When a path is given to constructor, only use that configuration file.

If that configuration file lacks options, the defaults will be used.

This will be used to make sure that tests always run with a fixed,
well-defined configuration, not influenced by how the user happened to
configure topydo.

topydo/lib/Config.py

@@ -27,6 +27,12 @@ class ConfigError(Exception):
 
 class _Config:
     def __init__(self, p_path=None):
+        """
+        Constructor.
+
+        If p_path is given, that is the only configuration file that will be
+        read.
+        """
         self.sections = ['topydo', 'tags', 'sort', 'ls']
 
         self.defaults = {
@@ -56,9 +62,17 @@ class _Config:
 
         self.cp = ConfigParser.SafeConfigParser(self.defaults)
 
-        files = ["/etc/topydo.conf", self._home_config_path(), ".topydo", "topydo.conf"]
+        files = [
+            "/etc/topydo.conf",
+            self._home_config_path(),
+            ".topydo",
+            "topydo.conf"
+        ]
+
+        # when a path is given, *only* use the values in that file, or the
+        # defaults listed above.
         if p_path != None:
-            files.append(p_path)
+            files = [p_path]
 
         self.cp.read(files)
 
@@ -140,8 +154,10 @@ class _Config:
 def config(p_path=None):
     """
     Retrieve the config instance.
+
     If a path is given, the instance is overwritten by the one that supplies an
-    additional filename (for testability).
+    additional filename (for testability). Moreover, no other configuration
+    files will be read when a path is given.
     """
     if not config.instance or p_path != None:
         try: