[英]How to pass a list element as reference?
I am passing a single element of a list to a function.我将列表的单个元素传递给函数。 I want to modify that element, and therefore, the list itself.
我想修改那个元素,也就是列表本身。
def ModList(element):
element = 'TWO'
l = list();
l.append('one')
l.append('two')
l.append('three')
print l
ModList(l[1])
print l
But this method does not modify the list.但是这种方法不会修改列表。 It's like the element is passed by value.
这就像元素是按值传递的。 The output is:
输出是:
['one','two','three']
['one','two','three']
I want that the second element of the list after the function call to be 'TWO':我希望函数调用后列表的第二个元素是“TWO”:
['one','TWO','three']
Is this possible?这可能吗?
The explanations already here are correct.已经在这里的解释是正确的。 However, since I have wanted to abuse python in a similar fashion, I will submit this method as a workaround.
但是,由于我想以类似的方式滥用 python,因此我将提交此方法作为解决方法。
Calling a specific element from a list directly returns a copy of the value at that element in the list.从列表中调用特定元素直接返回列表中该元素的值的副本。 Even copying a sublist of a list returns a new reference to an array containing copies of the values.
即使复制列表的子列表也会返回对包含值副本的数组的新引用。 Consider this example:
考虑这个例子:
>>> a = [1, 2, 3, 4]
>>> b = a[2]
>>> b
3
>>> c = a[2:3]
>>> c
[3]
>>> b=5
>>> c[0]=6
>>> a
[1, 2, 3, 4]
Neither b
, a value only copy, nor c
, a sublist copied from a
, is able to change values in a
. b
仅复制值, c
也不是从a
复制的子列表,都不能更改a
值。 There is no link, despite their common origin.尽管它们有共同的起源,但没有任何联系。
However, numpy arrays use a "raw-er" memory allocation and allow views of data to be returned.但是,numpy 数组使用“原始”内存分配并允许返回数据视图。 A view allows data to be represented in a different way while maintaining the association with the original data.
视图允许以不同的方式表示数据,同时保持与原始数据的关联。 A working example is therefore
因此,一个工作示例是
>>> import numpy as np
>>> a = np.array([1, 2, 3, 4])
>>> a
array([1, 2, 3, 4])
>>> b = a[2]
>>> b
3
>>> b=5
>>> a
array([1, 2, 3, 4])
>>> c = a[2:3]
>>> c
array([3])
>>> c[0]=6
>>> a
array([1, 2, 6, 4])
>>>
While extracting a single element still copies by value only, maintaining an array view of element 2
is referenced to the original element 2
of a
(although it is now element 0
of c
), and the change made to c
's value changes a
as well.虽然仅通过值提取单个元件仍然份,保持元件的阵列视图
2
为参考原始元件2
的a
(虽然现在元素0
的c
),并进行变更c
的值改变a
作为好。
Numpy ndarray
s have many different types, including a generic object type. Numpy
ndarray
有许多不同的类型,包括通用对象类型。 This means that you can maintain this "by-reference" behavior for almost any type of data, not only numerical values.这意味着您可以为几乎任何类型的数据维护这种“按引用”行为,而不仅仅是数值。
Python is a pass by value language hence you can't change the value by assignment in the function ModList
. Python 是一种按值传递的语言,因此您不能通过函数
ModList
赋值来更改值。 What you could do instead though is pass the list and index into ModList
and then modify the element that way您可以做的是将列表和索引传递给
ModList
,然后以这种方式修改元素
def ModList(theList, theIndex) :
theList[theIndex] = 'TWO'
ModList(l, 1)
Python doesn't do pass by reference . Python 不会通过引用传递。 Just do it explicitly:
只需明确地做到这一点:
l[1] = ModList(l[1])
Also, since this only changes one element, I'd suggest that ModList
is a confusing name.此外,由于这只会更改一个元素,因此我建议
ModList
是一个令人困惑的名称。
In many cases you can also consider to let the function both modify and return the modified list.在很多情况下,你也可以考虑让函数既修改又返回修改后的列表。 This makes the caller code more readable:
这使得调用者代码更具可读性:
def ModList(theList, theIndex) :
theList[theIndex] = 'TWO'
return theList
l = ModList(l, 1)
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