From 06dd749b9f671d646145ca89db3015c5bddef330 Mon Sep 17 00:00:00 2001
From: Marc Wouts <marc.wouts@gmail.com>
Date: Thu, 30 Jul 2020 23:42:40 +0200
Subject: [PATCH] MyST Markdown files are recognized as such even if `myst` is
 missing

---
 CHANGELOG.md                |  3 ++-
 jupytext/formats.py         | 13 ++++++++++++
 tests/test_ipynb_to_myst.py | 40 ++++++++++++++++++++++++++++++++-----
 3 files changed, 50 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c286c1f70..16149847c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,9 +16,10 @@ error will occur if the required dependencies, resp. `myst-parser` and `pandoc`,
 **Fixed**
 - Configured coverage targets in `codecov.yml`
 - Only scripts can have an encoding comment, not Markdown or R Markdown files (#576)
-- Support spaces in `--pipe` commands (#562)
+- Spaces in `--pipe` commands are supported (#562)
 - Use `>=` and `<` rather than `~=` in the `extras_require` of `setup.py` (#589)
 - Bash commands starting with special characters are now correctly detected, thanks to Aaron Gokaslan (#587)
+- MyST Markdown files are recognized as such even if `myst` is missing (#556)
 
 
 1.5.2 (2020-07-21)
diff --git a/jupytext/formats.py b/jupytext/formats.py
index e349c057f..10731eaad 100644
--- a/jupytext/formats.py
+++ b/jupytext/formats.py
@@ -5,6 +5,7 @@
 
 import os
 import re
+import yaml
 import warnings
 import nbformat
 from .header import header_to_metadata_and_cell, insert_or_test_version_number
@@ -261,6 +262,18 @@ def read_metadata(text, ext):
     if ext in [".r", ".R"] and not metadata:
         metadata, _, _, _ = header_to_metadata_and_cell(lines, "#'", ext)
 
+    # MyST has the metadata at the root level
+    if not metadata and ext in myst_extensions() and text.startswith("---"):
+        for header in yaml.safe_load_all(text):
+            if (
+                header.get("jupytext", {})
+                .get("text_representation", {})
+                .get("format_name")
+                == "myst"
+            ):
+                return header
+            return metadata
+
     return metadata
 
 
diff --git a/tests/test_ipynb_to_myst.py b/tests/test_ipynb_to_myst.py
index 89de264a9..12a6bb974 100644
--- a/tests/test_ipynb_to_myst.py
+++ b/tests/test_ipynb_to_myst.py
@@ -5,7 +5,7 @@
 from textwrap import dedent
 import pytest
 from nbformat.v4.nbbase import new_notebook
-
+from tornado.web import HTTPError
 from jupytext.formats import get_format_implementation, JupytextFormatError
 from jupytext.myst import (
     myst_to_notebook,
@@ -149,12 +149,42 @@ def test_add_source_map():
     assert notebook.metadata.source_map == [3, 5, 7, 12]
 
 
+PLEASE_INSTALL_MYST = "The MyST Markdown format requires 'myst_parser>=0.8'."
+
+
 @requires_no_myst
-def test_meaningfull_error_when_myst_is_missing(tmpdir):
+def test_meaningfull_error_write_myst_missing(tmpdir):
     nb_file = tmpdir.join("notebook.ipynb")
     jupytext.write(new_notebook(), str(nb_file))
 
-    with pytest.raises(
-        ImportError, match="The MyST Markdown format requires 'myst_parser>=0.8'."
-    ):
+    with pytest.raises(ImportError, match=PLEASE_INSTALL_MYST):
         jupytext_cli([str(nb_file), "--to", "md:myst"])
+
+
+@requires_no_myst
+def test_meaningfull_error_open_myst_missing(tmpdir):
+    md_file = tmpdir.join("notebook.md")
+    md_file.write(
+        """---
+jupytext:
+  text_representation:
+    extension: '.md'
+    format_name: myst
+kernelspec:
+  display_name: Python 3
+  language: python
+  name: python3
+---
+
+1 + 1
+"""
+    )
+
+    with pytest.raises(ImportError, match=PLEASE_INSTALL_MYST):
+        jupytext_cli([str(md_file), "--to", "ipynb"])
+
+    cm = jupytext.TextFileContentsManager()
+    cm.root_dir = str(tmpdir)
+
+    with pytest.raises(HTTPError, match=PLEASE_INSTALL_MYST):
+        cm.get("notebook.md")