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.

tests/test_yaml.py

@@ -0,0 +1,19 @@
+from tljh.yaml import yaml
+
+
+def test_no_empty_flow(tmpdir):
+    path = tmpdir.join("config.yaml")
+    with path.open("w") as f:
+        f.write("{}")
+    # load empty config file
+    with path.open("r") as f:
+        config = yaml.load(f)
+    # set a value
+    config["key"] = "value"
+    # write to a file
+    with path.open("w") as f:
+        yaml.dump(config, f)
+    # verify that it didn't use compact '{}' flow-style
+    with path.open("r") as f:
+        content = f.read()
+    assert content.strip() == "key: value"

tljh/config.py

@@ -13,14 +13,13 @@ tljh-config show firstlevel.second_level
 """
 
 import argparse
+from collections import Sequence, Mapping
 from copy import deepcopy
 import os
 import re
 import sys
 
-from ruamel.yaml import YAML
-from ruamel.yaml.comments import CommentedMap, CommentedSeq
-yaml = YAML(typ='rt')
+from .yaml import yaml
 
 
 INSTALL_PREFIX = os.environ.get('TLJH_INSTALL_PREFIX', '/opt/tljh')
@@ -211,11 +210,11 @@ def parse_value(value_str):
 
 
 def _is_dict(item):
-    return isinstance(item, (dict, CommentedMap))
+    return isinstance(item, Mapping)
 
 
 def _is_list(item):
-    return isinstance(item, (list, CommentedSeq))
+    return isinstance(item, Sequence)
 
 
 def main(argv=None):

tljh/installer.py

@@ -12,7 +12,6 @@ from urllib.error import HTTPError
 from urllib.request import urlopen, URLError
 
 import pluggy
-from ruamel.yaml import YAML
 
 from tljh import (
     apt,
@@ -23,8 +22,7 @@ from tljh import (
     traefik,
     user,
 )
-
-from tljh.config import (
+from .config import (
     CONFIG_DIR,
     CONFIG_FILE,
     HUB_ENV_PREFIX,
@@ -32,10 +30,10 @@ from tljh.config import (
     STATE_DIR,
     USER_ENV_PREFIX,
 )
+from .yaml import yaml
 
 HERE = os.path.abspath(os.path.dirname(__file__))
 
-rt_yaml = YAML()
 
 logger = logging.getLogger(__name__)
 
@@ -263,7 +261,7 @@ def ensure_admins(admins):
     config_path = CONFIG_FILE
     if os.path.exists(config_path):
         with open(config_path, 'r') as f:
-            config = rt_yaml.load(f)
+            config = yaml.load(f)
     else:
         config = {}
 
@@ -271,7 +269,7 @@ def ensure_admins(admins):
     config['users']['admin'] = list(admins)
 
     with open(config_path, 'w+') as f:
-        rt_yaml.dump(config, f)
+        yaml.dump(config, f)
 
 
 def ensure_jupyterhub_running(times=4):
@@ -388,7 +386,7 @@ def ensure_config_yaml(plugin_manager):
 
     if os.path.exists(CONFIG_FILE):
         with open(CONFIG_FILE, 'r') as f:
-            config = rt_yaml.load(f)
+            config = yaml.load(f)
     else:
         config = {}
 
@@ -396,7 +394,7 @@ def ensure_config_yaml(plugin_manager):
     hook.tljh_config_post_install(config=config)
 
     with open(CONFIG_FILE, 'w+') as f:
-        rt_yaml.dump(config, f)
+        yaml.dump(config, f)
 
 
 def main():

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