BUG: DataFrame.to_html validates formatters has the correct length (pandas-dev#28632)

Author: guipleite

doc/source/whatsnew/v1.0.0.rst

@@ -309,6 +309,7 @@ I/O
 - Bug in :func:`DataFrame.to_string` where values were truncated using display options instead of outputting the full content (:issue:`9784`)
 - Bug in :meth:`DataFrame.to_json` where a datetime column label would not be written out in ISO format with ``orient="table"`` (:issue:`28130`)
 - Bug in :func:`DataFrame.to_parquet` where writing to GCS would fail with `engine='fastparquet'` if the file did not already exist (:issue:`28326`)
+- Bug in :meth:`DataFrame.to_html` where the length of the ``formatters`` argument was not verified (:issue:`28469`)
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -561,7 +561,17 @@ def __init__(
         self.sparsify = sparsify
 
         self.float_format = float_format
-        self.formatters = formatters if formatters is not None else {}
+        if formatters is None:
+            self.formatters = {}
+        elif len(frame.columns) == len(formatters) or isinstance(formatters, dict):
+            self.formatters = formatters
+        else:
+            raise ValueError(
+                (
+                    "Formatters length({flen}) should match"
+                    " DataFrame number of columns({dlen})"
+                ).format(flen=len(formatters), dlen=len(frame.columns))
+            )
         self.na_rep = na_rep
         self.decimal = decimal
         self.col_space = col_space

pandas/tests/io/formats/test_to_html.py

@@ -235,6 +235,15 @@ def test_to_html_truncate(datapath):
     assert result == expected
 
 
+@pytest.mark.parametrize("size", [1, 5])
+def test_html_invalid_formatters_arg_raises(size):
+    # issue-28469
+    df = DataFrame(columns=["a", "b", "c"])
+    msg = "Formatters length({}) should match DataFrame number of columns(3)"
+    with pytest.raises(ValueError, match=re.escape(msg.format(size))):
+        df.to_html(formatters=["{}".format] * size)
+
+
 def test_to_html_truncate_formatter(datapath):
     # issue-25955
     data = [