Only require nbformat_minor for v4 (#342)

nbformat/v4/convert.py

@@ -40,10 +40,14 @@ def upgrade(nb, from_version=None, from_minor=None):
         from_version = nb["nbformat"]
     if not from_minor:
         if "nbformat_minor" not in nb:
-            raise validator.ValidationError(
-                "The notebook does not include the nbformat minor which is needed"
-            )
-        from_minor = nb["nbformat_minor"]
+            if from_version == 4:
+                raise validator.ValidationError(
+                    "The v4 notebook does not include the nbformat minor, which is needed."
+                )
+            else:
+                from_minor = 0
+        else:
+            from_minor = nb["nbformat_minor"]
 
     if from_version == 3:
         # Validate the notebook before conversion

tests/test3_no_min_version.ipynb

@@ -0,0 +1,12 @@
+{
+ "metadata": {
+  "name": ""
+ },
+ "nbformat": 3,
+ "worksheets": [
+  {
+   "metadata": {},
+   "cells": []
+  }
+ ]
+}

tests/test_validator.py

@@ -355,6 +355,12 @@ def test_notebook_invalid_without_min_version():
         validate(nb)
 
 
+def test_notebook_v3_valid_without_min_version():
+    with TestsBase.fopen("test3_no_min_version.ipynb", "r") as f:
+        nb = read(f, as_version=4)
+    validate(nb)
+
+
 def test_notebook_invalid_without_main_version():
     pass