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Title: Explain in the "Data Model" document why arguments to __init__ are ok when __new__ is not defined
Type: enhancement Stage:
Components: Documentation Versions: Python 3.6
Status: open Resolution:
Dependencies: Superseder:
Assigned To: docs@python Nosy List: docs@python, jsbueno
Priority: normal Keywords:

Created on 2016-11-12 05:50 by jsbueno, last changed 2022-04-11 14:58 by admin.

Messages (1)
msg280633 - (view) Author: João S. O. Bueno (jsbueno) * Date: 2016-11-12 05:50
There is an specific Python behavior on object instantiation that is "expected" but not explicit, even for avanced users:

When a custom class defines `__init__` with extra parameters, but do not overrides `__new__`, it simply works. But if `__new__`is defined it has to match `__init__`s signature.

This behavior is not documented anywhere.

I could found this issue was discussed in this thread earlier this year, starting with this e-mail:

I propose the following paragraph from a follow up e-mail by "eryksun at" to be added to the description of "__new__" in the Data Model documentation page:


[The implementation] knows whether a type overrides the __new__ or
__init__ methods. You're expected to consume additional arguments in
this case. However, excess arguments are ignored in object.__new__ if
a type overrides __init__ without overriding __new__ (i.e. your second
example). Excess arguments are also ignored in object.__init__ if a
type overrides __new__ without overriding __init__.
Date User Action Args
2022-04-11 14:58:39adminsetgithub: 72858
2016-11-12 05:50:12jsbuenocreate