[英]What does [[]]*2 do in python?
A = [[]]*2
A[0].append("a")
A[1].append("b")
B = [[], []]
B[0].append("a")
B[1].append("b")
print "A: "+ str(A)
print "B: "+ str(B)
Yields: 产量:
A: [['a', 'b'], ['a', 'b']]
B: [['a'], ['b']]
One would expect that the A list would be the same as the B list, this is not the case, both append statements were applied to A[0] and A[1]. 可以预期A列表与B列表相同,但事实并非如此,两个追加语句都应用于A [0]和A [1]。
Why? 为什么?
A = [[]]*2
creates a list with 2 identical elements: [[],[]]
. A = [[]]*2
创建一个包含2个相同元素的列表: [[],[]]
。 The elements are the same exact list. 元素是完全相同的列表。 So
所以
A[0].append("a")
A[1].append("b")
appends both "a"
and "b"
to the same list. 将
"a"
和"b"
都附加到同一列表中。
B = [[], []]
creates a list with 2 distinct elements. B = [[], []]
创建一个包含2个不同元素的列表。
In [220]: A=[[]]*2
In [221]: A
Out[221]: [[], []]
This shows that the two elements of A
are identical: 这表明
A
的两个元素是相同的:
In [223]: id(A[0])==id(A[1])
Out[223]: True
In [224]: B=[[],[]]
This shows that the two elements of B
are different objects. 这表明
B
的两个元素是不同的对象。
In [225]: id(B[0])==id(B[1])
Out[225]: False
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