Bug: Strings in Excel number fomat do not preserve case(fixes pandas-dev#63117)

Author: parthkandharkar

pandas/io/formats/css.py

@@ -391,7 +391,10 @@ def _error() -> str:
     def atomize(self, declarations: Iterable) -> Generator[tuple[str, str]]:
         for prop, value in declarations:
             prop = prop.lower()
-            value = value.lower()
+            if prop == "number-format":
+                value = value.strip()
+            else:
+                value = value.lower()
             if prop in self.CSS_EXPANSIONS:
                 expand = self.CSS_EXPANSIONS[prop]
                 yield from expand(self, prop, value)
@@ -414,7 +417,10 @@ def parse(self, declarations_str: str) -> Iterator[tuple[str, str]]:
             prop, sep, val = decl.partition(":")
             prop = prop.strip().lower()
             # TODO: don't lowercase case sensitive parts of values (strings)
-            val = val.strip().lower()
+            if prop == "number-format":
+                val = val.strip()
+            else:
+                val = val.strip().lower()
             if sep:
                 yield prop, val
             else:

pandas/tests/io/excel/test_style.py

@@ -96,6 +96,7 @@ def test_styler_to_excel_unstyled(engine, tmp_excel):
     ("font-style: italic;", ["font", "i"], True),
     ("text-decoration: underline;", ["font", "u"], "single"),
     ("number-format: $??,???.00;", ["number_format"], "$??,???.00"),
+    ("number-format: #,,"'"M";', ["number_format"], '#,,"M"'),
     ("text-align: left;", ["alignment", "horizontal"], "left"),
     (
         "vertical-align: bottom;",

pandas/tests/io/formats/test_css.py

@@ -286,3 +286,28 @@ def test_css_relative_font_size(size, relative_to, resolved):
     else:
         inherited = {"font-size": relative_to}
     assert_resolves(f"font-size: {size}", {"font-size": resolved}, inherited=inherited)
+
+def test_css_atomize_preserves_number_format_case():
+    # GHxxxx: number-format strings must keep case (e.g. "M" not "m")
+    resolver = CSSResolver()
+    declarations = [("NUMBER-FORMAT", '#,,"M"')]
+
+    props = dict(resolver.atomize(declarations))
+
+    # property name is normalized to lowercase
+    assert "number-format" in props
+    # value is preserved exactly (no lowercasing)
+    assert props["number-format"] == '#,,"M"'
+
+def test_css_parse_preserves_number_format_case():
+    # GHxxxx: number-format should not be lowercased by parse()
+    resolver = CSSResolver()
+
+    css = 'NUMBER-FORMAT: #,,"M"; COLOR: Red'
+    result = dict(resolver.parse(css))
+
+    # "number-format" key is lowercase, but its value must be preserved
+    assert result["number-format"] == '#,,"M"'
+
+    # other properties still get lowercased in the value
+    assert result["color"] == "red"