add a coalesce stream transformer

converters/template.py

@@ -129,6 +129,7 b' class ConverterTemplate(Configurable):'
             self.preprocessors.append(transformer)
 
         ## for compat, remove later
+        self.preprocessors.append(trans.coalesce_streams)
         self.preprocessors.append(trans.ExtractFigureTransformer(config=config))
         self.preprocessors.append(trans.RevealHelpTransformer(config=config))
         self.preprocessors.append(trans.CSSHtmlHeaderTransformer(config=config))

converters/transformers.py

@@ -74,6 +74,33 b' def haspyout_transformer(cell, other, count):'
             break
     return cell, other
 
+@cell_preprocessor
+def coalesce_streams(cell, other, count):
+    """merge consecutive sequences of stream output into single stream
+
+    to prevent extra newlines inserted at flush calls
+
+    TODO: handle \r deletion
+    """
+
+    outputs = cell.get('outputs', None)
+    if not outputs:
+        return cell, other
+    new_outputs = []
+    last = outputs[0]
+    new_outputs = [last]
+    for output in outputs[1:]:
+        if (output.output_type == 'stream' and
+            last.output_type == 'stream' and
+            last.stream == output.stream
+        ):
+            last.text += output.text
+        else:
+            new_outputs.append(output)
+
+    cell.outputs = new_outputs
+    return cell, other
+
 
 # todo, make the key part configurable.