diff --git a/dp_creator_ii/converters.py b/dp_creator_ii/converters.py
new file mode 100644
index 0000000..6572f08
--- /dev/null
+++ b/dp_creator_ii/converters.py
@@ -0,0 +1,23 @@
+from pathlib import Path
+from tempfile import TemporaryDirectory
+import subprocess
+
+
+def convert_py_to_nb(python_str):
+    '''
+    Given Python code as a string, returns a notebook as a string.
+    Calls jupytext as a subprocess:
+    Not ideal, but only the CLI is documented well.
+    '''
+    with TemporaryDirectory() as temp_dir:
+        temp_dir_path = Path(temp_dir)
+        py_path = temp_dir_path / 'input.py'
+        py_path.write_text(python_str)
+        nb_path = temp_dir_path / 'output.ipynb'
+        subprocess.run(
+            ['jupytext',
+             '--to', 'ipynb',  # Target format
+             '--output', nb_path.absolute(),  # Output
+             py_path.absolute()],  # Input
+            check=True)
+        return nb_path.read_text()
diff --git a/dp_creator_ii/template.py b/dp_creator_ii/template.py
index 8bf3eb0..c990e70 100644
--- a/dp_creator_ii/template.py
+++ b/dp_creator_ii/template.py
@@ -34,7 +34,7 @@ def __str__(self):
         if unfilled:
             raise Exception(
                 f'Template {self._path} has unfilled slots: '
-                f'{", ".join(unfilled)}\n\n{self._template}')
+                f'{", ".join(sorted(unfilled))}\n\n{self._template}')
         return self._template
 
 
diff --git a/dp_creator_ii/tests/fake.csv b/dp_creator_ii/tests/fixtures/fake.csv
similarity index 100%
rename from dp_creator_ii/tests/fake.csv
rename to dp_creator_ii/tests/fixtures/fake.csv
diff --git a/dp_creator_ii/tests/fixtures/fake.ipynb b/dp_creator_ii/tests/fixtures/fake.ipynb
new file mode 100644
index 0000000..b2f49b1
--- /dev/null
+++ b/dp_creator_ii/tests/fixtures/fake.ipynb
@@ -0,0 +1,39 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "3cf946cc",
+   "metadata": {},
+   "source": [
+    "Introduction"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "d5a25047",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "print(2+2)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "08581071",
+   "metadata": {},
+   "source": [
+    "Conclusion"
+   ]
+  }
+ ],
+ "metadata": {
+  "jupytext": {
+   "cell_metadata_filter": "-all",
+   "main_language": "python",
+   "notebook_metadata_filter": "-all"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}
diff --git a/dp_creator_ii/tests/fixtures/fake.py b/dp_creator_ii/tests/fixtures/fake.py
new file mode 100644
index 0000000..a314dee
--- /dev/null
+++ b/dp_creator_ii/tests/fixtures/fake.py
@@ -0,0 +1,5 @@
+# Introduction
+
+print(2+2)
+
+# Conclusion
diff --git a/dp_creator_ii/tests/test_converters.py b/dp_creator_ii/tests/test_converters.py
new file mode 100644
index 0000000..a4d134b
--- /dev/null
+++ b/dp_creator_ii/tests/test_converters.py
@@ -0,0 +1,18 @@
+import re
+from pathlib import Path
+from dp_creator_ii.converters import convert_py_to_nb
+
+
+def test_convert_py_to_nb():
+    fixtures_path = Path('dp_creator_ii/tests/fixtures')
+    python_str = (fixtures_path / 'fake.py').read_text()
+    actual_nb_str = convert_py_to_nb(python_str)
+    expected_nb_str = (fixtures_path / 'fake.ipynb').read_text()
+
+    def norm_nb(nb_str):
+        normed_nb_str = re.sub(r'"id": "[^"]+"', '"id": "12345678"', nb_str)
+        return normed_nb_str
+
+    normed_actual_nb_str = norm_nb(actual_nb_str)
+    normed_expected_nb_str = norm_nb(expected_nb_str)
+    assert normed_actual_nb_str == normed_expected_nb_str
diff --git a/dp_creator_ii/tests/test_template.py b/dp_creator_ii/tests/test_template.py
index 9a89ae9..0b975e5 100644
--- a/dp_creator_ii/tests/test_template.py
+++ b/dp_creator_ii/tests/test_template.py
@@ -1,10 +1,12 @@
 from tempfile import NamedTemporaryFile
 import subprocess
+import re
+import pytest
 import opendp.prelude as dp
 from dp_creator_ii.template import _Template, make_notebook, make_script
 
 
-fake_csv = 'dp_creator_ii/tests/fake.csv'
+fake_csv = 'dp_creator_ii/tests/fixtures/fake.csv'
 
 
 def test_fill_template():
@@ -18,6 +20,16 @@ def test_fill_template():
     assert f"data=pl.scan_csv('{fake_csv}')" in context_block
 
