Closes pandas-dev#9795 (Stata writer changes input frame)

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Author: kshedden

doc/source/whatsnew/v0.16.1.txt

@@ -66,7 +66,7 @@ Bug Fixes
 
 - Bug in ``scatter_matrix`` draws unexpected axis ticklabels (:issue:`5662`)
 
-
+- Fixed bug in ``StataWriter`` resulting in changes to input ``DataFrame`` upon save (:issue:`9795`).
 
 
 - Bug in ``transform`` causing length mismatch when null entries were present and a fast aggregator was being used (:issue:`9697`)

pandas/io/stata.py

@@ -1885,6 +1885,8 @@ def _prepare_pandas(self, data):
         #NOTE: we might need a different API / class for pandas objects so
         # we can set different semantics - handle this with a PR to pandas.io
 
+        data = data.copy()
+
         if self._write_index:
             data = data.reset_index()
 
@@ -2013,7 +2015,7 @@ def _write_variable_labels(self, labels=None):
                 self._write(_pad_bytes("", 81))
 
     def _prepare_data(self):
-        data = self.data.copy()
+        data = self.data
         typlist = self.typlist
         convert_dates = self._convert_dates
         # 1. Convert dates

pandas/io/tests/test_stata.py

@@ -290,6 +290,15 @@ def test_stata_doc_examples(self):
             df = DataFrame(np.random.randn(10, 2), columns=list('AB'))
             df.to_stata(path)
 
+    def test_write_preserves_original(self):
+        # 9795
+        np.random.seed(423)
+        df = pd.DataFrame(np.random.randn(5,4), columns=list('abcd'))
+        df.ix[2, 'a':'c'] = np.nan
+        df_copy = df.copy()
+        df.to_stata('test.dta', write_index=False)
+        tm.assert_frame_equal(df, df_copy)
+
     def test_encoding(self):
 
         # GH 4626, proper encoding handling