Set prompt number to None

IPython/nbconvert/preprocessors/clearoutput.py

@@ -24,6 +24,5 b' class ClearOutputPreprocessor(Preprocessor):'
         """
         if cell.cell_type == 'code':
             cell.outputs = []
-            if 'prompt_number' in cell:
-                del cell['prompt_number']
+            cell['prompt_number'] = None
         return cell, resources

IPython/nbconvert/preprocessors/tests/test_clearoutput.py

@@ -39,4 +39,4 b' class TestClearOutput(PreprocessorTestsBase):'
         preprocessor = self.build_preprocessor()
         nb, res = preprocessor(nb, res)
         assert nb.worksheets[0].cells[0].outputs == []
-        assert 'prompt_number' not in nb.worksheets[0].cells[0]
+        assert nb.worksheets[0].cells[0].prompt_number is None