Run dataclass' original __post_init__ before validation (#560)

* Run dataclass' original __post_init__ before validation

* Add unit test

* Update HISTORY.rst

HISTORY.rst

@@ -6,6 +6,8 @@ History
 v0.xx (xxxx-xx-xx)
 ..................
 * fix JSON Schema for ``list``, ``tuple``, and ``set``, #540 by @tiangolo
+* Change `_pydantic_post_init` to execute dataclass' original `__post_init__` before
+  validation, #560 by @HeavenVolkoff
 
 v0.26 (2019-05-22)
 ..................

pydantic/dataclasses.py

@@ -25,11 +25,11 @@ if TYPE_CHECKING:  # pragma: no cover
 
 
 def _pydantic_post_init(self: 'DataclassType') -> None:
+    if self.__post_init_original__:
+        self.__post_init_original__()
     d = validate_model(self.__pydantic_model__, self.__dict__, cls=self.__class__)[0]
     object.__setattr__(self, '__dict__', d)
     object.__setattr__(self, '__initialised__', True)
-    if self.__post_init_original__:
-        self.__post_init_original__()
 
 
 def _validate_dataclass(cls: Type['DataclassType'], v: Any) -> 'DataclassType':

tests/test_dataclasses.py

@@ -107,6 +107,25 @@ def test_post_init():
     assert post_init_called
 
 
+def test_post_init_assignment():
+    from dataclasses import field
+
+    # Based on: https://docs.python.org/3/library/dataclasses.html#post-init-processing
+    @pydantic.dataclasses.dataclass
+    class C:
+        a: float
+        b: float
+        c: float = field(init=False)
+
+        def __post_init__(self):
+            self.c = self.a + self.b
+
+    c = C(0.1, 0.2)
+    assert c.a == 0.1
+    assert c.b == 0.2
+    assert c.c == 0.30000000000000004
+
+
 def test_inheritance():
     @pydantic.dataclasses.dataclass
     class A: