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ericvsmith merged 5 commits into from
May 10, 2021
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bpo-43558: Add base class initialization to dataclass doc #25967

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Add base class initialization to dataclass doc
May 7, 2021
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21 changes: 21 additions & 0 deletions Doc/library/dataclasses.rst
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Expand Up @@ -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)

@ericvsmith ericvsmith May 7, 2021

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That's a great example.

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.

@ericvsmith ericvsmith May 8, 2021

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This looks good. How about instead of "the derived class", use "the derived dataclass"? I'm trying to draw the distinction that if a non-dataclass is derived from a dataclass, then the base class's __init__ should in fact be called.

@dhoekstra2000 dhoekstra2000 May 8, 2021

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If I understand PEP 557 correctly, the __init__ method needs to be called if a dataclass inherits from a non-dataclass or if a non-dataclass inherits from a dataclass. I have tried to reflect this in the new wording.

@ericvsmith ericvsmith May 8, 2021

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If I understand PEP 557 correctly, the __init__ method needs to be called if a dataclass inherits from a non-dataclass or if a non-dataclass inherits from a dataclass. I have tried to reflect this in the new wording.

If a dataclass inherits from a non-dataclass, then it's probably that the base non-dataclass's __init__ does need to be called. That's what __post_init__ is for, among other uses.

If a non-dataclass inherits from a dataclass, then the non-dataclass most likely does need to call the dataclass's __init__.

It's only the case where a dataclass inherits from a dataclass that the base class's __init__ most likely doesn't need to be called. I say "most likely" because you could construct examples where it does need to be called, if the base dataclass's __post_init__ does some processing. But it's probably too complex to mention that here, I'm just mentioning it here to help you understand the issue. Apologies if I'm not being helpful.

@dhoekstra2000 dhoekstra2000 May 9, 2021

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This is very helpful! Thank you!

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.
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1 change: 1 addition & 0 deletions Misc/ACKS
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Expand Up @@ -735,6 +735,7 @@ David Hobley
Tim Hochberg
Benjamin Hodgson
Joerg-Cyril Hoehle
Douwe Hoekstra
Gregor Hoffleit
Chris Hoffman
Tim Hoffmann
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Documentation/2021-05-07-12-27-09.bpo-43558.UGhA8R.rst
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@@ -0,0 +1,2 @@
Add the remark to :mod:`dataclasses` documentation that the :meth:`__init__` of any base class
has to be called in :meth:`__post_init__`, along with a code example.