WIP Recreate old behaviour of ExecutePreprocessor(kernel_name="")

nbconvert/preprocessors/execute.py

@@ -4,6 +4,8 @@
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
+import warnings
+
 from textwrap import dedent
 from contextlib import contextmanager
 
@@ -161,7 +163,7 @@ class ExecutePreprocessor(Preprocessor):
     @default('kernel_name')
     def _kernel_name_default(self):
         try:
-            return self.nb.metadata.get('kernelspec', {}).get('name', 'python')
+            return find_kernel_name(self.nb)
         except AttributeError:
             raise AttributeError('You did not specify a kernel_name for '
                                  'the ExecutePreprocessor and you have not set '
@@ -248,7 +250,19 @@ def start_new_kernel(self, **kwargs):
         kc : KernelClient
             Kernel client as created by the kernel manager `km`.
         """
-        km = self.kernel_manager_class(kernel_name=self.kernel_name,
+
+        # Because of an old API, an empty kernel string should be interpreted as indicating
+        # that you want to use the notebook's metadata specified kernel.
+        # We use find_kernel_name to do this.
+        if not self.kernel_name:
+            kernel_name = find_kernel_name(self.nb)
+            warnings.warn("Setting kernel_name to '' as a way to use the kernel in a notebook metadata's is deprecated as of 5.4. \n"
+                          "Instead, do not set kernel_name, this is now default behaviour.", DeprecationWarning, stacklevel=2)
+        else:
+            kernel_name = self.kernel_name
+
+
+        km = self.kernel_manager_class(kernel_name=kernel_name,
                                        config=self.config)
         km.start_kernel(extra_arguments=self.extra_arguments, **kwargs)
 
@@ -516,6 +530,20 @@ def run_cell(self, cell, cell_index=0):
 
         return exec_reply, outs
 
+def find_kernel_name(nb):
+    """This is a utility that finds kernel names from notebook metadata.
+
+    Parameters
+    ----------
+    nb : NotebookNode
+        The notebook that has a kernelspec defined in its metadata.
+
+    Returns
+    -------
+    kernel_name: str
+        The string representing the kernel name found in the metadata.
+    """
+    return nb.metadata.get('kernelspec', {}).get('name', 'python')
 
 def executenb(nb, cwd=None, km=None, **kwargs):
     """Execute a notebook's code, updating outputs within the notebook object.

nbconvert/preprocessors/tests/files/UnicodePy3.ipynb

@@ -0,0 +1,32 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "\u2603\n"
+     ]
+    }
+   ],
+   "source": [
+    "print('\u2603')"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}

nbconvert/preprocessors/tests/test_execute.py

@@ -19,12 +19,15 @@
 import pytest
 
 from .base import PreprocessorTestsBase
-from ..execute import ExecutePreprocessor, CellExecutionError, executenb
+from ..execute import ExecutePreprocessor, CellExecutionError, executenb, find_kernel_name
 
-from nbconvert.filters import strip_ansi
 from testpath import modified_env
+from traitlets import TraitError
+from jupyter_client.kernelspec import KernelSpecManager
 from ipython_genutils.py3compat import string_types
 
+from nbconvert.filters import strip_ansi
+
 addr_pat = re.compile(r'0x[0-9a-f]{7,9}')
 ipython_input_pat = re.compile(r'<ipython-input-\d+-[0-9a-f]+>')
 current_dir = os.path.dirname(__file__)
@@ -151,6 +154,21 @@ def test_empty_path(self):
         res['metadata']['path'] = ''
         input_nb, output_nb = self.run_notebook(filename, {}, res)
         self.assert_notebooks_equal(input_nb, output_nb)
+
+    @pytest.mark.xfail("python3" not in KernelSpecManager().find_kernel_specs(),
+                        reason="requires a python3 kernelspec")
+    def test_empty_kernel_name(self):
+        """Can kernel in nb metadata be found when an empty string is passed?
+
+        Note: this pattern should be discouraged in practice.
+        Passing in no kernel_name to ExecutePreprocessor is recommended instead.
+        """
+        filename = os.path.join(current_dir, 'files', 'UnicodePy3.ipynb')
+        res = self.build_resources()
+        input_nb, output_nb = self.run_notebook(filename, {"kernel_name": ""}, res)
+        self.assert_notebooks_equal(input_nb, output_nb)
+        with pytest.raises(TraitError):
+            input_nb, output_nb = self.run_notebook(filename, {"kernel_name": None}, res)
 
     def test_disable_stdin(self):
         """Test disabling standard input"""
@@ -265,6 +283,15 @@ def test_custom_kernel_manager(self):
         for method, call_count in expected:
             self.assertNotEqual(call_count, 0, '{} was called'.format(method))
 
+    def test_find_kernel_name(self):
+        current_dir = os.path.dirname(__file__)
+        filename = os.path.join(current_dir, 'files', 'UnicodePy3.ipynb')
+
+        with io.open(filename) as f:
+            input_nb = nbformat.read(f, 4)
+
+        assert find_kernel_name(input_nb) == "python3"
+
     def test_execute_function(self):
         # Test the executenb() convenience API
         current_dir = os.path.dirname(__file__)