Fix issue with to_csv na_rep when dtype=string

Author: jbman223

pandas/core/internals/blocks.py

@@ -1774,8 +1774,8 @@ def to_native_types(self, slicer=None, na_rep="nan", quoting=None, **kwargs):
         mask = isna(values)
 
         try:
-            values = values.astype(str)
             values[mask] = na_rep
+            values = values.astype(str)
         except Exception:
             # eg SparseArray does not support setitem, needs to be converted to ndarray
             return super().to_native_types(slicer, na_rep, quoting, **kwargs)

pandas/tests/io/formats/test_to_csv.py

@@ -204,6 +204,14 @@ def test_to_csv_na_rep(self):
         assert df.set_index("a").to_csv(na_rep="_") == expected
         assert df.set_index(["a", "b"]).to_csv(na_rep="_") == expected
 
+        # GH 29975
+        # Make sure full na_rep shows up when a dtype is provided
+        csv = pd.Series(["a", pd.NA, "c"]).to_csv(na_rep="ZZZZZ")
+        assert ",0\n0,a\n1,ZZZZZ\n2,c\n" == csv
+
+        csv = pd.Series(["a", pd.NA, "c"], dtype="string").to_csv(na_rep="ZZZZZ")
+        assert ",0\n0,a\n1,ZZZZZ\n2,c\n" == csv
+
     def test_to_csv_date_format(self):
         # GH 10209
         df_sec = DataFrame({"A": pd.date_range("20130101", periods=5, freq="s")})