Fix incorrectly parsed empty strings in config save --set ...=...

When running `config save --set KEY=""`, KEY was incorrectly interpreted
as null.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Note: Breaking changes between versions are indicated by "💥".
 
+## Unreleased
+
+- [Bugfix] Fix incorrectly parsed empty strings for `config save --set ...=...` commands
+
 ## 3.11.2 (2020-01-17)
 
 - [Bugfix] Make sure `docker-compose.override.yml` are loaded in dev and local contexts

tests/test_serialize.py

@@ -25,3 +25,11 @@ class SerializeTests(unittest.TestCase):
 
     def test_parse_weird_chars(self):
         self.assertEqual("*@google.com", serialize.parse("*@google.com"))
+
+    def test_parse_empty_string(self):
+        self.assertEqual("", serialize.parse("''"))
+
+    # def test_dump_null(self):#     # Unfortunately, this fails as the output is "null\n...\n"
+
+
+#     self.assertEqual("null", serialize.dumps(None))

tutor/commands/config.py

@@ -25,6 +25,10 @@ class YamlParamType(click.ParamType):
             k, v = value.split("=")
         except ValueError:
             self.fail("'{}' is not of the form 'key=value'.".format(value), param, ctx)
+        if not v:
+            # Empty strings are incorrectly interpreted as null values, which is
+            # incorrect.
+            v = "''"
         return k, serialize.parse(v)
 
 
@@ -93,6 +97,7 @@ def printroot(context):
 def printvalue(context, key):
     config = tutor_config.load(context.root)
     try:
+        # Note that this will incorrectly print None values
         fmt.echo(config[key])
     except KeyError:
         raise exceptions.TutorError("Missing configuration value: {}".format(key))

tutor/serialize.py

@@ -8,7 +8,11 @@ def load(stream):
 
 
 def dump(content, fileobj):
-    yaml.dump(content, fileobj, default_flow_style=False)
+    yaml.dump(content, stream=fileobj, default_flow_style=False)
+
+
+def dumps(content):
+    return yaml.dump(content, stream=None, default_flow_style=False)
 
 
 def parse(v):