New ?_json=colname argument for returning unescaped JSON

Also extracted docs for special JSON arguments into a new section.

Closes #31

datasette/views/base.py

@@ -183,6 +183,15 @@ class BaseView(RenderMixin):
                     forward_querystring=False,
                 )
 
+            # Handle the _json= parameter which may modify data["rows"]
+            json_cols = []
+            if "_json" in request.args:
+                json_cols = request.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,
+                )
+
             # Deal with the _shape option
             shape = request.args.get("_shape", "arrays")
             if shape == "arrayfirst":
@@ -323,3 +332,22 @@ class BaseView(RenderMixin):
             "canned_query": canned_query,
             "config": self.ds.config,
         }, templates
+
+
+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

docs/json_api.rst

@@ -131,6 +131,33 @@ this format.
 The ``object`` keys are always strings. If your table has a compound primary
 key, the ``object`` keys will be a comma-separated string.
 
+Special JSON arguments
+----------------------
+
+Every Datasette endpoint that can return JSON also accepts the following
+querystring arguments:
+
+``?_shape=SHAPE``
+    The shape of the JSON to return, documented above.
+
+``?_json=COLUMN1&_json=COLUMN2``
+    If any of your SQLite columns contain JSON values, you can use one or more
+    ``_json=`` parameters to request that those columns be returned as regular
+    JSON. Without this argument those columns will be returned as JSON objects
+    that have been double-encoded into a JSON string value.
+
+    Compare `this query without the argument <https://fivethirtyeight.datasettes.com/fivethirtyeight.json?sql=select+%27{%22this+is%22%3A+%22a+json+object%22}%27+as+d&_shape=array>`_ to `this query using the argument <https://fivethirtyeight.datasettes.com/fivethirtyeight.json?sql=select+%27{%22this+is%22%3A+%22a+json+object%22}%27+as+d&_shape=array&_json=d>`_
+
+``?_timelimit=MS``
+    Sets a custom time limit for the query in ms. You can use this for optimistic
+    queries where you would like Datasette to give up if the query takes too
+    long, for example if you want to implement autocomplete search but only if
+    it can be executed in less than 10ms.
+
+``?_ttl=SECONDS``
+    For how many seconds should this response be cached by HTTP proxies? Use
+    ``?_ttl=0`` to disable HTTP caching entirely for this request.
+
 Special table arguments
 -----------------------
 
@@ -163,16 +190,6 @@ The Datasette table view takes a number of special querystring arguments:
     You can pass multiple ``_group_count`` columns to return counts against
     unique combinations of those columns.
 
-``?_timelimit=MS``
-    Sets a custom time limit for the query in ms. You can use this for optimistic
-    queries where you would like Datasette to give up if the query takes too
-    long, for example if you want to implement autocomplete search but only if
-    it can be executed in less than 10ms.
-
-``?_ttl=SECONDS``
-    For how many seconds should this response be cached by HTTP proxies? Use
-    ``?_ttl=0`` to disable HTTP caching entirely for this request.
-
 ``?_next=TOKEN``
     Pagination by continuation token - pass the token that was returned in the
     ``"next"`` property by the previous page.

tests/test_api.py

@@ -1151,3 +1151,39 @@ def test_suggest_facets_off():
 def test_ttl_parameter(app_client, path, expected_cache_control):
     response = app_client.get(path, gather_request=False)
     assert expected_cache_control == response.headers['Cache-Control']
+
+
+test_json_columns_default_expected = [{
+    "intval": 1,
+    "strval": "s",
+    "floatval": 0.5,
+    "jsonval": "{\"foo\": \"bar\"}"
+}]
+
+
+@pytest.mark.parametrize("extra_args,expected", [
+    ("", test_json_columns_default_expected),
+    ("&_json=intval", test_json_columns_default_expected),
+    ("&_json=strval", test_json_columns_default_expected),
+    ("&_json=floatval", test_json_columns_default_expected),
+    ("&_json=jsonval", [{
+        "intval": 1,
+        "strval": "s",
+        "floatval": 0.5,
+        "jsonval": {
+            "foo": "bar"
+        }
+    }])
+])
+def test_json_columns(app_client, extra_args, expected):
+    sql = '''
+        select 1 as intval, "s" as strval, 0.5 as floatval,
+        '{"foo": "bar"}' as jsonval
+    '''
+    path = "/test_tables.json?" + urllib.parse.urlencode({
+        "sql": sql,
+        "_shape": "array"
+    })
+    path += extra_args
+    response = app_client.get(path, gather_request=False)
+    assert expected == response.json