a=[1,2,3]
b=[4,5,6]
c=[]
d=[]
这两个语句有什么区别?
c[:]=a
d=b[:]
但两者得到的结果相同。
c为[1,2,3],d为[4,5,6]
从功能上讲,它们是否有任何区别?
c[:] = a
的意思是将 c 中的所有元素替换为 a 中的元素。
>>> 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[:]
意味着创建一个浅拷贝的b并将其分配给d,它类似于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]]
就像Ashwini说的那样。 :) 我稍微补充一下:
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)
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?
Ashwini的答案准确描述了发生了什么,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
这两者没有太大区别。c[:]=a
直接在原有的列表上进行更新。而 d=b[:]
创建并返回一个新的列表,这个新的列表是 b 的副本(忽略第四行创建的旧列表)。在大多数应用中,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,使用 c[:]=...
版本时,您必须已经有一个名为 c
的列表。
c
的引用?这些引用也会受到更改的影响。考虑使用:c=[]; f=c; c[:]=a
-- 现在,由于f
和c
是同一个列表,所以f
也将具有与a
相同的元素。 - mgilsona
包含可变对象,则对该可变对象的更改将同时出现在变量c
中,参见上述示例,返回 @InternalServerError。 - Ashwini Chaudhary