W dniu 21.12.2016 o 02:51, Ethan Furman pisze:
On 12/20/2016 03:39 PM, Ben Finney wrote:
"Mr. Wrobel" writes:
Quick question, can anybody tell me when to use __init__ instead of
__new__ in meta programming?
Use ‘__new__’ to do the work of *creating* one instance from nothing;
allocating the storage, determining the type, etc. — anything that will
be *the same* for every instance. ‘__new__’ is the constructor.
Use ‘__init__’ to do the work of *configuring* one instance, after it
already exists. Any attributes that are special to an instance should be
manipulated in the ‘__init__’ method. ‘__init__’ is the initialiser.
That sounds like general object creation/class advice, which as a general
guideline is okay, but don't feel like it's the most important thing.
I only use `__new__` when the object being created is (or is based on)
an immutable type; otherwise I use `__init__`. Likewise, if I'm using
`__new__` then I do all my configuration in `__new__` unless I have a
really good reason not to (such as making it easier for subclasses to
modify/skip `__init__`).
As far as metaclasses go... the only time I recall writing an `__init__`
for a metaclass was to strip off the extra arguments so `type.__init__`
wouldn't fail.
--
~Ethan~
Hi,thanx for answers, let's imagine that we want to add one class
attribute for newly created classess with using __init__ in metaclass,
here's an example:
#!/usr/bin/env python
class MetaClass(type):
# __init__ manipulation:
def __init__(cls, name, bases, dct):
dct['added_in_init'] = 'test'
super(MetaClass, cls).__init__(name, bases, dct)
class BaseClass(object):
__metaclass__ = MetaClass
class NewBaseClass(BaseClass):
pass
print("Lets print attributes added in __init__ in base classes:")
print(BaseClass.added_in_init)
print(NewBaseClass.added_in_init)
after running it: AttributeError: type object 'BaseClass' has no
attribute 'added_in_init'
Adding the same in __new__ works. Can anyone explain me please what's wrong?
Cheers,
M
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