Add support for InitVar (#591)

* Add support for InitVar

* Annotate *initvars as Any

* Simplify tests

* Add line to HISTORY.rst

* Use dataclasses.fields which also ignores ClassVars

* tweak history

HISTORY.rst

@@ -3,6 +3,10 @@
 History
 -------
 
+v0.29 (unreleased)
+..................
+* support dataclasses.InitVar, #592 by @pfrederiks
+
 v0.28 (2019-06-06)
 ..................
 * fix support for JSON Schema generation when using models with circular references in Python 3.7, #572 by @tiangolo

pydantic/dataclasses.py

@@ -25,9 +25,9 @@ if TYPE_CHECKING:  # pragma: no cover
             pass
 
 
-def _pydantic_post_init(self: 'DataclassType') -> None:
+def _pydantic_post_init(self: 'DataclassType', *initvars: Any) -> None:
     if self.__post_init_original__:
-        self.__post_init_original__()
+        self.__post_init_original__(*initvars)
     d = validate_model(self.__pydantic_model__, self.__dict__, cls=self.__class__)[0]
     object.__setattr__(self, '__dict__', d)
     object.__setattr__(self, '__initialised__', True)
@@ -79,8 +79,8 @@ def _process_class(
     cls = dataclasses._process_class(_cls, init, repr, eq, order, unsafe_hash, frozen)  # type: ignore
 
     fields: Dict[str, Any] = {
-        name: (field.type, field.default if field.default != dataclasses.MISSING else Required)
-        for name, field in cls.__dataclass_fields__.items()
+        field.name: (field.type, field.default if field.default != dataclasses.MISSING else Required)
+        for field in dataclasses.fields(cls)
     }
     cls.__post_init_original__ = post_init_original
     cls.__post_init_post_parse__ = post_init_post_parse

tests/test_dataclasses.py

@@ -1,5 +1,6 @@
 import dataclasses
 from datetime import datetime
+from typing import ClassVar
 
 import pytest
 
@@ -380,3 +381,44 @@ def test_nested_schema():
             }
         },
     }
+
+
+def test_initvar():
+    InitVar = dataclasses.InitVar
+
+    @pydantic.dataclasses.dataclass
+    class TestInitVar:
+        x: int
+        y: InitVar
+
+    tiv = TestInitVar(1, 2)
+    assert tiv.x == 1
+    with pytest.raises(AttributeError):
+        tiv.y
+
+
+def test_derived_field_from_initvar():
+    InitVar = dataclasses.InitVar
+
+    @pydantic.dataclasses.dataclass
+    class DerivedWithInitVar:
+        plusone: int = dataclasses.field(init=False)
+        number: InitVar[int]
+
+        def __post_init__(self, number):
+            self.plusone = number + 1
+
+    derived = DerivedWithInitVar(1)
+    assert derived.plusone == 2
+    with pytest.raises(TypeError):
+        DerivedWithInitVar("Not A Number")
+
+
+def test_classvar():
+    @pydantic.dataclasses.dataclass
+    class TestClassVar:
+        klassvar: ClassVar = "I'm a Class variable"
+        x: int
+
+    tcv = TestClassVar(2)
+    assert tcv.klassvar == "I'm a Class variable"