_cols)
+    if not json_cols.intersection(columns):
+        return rows
+    new_rows = []
+    for row in rows:
+        new_row = []
+        for value, column in zip(row, columns):
+            if column in json_cols:
+                try:
+                    value = json.loads(value)
+                except (TypeError, ValueError) as e:
+                    print(e)
+                    pass
+            new_row.append(value)
+        new_rows.append(new_row)
+    return new_rows
+
+
+def json_renderer(args, data, view_name):
+    """ Render a response as JSON """
+    status_code = 200
+    # Handle the _json= parameter which may modify data["rows"]
+    json_cols = []
+    if "_json" in args:
+        json_cols = args["_json"]
+    if json_cols and "rows" in data and "columns" in data:
+        data["rows"] = convert_specific_columns_to_json(
+            data["rows"], data["columns"], json_cols
+        )
+
+    # unless _json_infinity=1 requested, replace infinity with None
+    if "rows" in data and not value_as_boolean(args.get("_json_infinity", "0")):
+        data["rows"] = [remove_infinites(row) for row in data["rows"]]
+
+    # Deal with the _shape option
+    shape = args.get("_shape", "arrays")
+    if shape == "arrayfirst":
+        data = [row[0] for row in data["rows"]]
+    elif shape in ("objects", "object", "array"):
+        columns = data.get("columns")
+        rows = data.get("rows")
+        if rows and columns:
+            data["rows"] = [dict(zip(columns, row)) for row in rows]
+        if shape == "object":
+            error = None
+            if "primary_keys" not in data:
+                error = "_shape=object is only available on tables"
+            else:
+                pks = data["primary_keys"]
+                if not pks:
+                    error = (
+                        "_shape=object not available for tables with no primary keys"
+                    )
+                else:
+                    object_rows = {}
+                    for row in data["rows"]:
+                        pk_string = path_from_row_pks(row, pks, not pks)
+                        object_rows[pk_string] = row
+                    data = object_rows
+            if error:
+                data = {"ok": False, "error": error}
+        elif shape == "array":
+            data = data["rows"]
+    elif shape == "arrays":
+        pass
+    else:
+        status_code = 400
+        data = {
+            "ok": False,
+            "error": "Invalid _shape: {}".format(shape),
+            "status": 400,
+            "title": None,
+        }
+    # Handle _nl option for _shape=array
+    nl = args.get("_nl", "")
+    if nl and shape == "array":
+        body = "\n".join(json.dumps(item) for item in data)
+        content_type = "text/plain"
+    else:
+        body = json.dumps(data, cls=CustomJSONEncoder)
+        content_type = "application/json"
+    return {
+        "body": body,
+        "status_code": status_code,
+        "content_type": content_type
+    }