a[:]=b 和 a=b[:] 有什么区别

Question

a=[1,2,3]
b=[4,5,6]
c=[]
d=[]

这两种说法有什么区别？

c[:]=a
d=b[:]

但两者都给出相同的结果。

c 是 [1,2,3]，d 是 [4,5,6]

在功能方面有什么不同吗？

Answer 1

c[:] = a表示将c的所有元素替换为a的元素

>>> l = [1,2,3,4,5]
>>> l[::2] = [0, 0, 0] #you can also replace only particular elements using this 
>>> l
[0, 2, 0, 4, 0]

>>> k = [1,2,3,4,5]
>>> g = ['a','b','c','d']
>>> g[:2] = k[:2] # only replace first 2 elements
>>> g
[1, 2, 'c', 'd']

>>> a = [[1,2,3],[4,5,6],[7,8,9]]
>>> c[:] = a      #creates a shallow copy
>>> a[0].append('foo') #changing a mutable object inside a changes it in c too
>>> a
[[1, 2, 3, 'foo'], [4, 5, 6], [7, 8, 9]]
>>> c
[[1, 2, 3, 'foo'], [4, 5, 6], [7, 8, 9]]

d = b[:] 表示创建 b 的浅拷贝并将其分配给 d ，类似于d = list(b)

>>> l = [1,2,3,4,5]
>>> m = [1,2,3]
>>> l = m[::-1] 
>>> l
[3,2,1]

>>> l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> m = l[:] #creates a shallow copy 
>>> l[0].pop(1) # a mutable object inside l is changed, it affects both l and m
2
>>> l
[[1, 3], [4, 5, 6], [7, 8, 9]]
>>> m
[[1, 3], [4, 5, 6], [7, 8, 9]]

Answer 2

阿什维尼说的话。 :) 我会详细说明一下：

In [1]: a=[1,2,3]

In [2]: b = a

In [3]: c = a[:]

In [4]: b, c
Out[4]: ([1, 2, 3], [1, 2, 3])

In [5]: a is b, a is c
Out[5]: (True, False)

反之：

In [1]: a = [1,2,3]

In [2]: aold = a

In [3]: a[:] = [4,5,6]

In [4]: a, aold
Out[4]: ([4, 5, 6], [4, 5, 6])

In [5]: a = [7,8,9]

In [6]: a, aold
Out[6]: ([7, 8, 9], [4, 5, 6])

看看会发生什么？

Answer 3

没有太大区别。 c[:]=a 更新了 c 引用的列表。 d=b[:] 创建一个新列表，它是 b 的副本（忘记您在第 4 行创建的旧列表）。在大多数应用程序中，除非您有其他对数组的引用，否则您不太可能看到差异。当然，对于c[:]=... 版本，您必须已经有一个列表c。

Answer 4

Ashwini's answer 准确地描述了正在发生的事情，以下是两种方法之间差异的几个示例：

a=[1,2,3]
b=[4,5,6]
c=[]
c2=c
d=[]
d2=d

c[:]=a                            # replace all the elements of c by elements of a
assert c2 is c                    # c and c2 should still be the same list
c2.append(4)                      # modifying c2 will also modify c
assert c == c2 == [1,2,3,4]
assert c is not a                 # c and a are not the same list

d=b[:]                            # create a copy of b and assign it to d
assert d2 is not d                # d and d2 are no longer the same list
assert d == [4,5,6] and d2 == []  # d2 is still an empty list
assert d is not b                 # d and b are not the same list