[英]Confusion in code using inheritance, instantiation, *args, **kwargs
class Pen(object):
def __init__(self, mean_radius = None, length = None):
self.usage = "Is used to apply ink trails to drawing surfaces"
self.mean_radius = mean_radius
self.length = length
class FountainPen(Pen):
def __init__(self, manufacturer = "Waterman", *args, **kwargs):
Pen.__init__(self, *args, **kwargs)
self.manufacturer = manufacturer
self.cartridge_state = "non-empty"
>>> instance_FP = FountainPen(5, 1)
>>> instance_FP.mean_radius
>>> print instance_FP.mean_radius
1
>>> print instance_FP.length
None
What happens with the integer 5
being passed as an argument in the instantiation of FountainPen
? 在
FountainPen
的实例化中将整数5
作为参数传递时会发生什么情况? Why does print instance_FP.mean_radius
return 1
and not 5
? 为什么
print instance_FP.mean_radius
返回1
而不是5
?
You have to think it like that: *args
and **kwargs
"eat" all the positional/keyword arguments left by the "regular" arguments that precede them. 您必须这样认为:
*args
和**kwargs
“吃掉”在它们前面的“常规”参数留下的所有位置/关键字参数。 If you put them in the end, they will only get whatever couldn't fit the "regular arguments". 如果把它们放在最后,它们只会得到不适合“常规参数”的任何东西。
So, when you write FountainPen(5,1)
what happens is that FountainPen.__init__
gets called like this: 因此,当您编写
FountainPen(5,1)
,会发生FountainPen.__init__
调用FountainPen.__init__
情况:
self
is set to the newly created instance; self
设置为新创建的实例; manufacturer
gets the first argument, which is 5; manufacturer
获得第一个参数,即5; *args
"eats" all the remaining positional arguments, ie just 1
; *args
“吃掉”所有剩余的位置参数,即仅1
; so args
is now set to [1]
; args
现在设置为[1]
; *kwargs
would eat any keyword argument left, but there's none, so it becomes {}
. *kwargs
会吃掉剩下的所有关键字参数,但是没有关键字,因此它变成{}
。 It's then clear that Pen.__init__
is called with just 1
as argument (besides self
), and length
remains set to its default value ( None
). 然后很明显,
Pen.__init__
1
作为参数(除了self
)调用了Pen.__init__
,而length
仍设置为其默认值( None
)。
Matteo Italia 's mentions what is wrong, basically manufacturer
is a positional argument. Matteo Italia的提法出了什么问题,基本上
manufacturer
是一个立场论点。 You can fix it by slightly changing FountainPen
's __init__
to the following: 您可以通过将
FountainPen
的__init__
稍微更改为以下内容来修复它:
def __init__(self, *args, manufacturer = "Waterman", **kwargs):
Doing this will make manufacturer
a keyword argument so to change it you will have to call: 这样做会使
manufacturer
成为关键字参数,因此要更改它,您必须调用:
FountainPen(manufacturer="newval")
Note: Unfortunately this syntax (of having keyword only arguments after the *args
parameter) is only valid in Python 3. Jon Clements has a solution that solves this problem for Python 2. 注意:不幸的是,这种语法(在
*args
参数后仅具有关键字参数)仅在Python 3中有效。Jon Clements的解决方案可解决Python 2的此问题。
A more elegant solution would be: 一个更优雅的解决方案是:
class FountainPen(Pen):
def __init__(self, *args, **kwargs):
Pen.__init__(self, *args, **kwargs)
self.manufacturer = kwargs.get('manufacturer', 'Waterman')
self.cartridge_state = "non-empty"
blah = FountainPen(5, 1, manufacturer='Waterman')
In your code 5 is assigned to the argument manufacturer
and 1 goes to *args
. 在您的代码中,将5分配给参数
manufacturer
,将1分配给*args
。
When passing arguments without keys to a function, you must first provide the explicit ones. 将不带键的参数传递给函数时,必须首先提供显式参数。 The rest is passed to *args.
其余的传递给* args。 To avoid this, provide keywords when calling your constructor:
为避免这种情况,请在调用构造函数时提供关键字:
>>> instance_FP = FountainPen(mean_radius=5, length=1)
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