Add ability for __post_init__ method to be defined.

If this method is defined for a class, it will get executed at the end
of ``__init__``.  Previously, there was no way to extend what was
done during ``__init__`` when using ``@attr.s``.

CHANGELOG.rst

@@ -13,6 +13,10 @@ Changes:
 
 - Don't overwrite ``__name__`` with ``__qualname__`` for ``attr.s(slots=True)`` classes.
   `#99 <https://github.com/hynek/attrs/issues/99>`_
+- Allow for a ``__post_init__`` method that, if defined, will get executed at
+  the end of the ``__init__`` that gets constructed for an ``@attr.s``-decorated
+  class.
+  `#111 <https://github.com/hynek/attrs/pull/111>`_
 
 
 ----

docs/examples.rst

@@ -551,6 +551,26 @@ You can still have power over the attributes if you pass a dictionary of name: `
    >>> i.y
    []
 
+Sometimes, you want to have your class's ``__init__`` method do more than just
+the initialization, validation, etc. that gets done for you automatically when
+using ``@attr.s``.
+To do this, just define a ``__post_init__`` method in your class.
+It will get called at the end of the ``__init__`` method that has been
+autogenerated.
+
+.. doctest::
+
+   >>> @attr.s
+   ... class C(object):
+   ...     x = attr.ib()
+   ...     y = attr.ib()
+   ...
+   ...     def __post_init__(self):
+   ...         self.z = self.x + self.y
+   >>> obj = C(x=1, y=2)
+   >>> obj.z
+   3
+
 Finally, you can exclude single attributes from certain methods:
 
 .. doctest::

src/attr/_make.py

@@ -446,7 +446,11 @@ def _add_init(cls, frozen):
         sha1.hexdigest()
     )
 
-    script, globs = _attrs_to_script(attrs, frozen)
+    script, globs = _attrs_to_script(
+        attrs,
+        frozen,
+        getattr(cls, '__post_init__', False),
+    )
     locs = {}
     bytecode = compile(script, unique_filename, "exec")
     attr_dict = dict((a.name, a) for a in attrs)
@@ -540,7 +544,7 @@ def validate(inst):
             a.validator(inst, a, getattr(inst, a.name))
 
 
-def _attrs_to_script(attrs, frozen):
+def _attrs_to_script(attrs, frozen, post_init):
     """
     Return a script of an initializer for *attrs* and a dict of globals.
 
@@ -680,6 +684,8 @@ def fmt_setter_with_converter(attr_name, value_var):
                                                     a.name))
         names_for_globals[val_name] = a.validator
         names_for_globals[attr_name] = a
+    if post_init:
+        lines.append("self.__post_init__()")
 
     return """\
 def __init__(self, {args}):

tests/test_make.py

@@ -302,6 +302,21 @@ class D(object):
         assert C.D.__name__ == "D"
         assert C.D.__qualname__ == C.__qualname__ + ".D"
 
+    def test_post_init(self):
+        """
+        Verify that __post_init__ gets called if defined.
+        """
+        @attributes
+        class C(object):
+            x = attr()
+            y = attr()
+
+            def __post_init__(self2):
+                self2.z = self2.x + self2.y
+
+        c = C(x=10, y=20)
+        assert 30 == getattr(c, 'z', None)
+
 
 @attributes
 class GC(object):

tests/utils.py

@@ -167,7 +167,13 @@ class HypClass:
     frozen_flag = draw(st.booleans()) if frozen is None else frozen
     slots_flag = draw(st.booleans()) if slots is None else slots
 
-    return make_class('HypClass', dict(zip(_gen_attr_names(), attrs)),
+    cls_dict = dict(zip(_gen_attr_names(), attrs))
+    post_init_flag = draw(st.booleans())
+    if post_init_flag:
+        def post_init(self):
+            pass
+        cls_dict['__post_init__'] = post_init
+    return make_class('HypClass', cls_dict,
                       slots=slots_flag, frozen=frozen_flag)
 
 # Ok, so st.recursive works by taking a base strategy (in this case,