Add ability to set font size in plot (#1879)

* Added a function to process font sizes

* Add ability to set font size in plot and test it

* formatting with black

* correct import order

* move arg to be the last and added a docstring

* Update folium/folium.py

Co-authored-by: Frank Anema <[email protected]>

* Change to accept em and px

made px the default in line with _parse_size.

* correct test units

* formatting with black

* formatting with black

* Update tests/test_utilities.py

* Update tests/test_utilities.py

---------

Co-authored-by: Frank Anema <[email protected]>

Author: this-josh

Diff for: folium/folium.py

@@ -17,6 +17,7 @@
     TypeBounds,
     TypeJsonValue,
     _parse_size,
+    parse_font_size,
     parse_options,
     temp_html_filepath,
     validate_location,
@@ -152,6 +153,9 @@ class Map(JSCSSMixin, MacroElement):
         rare environments) even if they're supported.
     zoom_control : bool, default True
         Display zoom controls on the map.
+    font_size : int or float or string (default: '1rem')
+        The font size to use for Leaflet, can either be a number or a
+        string ending in 'rem', 'em', or 'px'.
     **kwargs
         Additional keyword arguments are passed to Leaflets Map class:
         https://leafletjs.com/reference.html#map
@@ -186,7 +190,7 @@ class Map(JSCSSMixin, MacroElement):
                     left: {{this.left[0]}}{{this.left[1]}};
                     top: {{this.top[0]}}{{this.top[1]}};
                 }
-                .leaflet-container { font-size: 1rem; }
+                .leaflet-container { font-size: {{this.font_size}}; }
             </style>
         {% endmacro %}
 
@@ -253,6 +257,7 @@ def __init__(
         disable_3d: bool = False,
         png_enabled: bool = False,
         zoom_control: bool = True,
+        font_size: str = "1rem",
         **kwargs: TypeJsonValue,
     ):
         super().__init__()
@@ -276,6 +281,7 @@ def __init__(
         self.left = _parse_size(left)
         self.top = _parse_size(top)
         self.position = position
+        self.font_size = parse_font_size(font_size)
 
         max_bounds_array = (
             [[min_lat, min_lon], [max_lat, max_lon]] if max_bounds else None

Diff for: folium/utilities.py

@@ -428,3 +428,13 @@ def __init__(self, js_code: Union[str, "JsCode"]):
             self.js_code: str = js_code.js_code
         else:
             self.js_code = js_code
+
+
+def parse_font_size(value: Union[str, int, float]) -> str:
+    """Parse a font size value, if number set as px"""
+    if isinstance(value, (int, float)):
+        return f"{value}px"
+
+    if (value[-3:] != "rem") and (value[-2:] not in ["em", "px"]):
+        raise ValueError("The font size must be expressed in rem, em, or px.")
+    return value

Diff for: tests/test_folium.py

@@ -75,6 +75,7 @@ def setup_method(self):
             max_zoom=20,
             zoom_start=4,
             max_bounds=True,
+            font_size="1.5rem",
             attr=attr,
         )
         self.env = Environment(loader=PackageLoader("folium", "templates"))
@@ -94,6 +95,7 @@ def test_init(self):
         assert self.m.top == (0, "%")
         assert self.m.global_switches.no_touch is False
         assert self.m.global_switches.disable_3d is False
+        assert self.m.font_size == "1.5rem"
         assert self.m.to_dict() == {
             "name": "Map",
             "id": self.m._id,

Diff for: tests/test_utilities.py

@@ -12,6 +12,7 @@
     get_obj_in_upper_tree,
     if_pandas_df_convert_to_numpy,
     javascript_identifier_path_to_array_notation,
+    parse_font_size,
     parse_options,
     validate_location,
     validate_locations,
@@ -230,3 +231,26 @@ def test_js_code_init_js_code():
     js_code_2 = JsCode(js_code)
     assert isinstance(js_code_2, JsCode)
     assert isinstance(js_code_2.js_code, str)
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        (10, "10px"),
+        (12.5, "12.5px"),
+        ("1rem", "1rem"),
+        ("1em", "1em"),
+    ],
+)
+def test_parse_font_size_valid(value, expected):
+    assert parse_font_size(value) == expected
+
+
+invalid_values = ["1", "1unit"]
+expected_errors = "The font size must be expressed in rem, em, or px."
+
+
+@pytest.mark.parametrize("value,error_message", zip(invalid_values, expected_errors))
+def test_parse_font_size_invalid(value, error_message):
+    with pytest.raises(ValueError, match=error_message):
+        parse_font_size(value)