BUG: read_fwf raise if len colspec doesnt match len names

doc/source/whatsnew/v1.4.0.rst

@@ -258,7 +258,7 @@ I/O
 - Bug in :func:`read_excel` attempting to read chart sheets from .xlsx files (:issue:`41448`)
 - Bug in :func:`json_normalize` where ``errors=ignore`` could fail to ignore missing values of ``meta`` when ``record_path`` has a length greater than one (:issue:`41876`)
 - Bug in :func:`read_csv` with multi-header input and arguments referencing column names as tuples (:issue:`42446`)
--
+- Bug in :func:`read_fwf` where difference in length of ``colspecs`` and ``names`` did not throw any error (:issue:`40830`)
 
 Period
 ^^^^^^

pandas/io/parsers/readers.py

@@ -756,6 +756,26 @@ def read_fwf(
             colspecs.append((col, col + w))
             col += w
 
+    # GH#40830
+    # Ensure length of `colspecs` matches length of `names`
+    names = kwds.get("names")
+    if names is not None:
+        if len(names) != len(colspecs):
+            # maybe name of index(s) not present in `names`
+            len_index = 0
+            if kwds.get("index_col") is not None:
+                index_col = kwds.get("index_col")
+                if isinstance(index_col, int) or isinstance(index_col, str):
+                    len_index = 1
+                elif index_col is False:
+                    len_index = 0
+                else:
+                    # error: Argument 1 to "len" has incompatible type
+                    # "Optional[Any]"; expected "Sized"  [arg-type]
+                    len_index = len(index_col)  # type: ignore[arg-type]
+            if len(names) + len_index != len(colspecs):
+                raise ValueError("Length of colspecs must match length of names")
+
     kwds["colspecs"] = colspecs
     kwds["infer_nrows"] = infer_nrows
     kwds["engine"] = "python-fwf"

pandas/tests/io/parser/test_read_fwf.py

@@ -710,3 +710,20 @@ def test_encoding_mmap(memory_map):
     data.seek(0)
     df_reference = DataFrame([[1, "A", "Ä", 2]])
     tm.assert_frame_equal(df, df_reference)
+
+
+@pytest.mark.parametrize(
+    "colspecs, names, widths",
+    [
+        ([(0, 6), (6, 12), (12, 18), (18, None)], list("abcde"), None),
+        ([6] * 4, list("abcde"), None),
+        (None, list("abcde"), [6] * 4),
+    ],
+)
+def test_len_colspecs_len_names(colspecs, names, widths):
+    # GH#40830
+    data = """col1  col2  col3  col4
+    bab   ba    2"""
+    msg = "Length of colspecs must match length of names"
+    with pytest.raises(ValueError, match=msg):
+        read_fwf(StringIO(data), colspecs=colspecs, names=names, widths=widths)