Disable and add to the list by default

nbgrader/converters/autograde.py

@@ -9,7 +9,7 @@
 from .base import BaseConverter, NbGraderException
 from ..preprocessors import (
     AssignLatePenalties, ClearOutput, DeduplicateIds, OverwriteCells, SaveAutoGrades,
-    Execute, LimitOutput, OverwriteKernelspec, CheckCellMetadata)
+    Execute, LimitOutput, OverwriteKernelspec, CheckCellMetadata, IgnorePattern)
 from ..api import Gradebook, MissingEntry
 from .. import utils
 
@@ -61,6 +61,7 @@ def _output_directory(self) -> str:
     ]).tag(config=True)
     autograde_preprocessors = List([
         Execute,
+        IgnorePattern,
         ClearMetadataPreprocessor,
         LimitOutput,
         SaveAutoGrades,

nbgrader/preprocessors/ignorepattern.py

@@ -1,6 +1,6 @@
 from . import NbGraderPreprocessor
 
-from traitlets import Unicode
+from traitlets import Unicode, Bool
 from nbformat.notebooknode import NotebookNode
 from nbconvert.exporters.exporter import ResourcesDict
 from typing import Tuple
@@ -11,6 +11,7 @@ class IgnorePattern(NbGraderPreprocessor):
     """Preprocessor for removing cell outputs that match a particular pattern"""
 
     pattern = Unicode("", help="The regular expression to remove from stderr").tag(config=True)
+    enabled = Bool(False, help="Whether to use this preprocessor when running nbgrader").tag(config=True)
 
     def preprocess_cell(self,
                         cell: NotebookNode,