✨ NEW: nb_merge_streams configuration

If `True`, ensure all stdout / stderr output streams are merged into single outputs.
This ensures deterministic outputs.

docs/use/start.md

@@ -112,4 +112,7 @@ Then for parsing and output rendering:
 * - `nb_output_stderr`
   - `show`
   - One of 'show', 'remove', 'warn', 'error' or 'severe', [see here](use/format/stderr) for details.
+* - `nb_merge_streams`
+  - `False`
+  - If `True`, ensure all stdout / stderr output streams are merged into single outputs. This ensures deterministic outputs.
 `````

myst_nb/__init__.py

@@ -133,6 +133,7 @@ def visit_element_html(self, node):
     app.add_config_value("nb_render_plugin", "default", "env")
     app.add_config_value("nb_render_text_lexer", "myst-ansi", "env")
     app.add_config_value("nb_output_stderr", "show", "env")
+    app.add_config_value("nb_merge_streams", False, "env")
 
     # Register our post-transform which will convert output bundles to nodes
     app.add_post_transform(PasteNodesToDocutils)

myst_nb/render_outputs.py

@@ -1,5 +1,6 @@
 """A Sphinx post-transform, to convert notebook outpus to AST nodes."""
 import os
+import re
 from abc import ABC, abstractmethod
 from typing import List, Optional
 from unittest import mock
@@ -91,6 +92,58 @@ def load_renderer(name: str) -> "CellOutputRendererBase":
     raise MystNbEntryPointError(f"No Entry Point found for myst_nb.mime_render:{name}")
 
 
+RGX_CARRIAGERETURN = re.compile(r".*\r(?=[^\n])")
+RGX_BACKSPACE = re.compile(r"[^\n]\b")
+
+
+def coalesce_streams(outputs: List[NotebookNode]) -> List[NotebookNode]:
+    """Merge all stream outputs with shared names into single streams.
+
+    This ensure deterministic outputs.
+
+    Adapted from:
+    https://github.com/computationalmodelling/nbval/blob/master/nbval/plugin.py.
+    """
+    if not outputs:
+        return []
+
+    new_outputs = []
+    streams = {}
+    for output in outputs:
+        if output["output_type"] == "stream":
+            if output["name"] in streams:
+                streams[output["name"]]["text"] += output["text"]
+            else:
+                new_outputs.append(output)
+                streams[output["name"]] = output
+        else:
+            new_outputs.append(output)
+
+    # process \r and \b characters
+    for output in streams.values():
+        old = output["text"]
+        while len(output["text"]) < len(old):
+            old = output["text"]
+            # Cancel out anything-but-newline followed by backspace
+            output["text"] = RGX_BACKSPACE.sub("", output["text"])
+        # Replace all carriage returns not followed by newline
+        output["text"] = RGX_CARRIAGERETURN.sub("", output["text"])
+
+    # We also want to ensure stdout and stderr are always in the same consecutive order,
+    # because they are asynchronous, so order isn't guaranteed.
+    for i, output in enumerate(new_outputs):
+        if output["output_type"] == "stream" and output["name"] == "stderr":
+            if (
+                len(new_outputs) >= i + 2
+                and new_outputs[i + 1]["output_type"] == "stream"
+                and new_outputs[i + 1]["name"] == "stdout"
+            ):
+                stdout = new_outputs.pop(i + 1)
+                new_outputs.insert(i, stdout)
+
+    return new_outputs
+
+
 class CellOutputsToNodes(SphinxPostTransform):
     """Use the builder context to transform a CellOutputNode into Sphinx nodes."""
 
@@ -108,6 +161,8 @@ def run(self):
                 renderer_cls = load_renderer(node.renderer)
                 renderers[node.renderer] = renderer_cls
             renderer = renderer_cls(self.document, node, abs_dir)
+            if self.config.nb_merge_streams:
+                node._outputs = coalesce_streams(node.outputs)
             output_nodes = renderer.cell_output_to_nodes(self.env.nb_render_priority)
             node.replace_self(output_nodes)
 

tests/notebooks/merge_streams.ipynb

@@ -0,0 +1,82 @@
+{
+  "cells": [
+    {
+      "cell_type": "code",
