[BangPypers] [New bie question ] Clarification on "Multiply" operator applied to list(data structure)

[Aman Aggarwal]

Hello there

I am reading "How to think like a computer scientist" which is an
introductory test in Python.

I wanna clarify the behaviour of multiply operator(*) when applied to
list(data structure).

Consider the function make_matrix

def make_matrix(rows, columns):
"""
  >>> make_matrix(4, 2)
  [[0, 0], [0, 0], [0, 0], [0, 0]]
  >>> m = make_matrix(4, 2)
  >>> m[1][1] = 7
  >>> m
  [[0, 0], [0, 7], [0, 0], [0, 0]]
"""
return [[0] * columns] * rows




The actual output is

[[0, 7], [0, 7], [0, 7], [0, 7]]




The correct version of make_matrix is :



def make_matrix(rows, columns):
"""
  >>> make_matrix(3, 5)
  [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
  >>> make_matrix(4, 2)
  [[0, 0], [0, 0], [0, 0], [0, 0]]
  >>> m = make_matrix(4, 2)
  >>> m[1][1] = 7
  >>> m
  [[0, 0], [0, 7], [0, 0], [0, 0]]
"""
matrix = []
for row in range(rows):
    matrix += [[0] * columns]
return matrix


The reason why first version of make_matrix fails ( as explained in
the book at 9.8 ) is that

"...each row is an alias of the other rows..."

I wonder why

[[0] * columns] * rows

causes "...each row is an alias of the other rows..."

but not

[[0] * columns]

i.e. why each [0] in a row is not an alias of other row element.





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