Why is this list changing value?

Question

I have a list called ones that changes value after a block of code that shouldn't affect it. Why?

s = 3

ones = []
terms = []
for i in range (0, s):
    ones.append(1)
terms.append(ones)
print(terms)

twos = []
if len(ones) > 1:
    twos.append(ones)
    twos[-1].pop()
    twos[-1][-1] = 2
    print(twos)

print(terms)

Output:

[[1, 1, 1]]  # terms
[[1, 1, 2]]  # twos
[1, 1, 2]    # terms

For context, I'm trying to use this to begin to solve the problem on page 5 of this British Informatics Olympiad past exam: http://www.olympiad.org.uk/papers/2009/bio/bio09-exam.pdf.

Answer 1

Here:

twos.append(ones)

You are appending a reference to ones, not its values. See the difference:

In [1]: l1 = [1, 2, 3]

In [2]: l2 = []

In [3]: l2.append(l1)

In [4]: l2, l1
Out[4]: ([[1, 2, 3]], [1, 2, 3])

In [5]: l2[0][1] = 'test'

In [6]: l2, l1
Out[6]: ([[1, 'test', 3]], [1, 'test', 3])

In order to avoid this you can give a copy by using [:] operator:

In [7]: l1 = [1, 2, 3]

In [8]: l2 = []

In [9]: l2.append(l1[:])

In [10]: l2, l1
Out[10]: ([[1, 2, 3]], [1, 2, 3])

In [11]: l2[0][1] = 'test'

In [12]: l2, l1
Out[12]: ([[1, 'test', 3]], [1, 2, 3])

Answer 2

twos.append(ones) does not copy ones.

There is only ever one list ones in memory, which also goes by the following references:

1. terms[0]
2. twos[0]

and also terms[-1] and twos[-1] because terms and twos only have one element each, so the first is the last.

Now, when you mutate ones/terms[0]/terms[-1]/twos[0]/twos[-1] you are mutating the same list in memory.

I highly recommend watching Facts and Myths about Python names and values.