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

@@ -354,8 +354,13 @@ def test_noncallable_validators(
  """
  with pytest.raises(TypeError) as e:
  deep_iterable(member_validator, iterable_validator)
-
- e.match(r"\w* must be callable")
+ value = 42
+ assert (
+ "must be callable (got {value} that is a {type_}).".format(
+ value=value, type_=value.__class__
+ )
+ in e.value.args[0]
+ )
 
  def test_fail_invalid_member(self):
  """
@@ -466,7 +471,13 @@ 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")
+ value = 42
+ assert (
+ "must be callable (got {value} that is a {type_}).".format(
+ value=value, type_=value.__class__
+ )
+ in e.value.args[0]
+ )
 
  def test_fail_invalid_mapping(self):
  """
@@ -550,7 +561,13 @@ def test_fail(self):
  with pytest.raises(TypeError) as e:
  v(None, a, None)
 
- e.match("'test' must be callable")
+ value = None
+ assert (
+ "must be callable (got {value} that is a {type_}).".format(
+ value=value, type_=value.__class__
+ )
+ in e.value.args[0]
+ )
 
  def test_repr(self):
  """