+      "execution_count": 1,
+      "source": [
+        "import sys\n",
+        "print('stdout1', file=sys.stdout)\n",
+        "print('stdout2', file=sys.stdout)\n",
+        "print('stderr1', file=sys.stderr)\n",
+        "print('stderr2', file=sys.stderr)\n",
+        "print('stdout3', file=sys.stdout)\n",
+        "print('stderr3', file=sys.stderr)\n",
+        "1"
+      ],
+      "outputs": [
+        {
+          "output_type": "stream",
+          "name": "stdout",
+          "text": [
+            "stdout1\n",
+            "stdout2\n"
+          ]
+        },
+        {
+          "output_type": "stream",
+          "name": "stderr",
+          "text": [
+            "stderr1\n",
+            "stderr2\n"
+          ]
+        },
+        {
+          "output_type": "stream",
+          "name": "stdout",
+          "text": [
+            "stdout3\n"
+          ]
+        },
+        {
+          "output_type": "stream",
+          "name": "stderr",
+          "text": [
+            "stderr3\n"
+          ]
+        },
+        {
+          "output_type": "execute_result",
+          "data": {
+            "text/plain": [
+              "1"
+            ]
+          },
+          "metadata": {},
+          "execution_count": 1
+        }
+      ],
+      "metadata": {}
+    }
+  ],
+  "metadata": {
+    "kernelspec": {
+      "display_name": "Python 3",
+      "language": "python",
+      "name": "python3"
+    },
+    "language_info": {
+      "codemirror_mode": {
+        "name": "ipython",
+        "version": 3
+      },
+      "file_extension": ".py",
+      "mimetype": "text/x-python",
+      "name": "python",
+      "nbconvert_exporter": "python",
+      "pygments_lexer": "ipython3",
+      "version": "3.6.1"
+    }
+  },
+  "nbformat": 4,
+  "nbformat_minor": 2
+}

tests/test_render_outputs.py

@@ -72,6 +72,17 @@ def test_stderr_remove(sphinx_run, file_regression):
     file_regression.check(doctree.pformat(), extension=".xml", encoding="utf8")
 
 
+@pytest.mark.sphinx_params(
+    "merge_streams.ipynb",
+    conf={"jupyter_execute_notebooks": "off", "nb_merge_streams": True},
+)
+def test_merge_streams(sphinx_run, file_regression):
+    sphinx_run.build()
+    assert sphinx_run.warnings() == ""
+    doctree = sphinx_run.get_resolved_doctree("merge_streams")
+    file_regression.check(doctree.pformat(), extension=".xml", encoding="utf8")
+
+
 @pytest.mark.sphinx_params(
     "metadata_image.ipynb",
     conf={"jupyter_execute_notebooks": "off", "nb_render_key": "myst"},

tests/test_render_outputs/test_merge_streams.xml

@@ -0,0 +1,23 @@
+<document source="merge_streams">
+    <CellNode cell_type="code" classes="cell">
+        <CellInputNode classes="cell_input">
+            <literal_block language="ipython3" linenos="False" xml:space="preserve">
+                import sys
+                print('stdout1', file=sys.stdout)
+                print('stdout2', file=sys.stdout)
+                print('stderr1', file=sys.stderr)
+                print('stderr2', file=sys.stderr)
+                print('stdout3', file=sys.stdout)
+                print('stderr3', file=sys.stderr)
+                1
+        <CellOutputNode classes="cell_output">
+            <literal_block classes="output stream" language="myst-ansi" linenos="False" xml:space="preserve">
+                stdout1
+                stdout2
+                stdout3
+            <literal_block classes="output stderr" language="myst-ansi" linenos="False" xml:space="preserve">
+                stderr1
+                stderr2
+                stderr3
+            <literal_block classes="output text_plain" language="myst-ansi" linenos="False" xml:space="preserve">
+                1

tox.ini

@@ -11,9 +11,9 @@
 # then then deleting compiled files has been found to fix it: `find . -name \*.pyc -delete`
 
 [tox]
-envlist = py37-sphinx3
+envlist = py37-sphinx4
 
-[testenv:py{36,37,38,39}-sphinx{3,4}]
+[testenv:py{37,38,39}-sphinx{3,4}]
 extras = testing
 deps =
     sphinx3: sphinx>=3,<4