Set placeholder for IntInput (Fixes #8359) (#8360)

* Set placeholder for IntInput

* Add test and move placeholder onto SpinnerBase

---------

Co-authored-by: Philipp Rudiger <prudiger@anaconda.com>

Author: emunsing

panel/tests/widgets/test_input.py

@@ -483,3 +483,14 @@ def test_file_input_save_one_file(document, comm, tmpdir):
     assert fpath.exists()
     content = fpath.read_text()
     assert content == 'Some text\n'
+
+def test_int_input_placeholder(document, comm):
+
+    int_input = IntInput(placeholder='Foo', name='IntInput')
+
+    widget = int_input.get_root(document, comm=comm)
+
+    assert widget.placeholder == "Foo"
+
+    int_input.placeholder = "Bar"
+    assert widget.placeholder == "Bar"

panel/widgets/input.py

@@ -993,6 +993,9 @@ class _SpinnerBase(_NumericInputBase):
         Defines the multiplication factor applied to step when the page up
         and page down keys are pressed.""")
 
+    placeholder = param.String(default='', doc="""
+        Placeholder when the value is empty.""")
+
     wheel_wait = param.Integer(default=100, doc="""
         Defines the debounce time in ms before updating `value_throttled` when
         the mouse wheel is used to change the input.""")