Allow callables to be used as options for NestedSelect (#5969)

examples/reference/widgets/NestedSelect.ipynb

@@ -22,9 +22,9 @@
     "\n",
     "##### Core\n",
     "\n",
-    "* **``options``** (dict): The options to select from. The options may be nested dictionaries or lists.\n",
+    "* **``options``** (dict | callable): The options to select from. The options may be nested dictionaries, lists, or callables that return those types. If callables are used, the callables must accept `level` and `value` keyword arguments, where `level` is the level that updated and `value` is a dictionary of the current values, containing keys up to the level that was updated.\n",
     "* **``value``** (dict): The value from all the Select widgets; the keys are the levels names. If no levels names are specified, the keys are the levels indices.\n",
-    "* **``levels``** (list): Either a list of strings or a list of dictionaries. If a list of strings, the strings are used as the names of the levels. If a list of dictionaries, each dictionary may have a \"name\" key, which is used as the name of the level, a \"type\" key, which is used as the type of widget, and any corresponding widget keyword arguments.\n",
+    "* **``levels``** (list): Either a list of strings or a list of dictionaries. If a list of strings, the strings are used as the names of the levels. If a list of dictionaries, each dictionary may have a \"name\" key, which is used as the name of the level, a \"type\" key, which is used as the type of widget, and any corresponding widget keyword arguments. Must be specified if options is callable.\n",
     "\n",
     "##### Display\n",
     "\n",
@@ -207,6 +207,75 @@
     "select.value = {\"Model\": \"NAME\"}"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Alternatively, `options` can be a callable. Its nested levels too: e.g. `options={\"Daily\": list_options, \"Monthly\": list_options}`.\n",
+    "\n",
+    "If callables are used, the callables must accept `level` and `value` keyword arguments, where `level` is the level that updated and `value` is a dictionary of the current values, containing keys up to the level that was updated.\n",
+    "\n",
+    "Note, the callable can vary across `options`, and `levels` must be provided if any of the `options` is callable."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def list_options(level, value):\n",
+    "    if level == \"time_step\":\n",
+    "        options = {\"Daily\": list_options, \"Monthly\": list_options}\n",
+    "    elif level == \"level_type\":\n",
+    "        options = {f\"{value['time_step']}_upper\": list_options, f\"{value['time_step']}_lower\": list_options}\n",
+    "    else:\n",
+    "        options = [f\"{value['level_type']}.json\", f\"{value['level_type']}.csv\"]\n",
+    "\n",
+    "    return options\n",
+    "\n",
+    "pn.widgets.NestedSelect(\n",
+    "    options=list_options,\n",
+    "    levels=[\"time_step\", \"level_type\", \"file_type\"],\n",
+    ")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This is useful if you are trying to use options from a hosted source.\n",
+    "\n",
+    "Using `pn.cache` here can help improve user experience and reduce the risk of rate limits.\n",
+    "\n",
+    "```python\n",
+    "import panel as pn\n",
+    "pn.extension()\n",
+    "\n",
+    "@pn.cache()\n",
+    "def list_options(level, value):\n",
+    "    value_path = \"/\".join(list(value.values()))\n",
+    "    url = f\"https://downloads.psl.noaa.gov/Datasets/ncep.reanalysis/{value_path}\"\n",
+    "\n",
+    "    options = [var.rstrip(\"/\") for var in pd.read_html(url)[0][\"Name\"].dropna()[1:]]\n",
+    "    if level == \"time_step\":\n",
+    "        options = {option: list_options for option in options if option[0].isupper()}\n",
+    "    elif level == \"level_type\":\n",
+    "        options = {option: list_options for option in options if option[0].islower()}\n",
+    "    else:\n",
+    "        options = [option for option in options if option.endswith(\".nc\")]\n",
+    "\n",
+    "    return options\n",
+    "\n",
+    "\n",
+    "select = pn.widgets.NestedSelect(\n",
+    "    options=list_options,\n",
+    "    levels=[\"time_step\", \"level_type\", \"file\"],\n",
+    ")\n",
+    "select\n",
+    "```"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

panel/tests/widgets/test_select.py

@@ -545,6 +545,72 @@ def test_nested_select_custom_widgets(document, comm):
     assert widget_2.options == [1000, 925, 700, 500, 300]
 
 
+def test_nested_select_callable_top_level(document, comm):
+    def list_options(level, value):
+        if level == "time_step":
+            options = {"Daily": list_options, "Monthly": list_options}
+        elif level == "level_type":
+            options = {f"{value['time_step']}_upper": list_options, f"{value['time_step']}_lower": list_options}
+        else:
+            options = [f"{value['level_type']}.json", f"{value['level_type']}.csv"]
+
+        return options
+
+    select = NestedSelect(
+        options=list_options,
+        levels=["time_step", "level_type", "file"],
+    )
+    assert select._widgets[0].options == ["Daily", "Monthly"]
+    assert select._widgets[1].options == ["Daily_upper", "Daily_lower"]
+    assert select._widgets[2].options == ["Daily_upper.json", "Daily_upper.csv"]
+    assert select.value == {"time_step": "Daily", "level_type": "Daily_upper", "file": "Daily_upper.json"}
+    assert select._max_depth == 3
+
+    select.value = {"time_step": "Monthly"}
+    assert select._widgets[0].options == ["Daily", "Monthly"]
+    assert select._widgets[1].options == ["Monthly_upper", "Monthly_lower"]
+    assert select._widgets[2].options == ["Monthly_upper.json", "Monthly_upper.csv"]
+    assert select.value == {"time_step": "Monthly", "level_type": "Monthly_upper", "file": "Monthly_upper.json"}
+    assert select._max_depth == 3
+
+
+def test_nested_select_callable_mid_level(document, comm):
+    def list_options(level, value):
+        if level == "level_type":
+            options = {f"{value['time_step']}_upper": list_options, f"{value['time_step']}_lower": list_options}
+        else:
+            options = [f"{value['level_type']}.json", f"{value['level_type']}.csv"]
+
+        return options
+
+    select = NestedSelect(
+        options={"Daily": list_options, "Monthly": list_options},
+        levels=["time_step", "level_type", "file"],
+    )
+    assert select._widgets[0].options == ["Daily", "Monthly"]
+    assert select._widgets[1].options == ["Daily_upper", "Daily_lower"]
+    assert select._widgets[2].options == ["Daily_upper.json", "Daily_upper.csv"]
+    assert select.value == {"time_step": "Daily", "level_type": "Daily_upper", "file": "Daily_upper.json"}
+    assert select._max_depth == 3
+
+    select.value = {"time_step": "Monthly"}
+    assert select._widgets[0].options == ["Daily", "Monthly"]
+    assert select._widgets[1].options == ["Monthly_upper", "Monthly_lower"]
+    assert select._widgets[2].options == ["Monthly_upper.json", "Monthly_upper.csv"]
+    assert select.value == {"time_step": "Monthly", "level_type": "Monthly_upper", "file": "Monthly_upper.json"}
+    assert select._max_depth == 3
+
+
+def test_nested_select_callable_must_have_levels(document, comm):
+    def list_options(level, value):
+        pass
+
+    with pytest.raises(ValueError, match="levels must be specified"):
+        NestedSelect(
+            options={"Daily": list_options, "Monthly": list_options},
+        )
+
+
 @pytest.mark.parametrize('options', [[10, 20], dict(A=10, B=20)], ids=['list', 'dict'])
 @pytest.mark.parametrize('size', [1, 2], ids=['size=1', 'size>1'])
 def test_select_disabled_options_init(options, size, document, comm):