Add tests

readme.md

@@ -57,3 +57,67 @@ from undefined import Undefined
 
 I tend to be torn between lowercase, for simplicity, and Uppercase.
 
+
+# Why not `None`, difference with `None`
+
+`undefined` is likely slower, and as it is a regular Python object there are a few  on purpose (or not difference).
+
+### Unlike `None`, you can assign to it
+
+```
+>>> None = 3 
+SyntaxError: can't assign to keyword
+```
+
+```
+>>> undefined = 3
+>>> undefned
+3
+```
+
+### Unlike `None`, `undefined` is mutable
+
+```
+>>> undefined.value = 42
+>>> undefined.value
+42
+```
+
+(might be able to fix that with `__get_attr__`
+
+### Unlike `None`, `undefined` is neither true not false.
+
+If you test for boolean value of `undefind` if will raise. 
+That is to say: the following will fail:
+
+```
+value = undefined
+if value:
+   pass # will raise before reaching here.
+```
+
+You have to check for identity:
+
+```
+value = undefined
+other = 1
+if value is undefined:
+    pass # will execute
+```
+
+for info, undefined is not `True`,`False`, not undefined with respect to identity
+
+```
+>>> undefined is True
+False
+
+>>> undefined is False
+False
+
+>>>: undefined is None
+False
+```
+
+### String form
+
+`str(undefined)` raises. `repr(undefined)` is the unicode string `'Undefined'`

undefined/__init__.py

@@ -27,9 +27,10 @@ def __repr__(self):
         return self.__class__.__name__
 
     def __bool__(self):
-        return False
+        raise NotImplementedError('Undefined is not defined, neither True, nor False.')
 
-    __str__ = __repr__
+    def __str__(self):
+        raise NotImplementedError("Cannot represent undefined !")
 
 
 Undefined.__name__ = 'Undefined'

undefined/tests/test_undefined.py

@@ -1,7 +1,24 @@
 import undefined as ud
+import undefined
 from  undefined import Undefined as uc
 
 
 def test_undefined():
     assert ud is uc
 
+
+
+import unittest
+
+class TestUndefined(unittest.TestCase):
+
+
+  def test_bool(self):
+      with self.assertRaises(NotImplementedError):
+          if undefined:
+              pass
+
+  def test_str(self):
+      with self.assertRaises(NotImplementedError):
+          '%s' % undefined
+