From a2721fd602e43128314d4efd056dae56a89197bf Mon Sep 17 00:00:00 2001
From: Rushabh Vasani <vasanirushabh24@gmail.com>
Date: Mon, 3 Feb 2020 21:22:26 +0530
Subject: [PATCH] Raise error in read_csv when arguments header and prefix both
 are not None (#31383)

Closes https://github.com/pandas-dev/pandas/issues/27394
---
 doc/source/whatsnew/v1.1.0.rst        |  1 +
 pandas/io/parsers.py                  | 26 +++++++++++++++-----------
 pandas/tests/io/parser/test_common.py | 11 +++++++++++
 3 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/doc/source/whatsnew/v1.1.0.rst b/doc/source/whatsnew/v1.1.0.rst
index 9fdda83abe944..27b28c1c08e23 100644
--- a/doc/source/whatsnew/v1.1.0.rst
+++ b/doc/source/whatsnew/v1.1.0.rst
@@ -177,6 +177,7 @@ 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 the arguments `header` and `prefix` both are not `None`. (:issue:`27394`)
 - Bug in :meth:`DataFrame.to_json` was raising ``NotFoundError`` when ``path_or_buf`` was an S3 URI (:issue:`28375`)
 -
 
diff --git a/pandas/io/parsers.py b/pandas/io/parsers.py
index a33d81ff437bf..b38aa9770a73b 100755
--- a/pandas/io/parsers.py
+++ b/pandas/io/parsers.py
@@ -1399,17 +1399,21 @@ def __init__(self, kwds):
                         "index_col must only contain row numbers "
                         "when specifying a multi-index header"
                     )
-
-        # GH 16338
-        elif self.header is not None and not is_integer(self.header):
-            raise ValueError("header must be integer or list of integers")
-
-        # GH 27779
-        elif self.header is not None and self.header < 0:
-            raise ValueError(
-                "Passing negative integer to header is invalid. "
-                "For no header, use header=None instead"
-            )
+        elif self.header is not None:
+            # GH 27394
+            if self.prefix is not None:
+                raise ValueError(
+                    "Argument prefix must be None if argument header is not None"
+                )
+            # GH 16338
+            elif not is_integer(self.header):
+                raise ValueError("header must be integer or list of integers")
+            # GH 27779
+            elif self.header < 0:
+                raise ValueError(
+                    "Passing negative integer to header is invalid. "
+                    "For no header, use header=None instead"
+                )
 
         self._name_processed = False
 
diff --git a/pandas/tests/io/parser/test_common.py b/pandas/tests/io/parser/test_common.py
index 6c17f40b790ac..c19056d434ec3 100644
--- a/pandas/tests/io/parser/test_common.py
+++ b/pandas/tests/io/parser/test_common.py
@@ -2040,6 +2040,17 @@ def test_read_csv_memory_growth_chunksize(all_parsers):
             pass
 
 
+def test_read_csv_raises_on_header_prefix(all_parsers):
+    # gh-27394
+    parser = all_parsers
+    msg = "Argument prefix must be None if argument header is not None"
+
+    s = StringIO("0,1\n2,3")
+
+    with pytest.raises(ValueError, match=msg):
+        parser.read_csv(s, header=0, prefix="_X")
+
+
 def test_read_table_equivalency_to_read_csv(all_parsers):
     # see gh-21948
     # As of 0.25.0, read_table is undeprecated