ENH: Move CSSWarning to error/__init__.py per GH27656 (pandas-dev#47591)

* ENH: Move CSSWarning to error/__init__.py per GH27656

* ENH: fix typo

* ENH: reword sentence

Author: dataxerik

doc/source/reference/testing.rst

@@ -26,6 +26,7 @@ Exceptions and warnings
 
    errors.AbstractMethodError
    errors.AccessorRegistrationWarning
+   errors.CSSWarning
    errors.DataError
    errors.DtypeWarning
    errors.DuplicateLabelError

pandas/errors/__init__.py

@@ -395,3 +395,21 @@ def __init__(self, message: str) -> None:
         # attr only exists on Windows, so typing fails on other platforms
         message += f" ({ctypes.WinError()})"  # type: ignore[attr-defined]
         super().__init__(message)
+
+
+class CSSWarning(UserWarning):
+    """
+    Warning is raised when converting css styling fails.
+    This can be due to the styling not having an equivalent value or because the
+    styling isn't properly formatted.
+
+    Examples
+    --------
+    >>> df = pd.DataFrame({'A': [1, 1, 1]})
+    >>> df.style.applymap(lambda x: 'background-color: blueGreenRed;')
+    ...         .to_excel('styled.xlsx') # doctest: +SKIP
+    ... # CSSWarning: Unhandled color format: 'blueGreenRed'
+    >>> df.style.applymap(lambda x: 'border: 1px solid red red;')
+    ...         .to_excel('styled.xlsx') # doctest: +SKIP
+    ... # CSSWarning: Too many tokens provided to "border" (expected 1-3)
+    """

pandas/io/formats/css.py

@@ -12,11 +12,7 @@
 )
 import warnings
 
-
-class CSSWarning(UserWarning):
-    """
-    This CSS syntax cannot currently be parsed.
-    """
+from pandas.errors import CSSWarning
 
 
 def _side_expander(prop_fmt: str) -> Callable:

pandas/tests/io/formats/test_css.py

@@ -1,11 +1,10 @@
 import pytest
 
+from pandas.errors import CSSWarning
+
 import pandas._testing as tm
 
-from pandas.io.formats.css import (
-    CSSResolver,
-    CSSWarning,
-)
+from pandas.io.formats.css import CSSResolver
 
 
 def assert_resolves(css, props, inherited=None):

pandas/tests/io/formats/test_to_excel.py

@@ -6,11 +6,11 @@
 
 import pytest
 
+from pandas.errors import CSSWarning
 import pandas.util._test_decorators as td
 
 import pandas._testing as tm
 
-from pandas.io.formats.css import CSSWarning
 from pandas.io.formats.excel import (
     CssExcelCell,
     CSSToExcelConverter,

pandas/tests/test_errors.py

@@ -29,6 +29,7 @@
         "NumExprClobberingError",
         "IndexingError",
         "PyperclipException",
+        "CSSWarning",
     ],
 )
 def test_exception_importable(exc):