Fix unbound __str__ on Python 2 when slots=True (#199)

Previously:

```pycon
>>> import attr
>>> @attr.s(str=True, slots=True)
... class A(object):
...     a = attr.ib()
...
>>> a = A(1)
>>> str(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method repr_() must be called with A instance as first argument (got nothing instead)
```

Fixes #198.

CHANGELOG.rst

@@ -24,7 +24,8 @@ Deprecations:
 Changes:
 ^^^^^^^^
 
-*none*
+- Fix *str* on Python 2 when ``slots=True``.
+  `#198 <https://github.com/python-attrs/attrs/issues/198>`_
 
 
 ----

src/attr/_make.py

@@ -343,9 +343,7 @@ def wrap(cls):
         # Can't compare function objects because Python 2 is terrible. :(
         effectively_frozen = _has_frozen_superclass(cls) or frozen
         if repr is True:
-            cls = _add_repr(cls, ns=repr_ns)
-        if str is True:
-            cls.__str__ = cls.__repr__
+            cls = _add_repr(cls, ns=repr_ns, str=str)
         if cmp is True:
             cls = _add_cmp(cls)
 
@@ -516,9 +514,9 @@ def ge(self, other):
     return cls
 
 
-def _add_repr(cls, ns=None, attrs=None):
+def _add_repr(cls, ns=None, attrs=None, str=False):
     """
-    Add a repr method to *cls*.
+    Add a repr method to *cls*. If *str* is True, also add __str__.
     """
     if attrs is None:
         attrs = [a for a in cls.__attrs_attrs__ if a.repr]
@@ -543,6 +541,8 @@ def repr_(self):
                       for a in attrs)
         )
     cls.__repr__ = repr_
+    if str is True:
+        cls.__str__ = repr_
     return cls
 
 

tests/test_dunders.py

@@ -198,15 +198,15 @@ class C(object):
 
         assert "C(_x=42)" == repr(i)
 
-    @pytest.mark.parametrize("add_str", [True, False])
-    def test_str(self, add_str):
+    @given(add_str=booleans(), slots=booleans())
+    def test_str(self, add_str, slots):
         """
         If str is True, it returns the same as repr.
 
         This only makes sense when subclassing a class with an poor __str__
         (like Exceptions).
         """
-        @attributes(str=add_str)
+        @attributes(str=add_str, slots=slots)
         class Error(Exception):
             x = attr()