consolidate yaml configuration

workaround ruamel.yaml issue 255,
where once an empty dict or list has been written,
'flow' style is used thereafter, using dense `{key: value}` form
instead of traditional yaml block style.

tljh/yaml.py

@@ -0,0 +1,32 @@
+"""consolidated yaml API
+
+ensures the same yaml settings for reading/writing
+throughout tljh
+"""
+from ruamel.yaml.composer import Composer
+from ruamel.yaml import YAML
+
+
+class _NoEmptyFlowComposer(Composer):
+    """yaml composer that avoids setting flow_style on empty
+    containers.
+
+    workaround ruamel.yaml issue #255
+    """
+
+    def compose_mapping_node(self, anchor):
+        node = super().compose_mapping_node(anchor)
+        if not node.value:
+            node.flow_style = False
+        return node
+
+    def compose_sequence_node(self, anchor):
+        node = super().compose_sequence_node(anchor)
+        if not node.value:
+            node.flow_style = False
+        return node
+
+
+# create the global yaml object:
+yaml = YAML(typ="rt")
+yaml.Composer = _NoEmptyFlowComposer