 
+def test_fill_template_unfilled_slots():
+    context_template = _Template('context.py')
+    with pytest.raises(
+        Exception,
+        match=re.escape(
+            'context.py has unfilled slots: CSV_PATH, LOSS, UNIT, WEIGHTS')
+    ):
+        str(context_template.fill_values())
+
+
 def test_make_notebook():
     notebook = make_notebook(
         csv_path=fake_csv,
diff --git a/pyproject.toml b/pyproject.toml
index 1e69e4b..2128216 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,7 @@ dependencies = [
     "shiny",
     "shinywidgets",
     "opendp[polars]",
+    "jupytext",
 ]
 
 [project.scripts]
diff --git a/requirements-dev.in b/requirements-dev.in
index f0b9fa3..6ca0900 100644
--- a/requirements-dev.in
+++ b/requirements-dev.in
@@ -1,22 +1,29 @@
 # After making changes here, run:
-#    pip-compile requirements-dev.in
+#    pip-compile requirements-dev.in && pip install -r requirements-dev.txt
 
+# Developer tools:
 pip-tools
 flit
 autoflake
 
-# opendp
-opendp[polars]
-# For Python 3.9:
-scipy<1.14
-
-
-# testing:
+# Testing:
 pytest
 flake8
 mypy
 coverage
 
-# shiny:
+
+
+# Everything below should also be listed in pyproject.toml:
+
+# OpenDP:
+opendp[polars]
+# For Python 3.9:
+scipy<1.14
+
+# Conversion:
+jupytext
+
+# Shiny:
 shiny
 shinywidgets
\ No newline at end of file
diff --git a/requirements-dev.txt b/requirements-dev.txt
index c7eb48e..3842683 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -14,6 +14,10 @@ asgiref==3.8.1
     # via shiny
 asttokens==2.4.1
     # via stack-data
+attrs==24.2.0
+    # via
+    #   jsonschema
+    #   referencing
 autoflake==2.3.1
     # via -r requirements-dev.in
 build==1.2.2
@@ -37,6 +41,8 @@ docutils==0.21.2
     # via flit
 executing==2.1.0
     # via stack-data
+fastjsonschema==2.20.0
+    # via nbformat
 flake8==7.1.1
     # via -r requirements-dev.in
 flit==3.9.0
@@ -61,14 +67,23 @@ jedi==0.19.1
     # via ipython
 joblib==1.4.2
     # via scikit-learn
+jsonschema==4.23.0
+    # via nbformat
+jsonschema-specifications==2023.12.1
+    # via jsonschema
 jupyter-core==5.7.2
-    # via shinywidgets
+    # via
+    #   nbformat
+    #   shinywidgets
 jupyterlab-widgets==3.0.13
     # via ipywidgets
+jupytext==1.16.4
+    # via -r requirements-dev.in
 linkify-it-py==2.0.3
     # via shiny
 markdown-it-py==3.0.0
     # via
+    #   jupytext
     #   mdit-py-plugins
     #   shiny
 matplotlib-inline==0.1.7
@@ -76,13 +91,17 @@ matplotlib-inline==0.1.7
 mccabe==0.7.0
     # via flake8
 mdit-py-plugins==0.4.2
-    # via shiny
+    # via
+    #   jupytext
+    #   shiny
 mdurl==0.1.2
     # via markdown-it-py
 mypy==1.11.2
     # via -r requirements-dev.in
 mypy-extensions==1.0.0
     # via mypy
+nbformat==5.10.4
+    # via jupytext
 numpy==1.26.4
     # via
     #   opendp
@@ -96,6 +115,7 @@ packaging==24.1
     # via
     #   build
     #   htmltools
+    #   jupytext
     #   pytest
     #   shiny
 parso==0.8.4
@@ -139,12 +159,22 @@ python-dateutil==2.9.0.post0
     # via shinywidgets
 python-multipart==0.0.9
     # via shiny
+pyyaml==6.0.2
+    # via jupytext
 questionary==2.0.1
     # via shiny
 randomgen==2.0.1
     # via opendp
+referencing==0.35.1
+    # via
+    #   jsonschema
+    #   jsonschema-specifications
 requests==2.32.3
     # via flit
+rpds-py==0.20.0
+    # via
+    #   jsonschema
+    #   referencing
 scikit-learn==1.5.2
     # via opendp
 scipy==1.13.1
@@ -178,6 +208,7 @@ traitlets==5.14.3
     #   ipywidgets
     #   jupyter-core
     #   matplotlib-inline
+    #   nbformat
 typing-extensions==4.12.2
     # via
     #   htmltools