ENH/TST: Raise error if invalid value passed to if_exists argument

The if_exists argument in io.sql.write_frame needed data validation
because the logic of the function implicitly used 'append' if the argument
value was any string that was not either 'fail' or 'replace'.
I added a new unit test to support the requirement.

Author: davidshinn

pandas/io/sql.py

@@ -196,6 +196,10 @@ def write_frame(frame, name, con, flavor='sqlite', if_exists='fail', **kwargs):
             if_exists='append'
         else:
             if_exists='fail'
+    
+    if if_exists not in ('fail', 'replace', 'append'):
+        raise ValueError, "'%s' is not valid for if_exists" % if_exists
+
     exists = table_exists(name, con, flavor)
     if if_exists == 'fail' and exists:
         raise ValueError, "Table '%s' already exists." % name

pandas/io/tests/test_sql.py

@@ -240,6 +240,24 @@ def test_onecolumn_of_integer(self):
         result = sql.read_frame("select * from mono_df",con_x)
         tm.assert_frame_equal(result,mono_df)
 
+    def test_if_exists(self):
+        df_if_exists_1 = DataFrame({'col1': [1, 2], 'col2': ['A', 'B']})
+        df_if_exists_2 = DataFrame({'col1': [3, 4, 5], 'col2': ['C', 'D', 'E']})
+        table_name = 'table_if_exists'
+
+        # test if invalid value for if_exists raises appropriate error
+        self.assertRaises(ValueError,
+                          sql.write_frame,
+                          frame=df_if_exists_1,
+                          con=self.db,
+                          name=table_name,
+                          flavor='sqlite',
+                          if_exists='notvalidvalue')
+        if sql.table_exists(table_name, self.db, flavor='sqlite'):
+            cur = self.db.cursor()
+            cur.execute("DROP TABLE %s" % table_name)
+            cur.close()
+
 
 class TestMySQL(unittest.TestCase):