__init__ is the initialiser

Terry Reedy tjreedy at udel.edu
Sat Feb 1 03:28:02 CET 2014

On 1/31/2014 3:41 PM, Ethan Furman wrote:
> On 01/31/2014 11:52 AM, Ned Batchelder wrote:
>> On 1/31/14 2:33 PM, Mark Lawrence wrote:
>>>  From http://docs.python.org/3/reference/datamodel.html#object.__init__
>>> which states:-
>>> "
>>> Called when the instance is created. The arguments are those passed to
>>> the class constructor expression. If a base class has an __init__()
>>> method, the derived class’s __init__() method, if any, must explicitly
>>> call it to ensure proper initialization of the base class part of the
>>> instance; for example: BaseClass.__init__(self, [args...]). As a special
>>> constraint on constructors, no value may be returned; doing so will
>>> cause a TypeError to be raised at runtime.
>>> "
>>> Should the wording of the above be changed to clearly reflect that we
>>> have an initialiser here and that __new__ is the constructor?

Construct a house:
   Clear and grade house site.
   Bring in power and water.
   Built temporary structures.
   Built foundation.
   Built house on foundation.

For most classes, __new__ stops with the foundation -- the bare object 
object (with the class name slapped onto it). A *house* is not 
constructed until the house is constructed on top of the foundation.

>> I'm torn about that.  The fact is, that for 95% of the reasons you
>> want to say "constructor", the thing you're
>> describing is __init__.

Right: __init__ usually does the stuff that makes the object an instance 
of a particular class rather than just a bare bones object. In English, 
constructing a 'something' includes constructing the features that make 
the object a 'something' rather than something else.

>>>  Most classes have __init__, only very very few have __new__.

*Every* class has .__new__. Mutable builtins and almost all user classes 
inherit .__new__ from object. Every class also has .__init__, but it is 
mainly inherited from object by immutable builtins.
 >>> tuple.__init__
<slot wrapper '__init__' of 'object' objects>
User classes lacking .__init__ usually inherit it from something other 
than object. So objects are constructed by first calling .__new__ and 
then passing the result to .__init__. The Python 3 doc should say so.

> My problem is with the first sentence, as it makes it sound like there
> is no __new__ and everything you need is in __init__.

.__new__ allocates a bare object and initializes immutable builtins.

   Figuring out how
> __new__ and __init__ were tied together took a long time because of that.
>> Why can't we call __init__ the constructor and __new__ the allocator?

Terry Jan Reedy

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