config: Add a Path config value and an ExpandedPath wrapper.

Allows us to easily use expanded paths, without losing the original value for
display and storage. In theory we could be using same trick for passwords.

mopidy/utils/config.py

@@ -5,6 +5,7 @@ import re
 import socket
 
 from mopidy import exceptions
+from mopidy.utils import path
 
 
 def validate_required(value, required):
@@ -93,7 +94,7 @@ class ConfigValue(object):
 class String(ConfigValue):
     """String values.
 
-    Supports: optional choices and secret.
+    Supports: optional, choices and secret.
     """
     def deserialize(self, value):
         value = value.strip()
@@ -209,6 +210,34 @@ class Port(Integer):
         self.maximum = 2 ** 16 - 1
 
 
+class ExpandedPath(bytes):
+    def __new__(self, value):
+        expanded = path.expand_path(value)
+        return super(ExpandedPath, self).__new__(self, expanded)
+
+    def __init__(self, value):
+        self.original = value
+
+
+class Path(ConfigValue):
+    """File system path that will be expanded with mopidy.utils.path.expand_path
+
+    Supports: optional, choices and secret.
+    """
+    def deserialize(self, value):
+        value = value.strip()
+        validate_required(value, not self.optional)
+        validate_choice(value, self.choices)
+        if not value:
+            return None
+        return ExpandedPath(value)
+
+    def serialize(self, value):
+        if isinstance(value, ExpandedPath):
+            return value.original
+        return value
+
+
 class ConfigSchema(object):
     """Logical group of config values that correspond to a config section.
 
@@ -231,6 +260,8 @@ class ConfigSchema(object):
         return self._schema[key]
 
     def format(self, name, values):
+        # TODO: should the output be encoded utf-8 since we use that in
+        # serialize for strings?
         lines = ['[%s]' % name]
         for key in self._order:
             value = values.get(key)