Allow cell tags as alternative options source

Reads cell metadata tags as an alternative to cell comments as a source
of options.

nbval/plugin.py

@@ -10,6 +10,7 @@
 import sys
 import re
 import hashlib
+import warnings
 from collections import OrderedDict, defaultdict
 
 # for python 3 compatibility
@@ -105,6 +106,12 @@ def pytest_collect_file(path, parent):
     'NBVAL_RAISES_EXCEPTION': 'check_exception',
 }
 
+metadata_tags = {
+    k.lower().replace('_', '-'): v
+    for (k, v) in comment_markers.items()
+}
+
+
 def find_comment_markers(cellsource):
     """Look through the cell source for comments which affect nbval's behaviour
 
@@ -120,6 +127,18 @@ def find_comment_markers(cellsource):
                 yield (comment_markers[comment], True)
 
 
+def find_metadata_tags(cell_metadata):
+    tags = cell_metadata.get('tags', None)
+    if tags is None:
+        return
+    elif not isinstance(tags, list):
+        warnings.warn("Cell tags is not a list, ignoring.")
+        return
+    for tag in tags:
+        if tag in metadata_tags:
+            yield (metadata_tags[tag], True)
+
+
 class IPyNbFile(pytest.File):
     """
     This class represents a pytest collector object.
@@ -212,7 +231,8 @@ def collect(self):
                 # The cell may contain a comment indicating that its output
                 # should be checked or ignored. If it doesn't, use the default
                 # behaviour. The --nbval option checks unmarked cells.
-                options = defaultdict(bool, find_comment_markers(cell.source))
+                options = defaultdict(bool, find_metadata_tags(cell.metadata))
+                options.update(find_comment_markers(cell.source))
                 options.setdefault('check', self.compare_outputs)
                 yield IPyNbCell('Cell ' + str(cell_num), self, cell_num,
                                 cell, options)
@@ -267,6 +287,9 @@ def reportinfo(self):
         description = "cell %d" % self.cell_num
         return self.fspath, 0, description
 
+    def should_compare_outputs(self):
+        return self.options['check'] and not self.options['ignore']
+
     def compare_outputs(self, test, ref, skip_compare=None):
         # Use stored skips unless passed a specific value
         skip_compare = skip_compare or self.parent.skip_compare
@@ -570,7 +593,7 @@ def runtest(self):
         #     self.diff_number_outputs(outs, self.cell.outputs)
         #     failed = True
         failed = False
-        if self.options['check'] and not self.options['ignore']:
+        if self.should_compare_outputs():
             if not self.compare_outputs(outs, self.cell.outputs):
                 failed = True
 

tests/sample_notebook.ipynb

@@ -44,6 +44,13 @@
     "from datetime import datetime"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Different ways to ignore output differences."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 6,
@@ -67,6 +74,76 @@
     "datetime.now()"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "datetime.datetime(2017, 1, 3, 16, 6, 20, 529355)"
+      ]
+     },
+     "execution_count": 4,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "# NBVAL_IGNORE_OUTPUT\n",
+    "datetime.now()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "metadata": {
+    "collapsed": false,
+    "tags": [
+     "nbval-ignore-output"
+    ]
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "datetime.datetime(2017, 1, 3, 16, 6, 20, 543350)"
+      ]
+     },
+     "execution_count": 5,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "datetime.now()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "metadata": {
+    "collapsed": false,
+    "tags": [
+     "nbval-check-output"
+    ]
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "5\n"
+     ]
+    }
+   ],
+   "source": [
+    "print(5)"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 31,