Mel wrote:
Ethan Furman wrote:
Emile van Sebille wrote:
Ethan Furman wrote:
--> d25._int = (1, 5)
Python considers names that start with a leading underscore as internal
or private, and that abuse is the burden of the abuser...
Is bytecodehacks still around? That was serious abuse :)
Good point. What I'm curious about, though, is the comment in the code
about making the Decimal instance immutable. I was unable to find docs
on that issue.
There's something in the Language Reference, chapter 3.1 'Objects, Values
and Types'.
Mel.
Thanks, Mel.
I had actually read that once already, but your post caused me to reread
it, and evidently the ideas there had had enough time to percolate
through my mind.
--> from decimal import Decimal
--> d25 = Decimal(25)
--> d25
Decimal("25")
--> d25.testing = 'immutable'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Decimal' object has no attribute 'testing'
Decimals are immutable in that we cannot add new attributes to them.
The documentation in Language Reference 3.4.2.4 '__slots__' has this to say:
If defined in a new-style class, __slots__ reserves space for
the declared variables and prevents the automatic creation of
__dict__ and __weakref__ for each instance.
and
Without a __dict__ variable, instances cannot be assigned new
variables not listed in the __slots__ definition. Attempts to
assign to an unlisted variable name raises AttributeError.
So the question I have now is this: is __new__ necessary, or would
__init__ have also worked? Let's see...
class tester(object):
__slots__ = ['test1', 'test2', 'test3']
def __init__(self, value1, value2, value3):
self.test1 = value1
self.test2 = value2
self.test3 = value3
--> import tester
--> testee = tester.tester(1, 2, 3)
--> testee
<tester.tester object at 0x009E7328>
--> dir(testee)
['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__',
'__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__setattr__', '__slots__', '__str__', 'test1', 'test2',
'test3']
--> testee.test4 = 4
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'tester' object has no attribute 'test4'
For this simple test, it looks like __init__ works just fine. So,
besides consistency (which is important) am I missing any other reasons
to use __new__ instead of __init__?
