From 79ed78fda03eaab649912dc630b44e941debf553 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?V=C3=ADtor=20Augusto=20da=20Silva=20Vasconcellos?=
 <vasconcellos.dev@gmail.com>
Date: Wed, 29 May 2019 14:14:36 -0300
Subject: [PATCH] Run dataclass' original __post_init__ before validation
 (#560)

* Run dataclass' original __post_init__ before validation

* Add unit test

* Update HISTORY.rst
---
 HISTORY.rst               |  2 ++
 pydantic/dataclasses.py   |  4 ++--
 tests/test_dataclasses.py | 19 +++++++++++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/HISTORY.rst b/HISTORY.rst
index 69fa838..3d3dcd0 100644
--- a/HISTORY.rst
+++ b/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)
 ..................
diff --git a/pydantic/dataclasses.py b/pydantic/dataclasses.py
index 0207489..83e3a98 100644
--- a/pydantic/dataclasses.py
+++ b/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':
diff --git a/tests/test_dataclasses.py b/tests/test_dataclasses.py
index 7cdd4b1..250c9eb 100644
--- a/tests/test_dataclasses.py
+++ b/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: