New submission from Aaron Staley:
The documentation for __new__ at
http://docs.python.org/reference/datamodel.html#object.__new__ is:
"""
object.__new__(cls[, ...])
Called to create a new instance of class cls. __new__() is a static method
(special-cased so you need not declare it as such) that takes the class of
which an instance was requested as its first argument. The remaining arguments
are those passed to the object constructor expression (the call to the class).
The return value of __new__() should be the new object instance (usually an
instance of cls).
Typical implementations create a new instance of the class by invoking the
superclass’s __new__() method using super(currentclass, cls).__new__(cls[,
...]) with appropriate arguments and then modifying the newly-created instance
as necessary before returning it.
If __new__() returns an instance of cls, then the new instance’s __init__()
method will be invoked like __init__(self[, ...]), where self is the new
instance and the remaining arguments are the same as were passed to __new__().
If __new__() does not return an instance of cls, then the new instance’s
__init__() method will not be invoked.
__new__() is intended mainly to allow subclasses of immutable types (like int,
str, or tuple) to customize instance creation. It is also commonly overridden
in custom metaclasses in order to customize class creation.
"""
The problem is in this line: "If __new__() returns an instance of cls, then the
new instance’s __init__() method will be invoked like __init__(self[, ...]),
where self is the new instance and the remaining arguments are the same as were
passed to __new__()."
This is only true in the context of a constructor. In particular, directly
calling cls.__new__(cls) will NOT call __init__.
If I define a class:
class C(object):
def __new__(*args, **kwargs):
print 'new', args, kwargs
return object.__new__(*args,**kwargs)
def __init__(self):
print 'init'
C() will result in __new__ and __init__ being both executed, but C.__new__(C)
will only create the instance of C; it will not call __init__!
The original documentation described in http://bugs.python.org/issue1123716 was
more correct:
"__new__ must return an object... If you return an
existing object, the constructor call will still call
its __init__ method unless the object is an instance of
a different class..."
That is __init__ is only being called in the context of an external constructor
call.
Proposed phrasing:
"If __new__() is invoked during object construction (cls()) and it returns an
instance of cls, then the new instance’s __init__() method will be invoked like
__init__(self[, ...]), where self is the new instance and the remaining
arguments are the same as were passed to the object constructor."
----------
assignee: docs@python
components: Documentation
messages: 167267
nosy: Aaron.Staley, docs@python
priority: normal
severity: normal
status: open
title: Documentation incorrectly suggests __init__ called after direct __new__
call
versions: Python 2.7
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Python tracker <[email protected]>
<http://bugs.python.org/issue15542>
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