[Python-Dev] Re: Documentation for __new__
steve at holdenweb.com
Mon Mar 7 21:53:15 CET 2005
Greg Ward wrote:
> On 05 March 2005, Nick Coghlan said:
>>Steven Bethard has put together some text to add __new__ to the list of
>>Basic Customisation methods in the language reference. Would one of the
>>documentation folks care to take a look at it?
> I've tried to tighten up the text there and hopefully make it a smidgeon
> clearer and more accurate. Here's my best effort:
> __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 to create an instance of as
> the first argument. The remaining arguments are those
> passed to the object constructor expression. The return
> value of __new__() should be the new object instance.
Just to offer alternatives:
__new__() is a static method (special-cased so you need
not declare it as such) whose fist argumen is the class
of which a new instance is required. 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 instance object, which is not constrained to be of
> Typical usage is to create a new instance of the class by
> invoking the superclass's __new__() method using
> "super(currentclass, cls).__new__([...])" with appropriate
> arguments, modifying the returned instance if necessary, and
> then returning it. If the returned value is an instance of
> 'cls', its __init__() will be invoked like
> "__init__(self[, ...])", where the extra arguments are the
> same as were passed to __new__().
Typical usage creates a new instance of the required
class by invoking the superclass's __new__() method
using "super(currentclass, cls).__new__(...)" with
appropriate argumnents 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 called with the
instance itself as the first argument and the remaining
arguments being the second and subsequent arguments to
> You do need not to return an instance of 'cls', but if you
> do not, the new instance's __init__() method will not be
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.
> Feedback welcome. Has anyone volunteered to render this in LaTeX yet?
> If not, I might.
I decided some time ago that documenting Python in LaTex wasn't my forte ...
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