a [：] = b和a = b之間的區別是什麼？

Question

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

這兩個聲明有什麼區別？

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

但是兩者都給出相同的結果。

c爲[1,2,3]，d爲[4,5,6]

而沒有任何區別的功能明智？

Answer 1

c[:] = a它意味着替換C的所有元件由一個

>>> 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[:]裝置元件創建的B A 淺拷貝並將其分配給d，它類似於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

沒有太大的區別。 c[:]=a更新c提到的列表。 d=b[:]創建一個新的列表，它是b的副本（忘記您在第4行創建的舊列表）。在大多數應用程序中，除非您有其他引用數組的參考，否則不太可能會看到差異。當然，在c[:]=...版本中，您必須已經有一個名單c。

Answer 3

Ashwini說什麼。 :)我會詳細說明一點點：

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) 

另一路：

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]) 

看看會發生什麼？

Answer 4

Ashwini's answer準確地描述發生了什麼，這裏有兩個方法之間的差異的幾個例子：

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