BUG: raise ValueError when sep and delimiter are defined in read_csv

doc/source/whatsnew/v1.3.0.rst

@@ -922,6 +922,7 @@ I/O
 - Bug in :func:`read_orc` always raising ``AttributeError`` (:issue:`40918`)
 - Bug in :func:`read_csv` and :func:`read_table` silently ignoring ``prefix`` if ``names`` and ``prefix`` are defined, now raising ``ValueError`` (:issue:`39123`)
 - Bug in :func:`read_csv` and :func:`read_excel` not respecting dtype for duplicated column name when ``mangle_dupe_cols`` is set to ``True`` (:issue:`35211`)
+- Bug in :func:`read_csv` silently ignoring ``sep`` if ``delimiter`` and ``sep`` are defined, now raising ``ValueError`` (:issue:`39823`)
 - Bug in :func:`read_csv` and :func:`read_table` misinterpreting arguments when ``sys.setprofile`` had been previously called (:issue:`41069`)
 - Bug in the conversion from pyarrow to pandas (e.g. for reading Parquet) with nullable dtypes and a pyarrow array whose data buffer size is not a multiple of dtype size (:issue:`40896`)
 - Bug in :func:`read_excel` would raise an error when pandas could not determine the file type, even when user specified the ``engine`` argument (:issue:`41225`)

pandas/io/parsers/readers.py

@@ -1255,6 +1255,9 @@ def _refine_defaults_read(
             sep is lib.no_default or sep == delim_default
         )
 
+    if delimiter and (sep is not lib.no_default):
+        raise ValueError("Specified a sep and a delimiter; you can only specify one.")
+
     if names is not lib.no_default and prefix is not lib.no_default:
         raise ValueError("Specified named and prefix; you can only specify one.")
 

pandas/tests/io/parser/common/test_common_basic.py

@@ -724,6 +724,15 @@ def test_read_csv_delim_whitespace_non_default_sep(all_parsers, delimiter):
         parser.read_csv(f, delim_whitespace=True, delimiter=delimiter)
 
 
+def test_read_csv_delimiter_and_sep_no_default(all_parsers):
+    # GH#39823
+    f = StringIO("a,b\n1,2")
+    parser = all_parsers
+    msg = "Specified a sep and a delimiter; you can only specify one."
+    with pytest.raises(ValueError, match=msg):
+        parser.read_csv(f, sep=" ", delimiter=".")
+
+
 @pytest.mark.parametrize("delimiter", [",", "\t"])
 def test_read_table_delim_whitespace_non_default_sep(all_parsers, delimiter):
     # GH: 35958