bpo-43558: Add note about base class initialization to dataclasses doc (GH-25967) (GH-26018)

(cherry picked from commit 2a031723eefcf6f1d678bbb4f6018610d43623c4)

Co-authored-by: dhoekstra2000 <douwe.hoekstra2512@gmail.com>

Co-authored-by: dhoekstra2000 <douwe.hoekstra2512@gmail.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/Doc/library/dataclasses.rst b/Doc/library/dataclasses.rst
index bfba15e..e203a1a 100644
--- a/Doc/library/dataclasses.rst
+++ b/Doc/library/dataclasses.rst
@@ -491,6 +491,27 @@ depend on one or more other fields.  For example::
         def __post_init__(self):
             self.c = self.a + self.b
 
+The :meth:`__init__` method generated by :func:`dataclass` does not call base
+class :meth:`__init__` methods. If the base class has an :meth:`__init__` method
+that has to be called, it is common to call this method in a
+:meth:`__post_init__` method::
+
+    @dataclass
+    class Rectangle:
+        height: float
+        width: float
+
+    @dataclass
+    class Square(Rectangle):
+        side: float
+
+        def __post_init__(self):
+            super().__init__(self.side, self.side)
+
+Note, however, that in general the dataclass-generated :meth:`__init__` methods
+don't need to be called, since the derived dataclass will take care of
+initializing all fields of any base class that is a dataclass itself.
+
 See the section below on init-only variables for ways to pass
 parameters to :meth:`__post_init__`.  Also see the warning about how
 :func:`replace` handles ``init=False`` fields.