consistencies can make a warning instead of raises

for that, you have to set something like:
  a.impl_add_consistency('not_equal', b, warnings_only=True)

warning product now adapted message

test/test_option_consistency.py

@@ -5,7 +5,8 @@ from tiramisu.setting import owners, groups
 from tiramisu.config import Config
 from tiramisu.option import IPOption, NetworkOption, NetmaskOption, IntOption,\
     BroadcastOption, SymLinkOption, OptionDescription
-from tiramisu.error import ConfigError
+from tiramisu.error import ConfigError, ValueWarning
+import warnings
 
 
 def test_consistency():
@@ -19,6 +20,19 @@ def test_consistency():
     raises(ConfigError, "a.impl_add_consistency('not_equal', 'a')")
 
 
+def test_consistency_warnings_only():
+    a = IntOption('a', '')
+    b = IntOption('b', '')
+    od = OptionDescription('od', '', [a, b])
+    a.impl_add_consistency('not_equal', b, warnings_only=True)
+    c = Config(od)
+    c.a = 1
+    warnings.simplefilter("always", ValueWarning)
+    with warnings.catch_warnings(record=True) as w:
+        c.b = 1
+    assert w != []
+
+
 def test_consistency_not_equal():
     a = IntOption('a', '')
     b = IntOption('b', '')