perf improvements for other native type writers

Author: jreback

doc/source/whatsnew/v0.16.1.txt

@@ -89,7 +89,8 @@ API changes
 Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-- Improved csv write performance with mixed dtypes, including datetimes (:issue:`9940`)
+- Improved csv write performance with mixed dtypes, including datetimes by up to 5x (:issue:`9940`)
+- Improved csv write performance generally by 2x (:issue:`9940`)
 
 
 

pandas/core/internals.py

@@ -490,8 +490,8 @@ def to_native_types(self, slicer=None, na_rep='', **kwargs):
         values = self.values
         if slicer is not None:
             values = values[:, slicer]
-        values = np.array(values, dtype=object)
         mask = isnull(values)
+        values = values.astype(str)
         values[mask] = na_rep
         return values.tolist()
 
@@ -1227,20 +1227,22 @@ def to_native_types(self, slicer=None, na_rep='', float_format=None, decimal='.'
         values = self.values
         if slicer is not None:
             values = values[:, slicer]
-        values = np.array(values, dtype=object)
         mask = isnull(values)
-        values[mask] = na_rep
-
 
+        formatter = None
         if float_format and decimal != '.':
             formatter = lambda v : (float_format % v).replace('.',decimal,1)
         elif decimal != '.':
             formatter = lambda v : ('%g' % v).replace('.',decimal,1)
         elif float_format:
             formatter = lambda v : float_format % v
+
+        if formatter is None:
+            values = values.astype(str)
         else:
-            formatter = None
+            values = np.array(values, dtype='object')
 
+        values[mask] = na_rep
         if formatter:
             imask = (~mask).ravel()
             values.flat[imask] = np.array(
@@ -1770,8 +1772,8 @@ def to_native_types(self, slicer=None, na_rep='', **kwargs):
         if slicer is not None:
             # Categorical is always one dimension
             values = values[slicer]
-        values = np.array(values, dtype=object)
         mask = isnull(values)
+        values = values.astype(str)
         values[mask] = na_rep
         # Blocks.to_native_type returns list of lists, but we are always only a list
         return [values.tolist()]