Add an extra safety measure: slots=True

This makes sure that we did not define extra or different
attributes in __init__.

eli5/base.py

@@ -5,6 +5,10 @@
 from .base_utils import attrs
 
 
+# @attrs decorator used in this file calls @attr.s(slots=True),
+# creating attr.ib entries based on the signature of __init__.
+
+
 @attrs
 class Explanation(object):
     """ An explanation for classifier or regressor,

eli5/base_utils.py

@@ -4,7 +4,10 @@
 
 
 def attrs(class_):
-    """ Like attr.s, but with attributes extracted from __init__ method signature.
+    """ Like attr.s with slots=True,
+    but with attributes extracted from __init__ method signature.
+    slots=True ensures that signature matches what really happens
+    (we can't define different attributes on self).
     It is useful if we still want __init__ for proper type-checking and
     do not want to repeat attribute definitions in the class body.
     """
@@ -21,4 +24,4 @@ def attrs(class_):
         if idx >= defaults_shift:
             attrib_kwargs['default'] = init_args.defaults[idx - defaults_shift]
         these[arg] = attr.ib(**attrib_kwargs)
-    return attr.s(class_, these=these, init=False, **attrs_kwargs)
+    return attr.s(class_, these=these, init=False, slots=True, **attrs_kwargs)

tests/test_base_utils.py

@@ -1,4 +1,5 @@
 import attr
+import pytest
 
 from eli5.base_utils import attrs
 
@@ -48,3 +49,14 @@ def __repr__(self):
 
     assert hash(WithRepr(1)) == hash(WithRepr(1))
     assert repr(WithRepr(2)) == 'foo'
+
+
+def test_bad_init():
+
+    @attrs
+    class BadInit(object):
+        def __init__(self, x):
+            self._x = x
+
+    with pytest.raises(AttributeError):
+        BadInit(1)