make requested changes

Author: rushabh-v

pandas/io/parsers.py

@@ -1400,14 +1400,14 @@ def __init__(self, kwds):
                         "when specifying a multi-index header"
                     )
         elif self.header is not None:
-            # GH 16338
-            if not is_integer(self.header):
-                raise ValueError("header must be integer or list of integers")
             # GH 27394
-            elif self.prefix is not None:
+            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(

pandas/tests/io/parser/test_common.py

@@ -2043,7 +2043,7 @@ def test_read_csv_memory_growth_chunksize(all_parsers):
 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")
+    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")