python create two-dimensional array

To create a two-dimensional array in Python:

aList = [[0] * cols for i inrange(rows)]

If we wanted to create a 2D array in Python, how would we write it?

>>> A = [0]* 3 * 4
>>> B = [[0]*3] * 4

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Is it A or B? Of course it's B! Or take a look at the output first:

>>> A
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
>>> B
[[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

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Unsurprisingly, we should B = [[0]*3]*4create a 2D array as follows. 
! ! ! ! ! ! ! ! BUT ! ! ! ! ! ! ! ! ! 
When you create a two-dimensional array in the above way, if you change a number in the two-dimensional array, what will the output be? Let's try it. For example, we change the second number in the first line to 2, B[0][1] = 2, and output:

>>> B
[[0, 2, 0], [0, 2, 0], [0, 2, 0], [0, 2, 0]]

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Why it came out like this? ! ! 
Because list is a variable type in Python, dear! Creating a two-dimensional array as follows B = [[0]*3]*4is just 4 references to the elements of this empty list, and modifying any one element will change the entire list. 
Another chestnut:

>>> A = [0]*3
>>> B = A
>>> B[0] = 1
>>> A
[1, 0, 0]

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Pit father! ! 
So, to create a two-dimensional array in Python you should write:

>>> C = [[0]*3 for i in range(4)]
>>> C 
[[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> C[0][1] = 2
>>> C
[[0, 2, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

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So next time you create a 2D array in Python remember this:

aList = [[0] * cols for i in range(rows)]

• #output the number of rows and columns of the array 
  print x.shape   # (4, 3) 
  #only output the number of rows print x.shape[0] # 4 
  #only output the number of columns print x.shape [1] # 3