add whatsnew

Author: rushabh-v

doc/source/whatsnew/v1.1.0.rst

@@ -162,7 +162,7 @@ MultiIndex
 I/O
 ^^^
 - Bug in :meth:`read_json` where integer overflow was occuring when json contains big number strings. (:issue:`30320`)
--
+- `read_csv` will now raise a ``ValueError`` when arguments `header` and `prefix` both are not None. (:issue:`27394`)
 -
 
 Plotting

pandas/tests/frame/test_to_csv.py

@@ -575,10 +575,10 @@ def test_to_csv_headers(self):
             recons.reset_index(inplace=True)
             tm.assert_frame_equal(to_df, recons)
 
-    def test_to_csv_raises_on_header_prefix():
+    def test_to_csv_raises_on_header_prefix(self):
         msg = "Argument prefix must be None if argument header is not None"
+        s = StringIO("0,1\n2,3")
         with pytest.raises(ValueError, match=msg):
-            s = StringIO("0,1\n2,3")
             read_csv(s, header=0, prefix="_X")
 
     def test_to_csv_multiindex(self, float_frame, datetime_frame):