Improve the error message informing the type that it got it

docs/api.rst

@@ -397,7 +397,7 @@ Validators
         >>> C("not a callable")
         Traceback (most recent call last):
             ...
-        TypeError: 'x' must be callable
+        TypeError: 'x' must be callable (got 'not a callable' that is a <class 'str'>).
 
 
 .. autofunction:: attr.validators.deep_iterable

src/attr/validators.py

@@ -186,7 +186,12 @@ def __call__(self, inst, attr, value):
         We use a callable class to be able to change the ``__repr__``.
         """
         if not callable(value):
-            raise TypeError("'{name}' must be callable".format(name=attr.name))
+            raise TypeError(
+                "'{name}' must be callable (got {value!r} that is a "
+                "{actual!r}).".format(
+                    name=attr.name, value=value, actual=value.__class__
+                )
+            )
 
     def __repr__(self):
         return "<is_callable validator>"
@@ -200,7 +205,8 @@ def is_callable():
     .. versionadded:: 19.1.0
 
     :raises TypeError: With a human readable error message containing the
-        attribute (of type :class:`attr.Attribute`) name.
+        attribute (of type :class:`attr.Attribute`) name, and the value it
+        got.
     """
     return _IsCallableValidator()
 

tests/test_validators.py

@@ -355,7 +355,10 @@ def test_noncallable_validators(
         with pytest.raises(TypeError) as e:
             deep_iterable(member_validator, iterable_validator)
 
-        e.match(r"\w* must be callable")
+        assert (
+            "must be callable (got 42 that is a <type 'int'>)."
+            in e.value.args[0]
+        )
 
     def test_fail_invalid_member(self):
         """
@@ -466,7 +469,10 @@ def test_noncallable_validators(
         with pytest.raises(TypeError) as e:
             deep_mapping(key_validator, value_validator, mapping_validator)
 
-        e.match(r"\w* must be callable")
+        assert (
+            "must be callable (got 42 that is a <type 'int'>)."
+            in e.value.args[0]
+        )
 
     def test_fail_invalid_mapping(self):
         """
@@ -550,7 +556,10 @@ def test_fail(self):
         with pytest.raises(TypeError) as e:
             v(None, a, None)
 
-        e.match("'test' must be callable")
+        assert (
+            "must be callable (got None that is a <type 'NoneType'>)."
+            in e.value.args[0]
+        )
 
     def test_repr(self):
         """