I wrote some code for my project and found that the result isn’t correct. So I spent quite some time debugging the whole working process this program and found that it is because I have initialized a list wrongly.

When we do the following in Python:

x = [[]] * 3
x[0].append(1.0)

I expect x now becomes [[1.0], [], []]. Instead, it becomes [[1.0], [1.0], [1.0]]. This is because when we use multiplication to create x, we actually created 3 references to the empty list. List is a mutable object in Python. When we append values to a list, we haven’t changed its identity. As a result, changing either one of the 3 sub-list will change the others since they refer to the same list.

It is different when we create a list of same immutable objects. For example, if we create a list of same int, and then change one of the int, other elements will not change. It is because we can not change the value of immutable types, assigning a new int to a list element will make it point to another address.

In [18]: a = [1] * 2

In [19]: a
Out[19]: [1, 1]

In [20]: print(id(a[0]), id(a[1]))
4438635584 4438635584

In [21]: a[0] = 2

In [22]: print(id(a[0]), id(a[1]))
4438635616 4438635584

To create a list 3 empty lists, we may use list comprehension instead:

x = [[] for _ in range(3)]

In each iteration, a new empty list is created. So the 3 sub-list are independent of each other.

Author jdhao

LastMod 2020-11-22

License CC BY-NC-ND 4.0