Python: List is overwritten. But Why?

Question

The following code:

file_path = 'some_path/data.txt'
exp = loadtxt(file_path)

signal_exp = []
signal_exp.append(exp[1, :])

signal_exp_new = []
signal_exp_new.append(signal_exp[0])

signal_exp_new[0][0:800] = 0.0

will result in signal_exp beeing overwritten at the first 800 elements as well as signal_exp_new. I found the solution but I do not understand why the next one works as expected (of me at least):

file_path = 'some_path/data.txt'
exp = loadtxt(file_path)

signal_exp = []
signal_exp.append(exp[0, :].tolist())

signal_exp_new = []
signal_exp_new.append(signal_exp[0][:])

for l in range(800):
    signal_exp_new[0][l] = 0.0

Can anyone give me an explanation why in the latter case, the original list is not overwritten but in the first case it is?

Answer 1

NumPy array slicing and Python list slicing work differently. Slicing on Python lists returns a shallow copy of the list, while on NumPy arrays it simply returns a view of the array items.

From NumPy docs:

Note that slices of arrays do not copy the internal array data but also produce new views of the original data.

So, in your first solution you can use .copy to get a copy of the array:

signal_exp_new.append(signal_exp[0].copy())