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Author jsbueno
Recipients docs@python, jsbueno
Date 2016-11-12.05:50:11
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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
2016-11-12 05:50:12jsbuenosetrecipients: + jsbueno, docs@python
2016-11-12 05:50:12jsbuenosetmessageid: <>
2016-11-12 05:50:12jsbuenolinkissue28672 messages
2016-11-12 05:50:11jsbuenocreate