[dtypes] Default value use on Empty and None

Closes G-Node#245

odml/dtypes.py

@@ -131,6 +131,8 @@ def float_get(string):
 
 
 def str_get(string):
+    if not string:
+        return default_values("string")
     if sys.version_info < (3, 0):
         return unicode(string)
     return str(string)
@@ -145,7 +147,7 @@ def str_get(string):
 
 def time_get(string):
     if not string:
-        return None
+        return default_values("time")
     if type(string) is datetime.time:
         return datetime.datetime.strptime(string.strftime('%H:%M:%S'),
                                           '%H:%M:%S').time()
@@ -155,15 +157,15 @@ def time_get(string):
 
 def time_set(value):
     if not value:
-        return None
+        return default_values("time")
     if type(value) is datetime.time:
         return value.strftime("%H:%M:%S")
     return value.isoformat()
 
 
 def date_get(string):
     if not string:
-        return None
+        return default_values("date")
     if type(string) is datetime.date:
         return datetime.datetime.strptime(string.isoformat(),
                                           '%Y-%m-%d').date()
@@ -176,7 +178,7 @@ def date_get(string):
 
 def datetime_get(string):
     if not string:
-        return None
+        return default_values("datetime")
     if type(string) is datetime.datetime:
         return datetime.datetime.strptime(string.strftime('%Y-%m-%d %H:%M:%S'),
                                           '%Y-%m-%d %H:%M:%S')
@@ -186,16 +188,16 @@ def datetime_get(string):
 
 def datetime_set(value):
     if not value:
-        return None
+        return default_values("datetime")
     if type(value) is datetime.datetime:
         return value.strftime('%Y-%m-%d %H:%M:%S')
     else:
         return datetime.datetime.strptime(value, '%Y-%m-%d %H:%M:%S')
 
 
 def boolean_get(string):
-    if string is None:
-        return None
+    if not string:
+        return default_values("boolean")
     if type(string) in (unicode, str):
         string = string.lower()
     truth = ["true", "1", True, "t"]  # be kind, spec only accepts True / False
@@ -209,6 +211,7 @@ def boolean_get(string):
 
 # Alias boolean_set to boolean_get. Both perform same function.
 
+
 boolean_set = boolean_get
 bool_get = boolean_get
 bool_set = boolean_set