Bruno Desthuilliers wrote:
Andrew MacKeith a écrit :
I create a class like this in Python-2.6
>>> class Y(str):
... def __init__(self, s):
... pass
...
>>> y = Y('giraffe')
>>> y
'giraffe'
>>>
How does the base class (str) get initialized with the value passed to
Y.__init__() ?
It happens in the __new__ method (which is the proper "constructor")
class Y(str):
def __new__(cls, value, foo="foo"):
instance = str.__new__(cls, value)
instance.foo = foo
return instance
def __repr__(self):
return "<%s(%s, %s)>" % (type(self).__name__, self, self.foo)
Is this behavior specific to the str type, or do base classes not need
to be explicitly initialized?
When you override a method in a derived class, it's your responsability
to call on the parent(s) class(es) implementation. __init__ is not an
exception to that rule. The point is that since __init__ works by
mutating the newly created instance, it makes no sense to have a
distinct __init__ for immutable types - which have their "value" set
once for all at creation time.
Thanks for the explanation, Bruno.
I have been successfully using a class derived from str, and in that
class I add a lot of extra data to the derived class instance in the
__init__ method of the derived class, so it is possible to mutate the
derived class __dict__, even if the base class data remains immutable.
See example below.
The __init__ must have the same arguments as the base class.
>>> class Y(str):
... def __init__(self):
... self.color = 'blue'
...
>>> y = Y('Parrot')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 1 argument (2 given)
>>>
>>> class Y(str):
... def __init__(self, text):
... self.color = 'blue'
...
>>> y = Y('Parrot')
>>> y
'Parrot'
>>> y.color
'blue'
>>> y[:3]
'Par'
>>> y.size = 'small'
>>>
>>> y.__class__
<class '__main__.Y'>
>>>
But you can get bitten if you do something that returns a new object
>>> y += 'Tail'
>>> y
'ParrotTail'
>>> y.color
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'color'
>>>
>>> y.__class__
<type 'str'>
>>>
Andrew
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