From 36576f36d0b018c24c5590866a5569f80a767af7 Mon Sep 17 00:00:00 2001
From: Yan Wong <yan.wong@bdi.ox.ac.uk>
Date: Thu, 2 Mar 2023 15:22:21 +0000
Subject: [PATCH] str(null_schema) returns "Null_schema" not None

---
 python/tests/test_tables.py | 13 ++++++-------
 python/tskit/metadata.py    |  7 +++++--
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/python/tests/test_tables.py b/python/tests/test_tables.py
index 919f2309ef..7e56e516ff 100644
--- a/python/tests/test_tables.py
+++ b/python/tests/test_tables.py
@@ -1,6 +1,6 @@
 # MIT License
 #
-# Copyright (c) 2018-2022 Tskit Developers
+# Copyright (c) 2018-2023 Tskit Developers
 # Copyright (c) 2017 University of Oxford
 #
 # Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -1321,9 +1321,8 @@ def test_metadata_schema(self, table_5row):
             table2.metadata_schema = tskit.MetadataSchema({"codec": "json"})
             with pytest.raises(
                 AssertionError,
-                match=f"{type(table_5row).__name__} metadata schemas differ: self=None "
-                f"other=OrderedDict([('codec', "
-                "'json')])",
+                match=f"{type(table_5row).__name__} metadata schemas differ: "
+                "self=Null_schema other=OrderedDict([('codec', 'json')])",
             ):
                 table_5row.assert_equals(table2)
             table_5row.assert_equals(table2, ignore_metadata=True)
@@ -3990,7 +3989,7 @@ def test_metadata_schema(self, t1, t2):
             AssertionError,
             match=re.escape(
                 "Metadata schemas differ: self=OrderedDict([('codec', 'json')]) "
-                "other=None"
+                "other=Null_schema"
             ),
         ):
             t1.assert_equals(t2)
@@ -4039,7 +4038,7 @@ def test_ignore_metadata(self, t1, t2, table_name):
                 AssertionError,
                 match=re.escape(
                     f"{type(table).__name__} metadata schemas differ: "
-                    f"self=OrderedDict([('codec', 'json')]) other=None"
+                    f"self=OrderedDict([('codec', 'json')]) other=Null_schema"
                 ),
             ):
                 t1.assert_equals(t2)
@@ -4100,7 +4099,7 @@ def test_ignore_reference_sequence(self, t1, t2):
             AssertionError,
             match=re.escape(
                 "Metadata schemas differ: "
-                "self=OrderedDict([('codec', 'json')]) other=None"
+                "self=OrderedDict([('codec', 'json')]) other=Null_schema"
             ),
         ):
             t1.assert_equals(t2)
diff --git a/python/tskit/metadata.py b/python/tskit/metadata.py
index b4c978d51c..78e6b97269 100644
--- a/python/tskit/metadata.py
+++ b/python/tskit/metadata.py
@@ -1,6 +1,6 @@
 # MIT License
 #
-# Copyright (c) 2020-2022 Tskit Developers
+# Copyright (c) 2020-2023 Tskit Developers
 #
 # Permission is hereby granted, free of charge, to any person obtaining a copy
 # of this software and associated documentation files (the "Software"), to deal
@@ -654,7 +654,10 @@ def __repr__(self) -> str:
         return self._string
 
     def __str__(self) -> str:
-        return pprint.pformat(self._schema)
+        if self._schema is None:
+            return "Null_schema"
+        else:
+            return pprint.pformat(self._schema)
 
     def __eq__(self, other) -> bool:
         return self._string == other._string