Note about difference between __new__ and __init__
Before explaining missing functionality let's get back to definition of __new__
and __init__
:
__new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class.
However, __init__ doesn't return anything; it's only responsible for
initializing the instance after it's been created.
Consequences of replacing __init__ with __new__
Mainly you would lose out on flexibility. You would get a lot of semantics headaches and loose separation of initializatin and construction (by joining __new__ and
init we are to joining construction and initialization into one step...).
Let's take a look on snippet below:
class A(object):
some_property = 'some_value'
def __new__(cls, *args, **kwargs):
obj = object.__new__(cls, *args, **kwargs)
obj.some_property = cls.some_property
return obj
class B(A):
some_property = 2
def __new__(cls, *args, **kwargs):
obj = super(B, cls).__new__(cls)
return obj
Consequences of moving __init__
actions into __new__
:
Initialize B
before A
: When you are using __new__
method instead of __init__
your first step of creating new instance of B is calling A.__new__
as side effect you cannot initialize B
before A
is initialized ( access and assign some properties to new B instance). Using of __init__
gives you such flexability.
Loose control on initializing order: let's imagine that you have B_N
inherited from two classes (A_N1
, A_N2
), now you would miss controlling of order of initializing new instance of B_N
(what is the order you are going to initialize instances ? it could be matter... what is weird.)
Properties and methods mess: you would miss access to A.some_property
(cls
would be equal to B
while instantiating new instance of B
. However directly accessing of A.some_property
is possible, but my guess it's at least weird to access properties within class throught class name and not by using classmethods
).
You cannot re-initialize an existed instance without creating new one or implementation special logic for this ( thanks to @platinhom for idea )
What can __init__
do that __new__
cannot?
There are no actions that cannot be done in __new__
and can in __init__
, because actions that __init__
performs is a subset of the actions that can be performed by __new__
.
An interesting moment from Python Docs, Pickling and unpickling normal class instances#object.getinitargs regarding when __init__
could be usefull:
When a pickled class instance is unpickled, its init() method is normally not invoked.
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