Python overwrites list while changing a copy. How can I keep the original list stored

[Dominik]

Hey guys,

hope somebody can help me out of my predicament.

Basically I have a list whose objects are lists of numbers and I need to use some numerical mathematics on it while keeping the old list to define the break criteria. This is my code so far:

def Numerik(n,m,Field):
    epsilon=1
    Field_new=Field
    while epsilon>0.001:
#inner nodes (explicit)
        for j in range(1,m):
            Horizontal_North=Field[j-1]
            Horizontal_West_East=Field[j]
            Horizontal_South=Field[j+1]
            Horizontal=Field_new[j]
            for i in range(1,n):
                North=Horizontal_Nord[i]
                West=Horizontal_West_East[i-1]
                East=Horizontal_West_East[i+1]
                South=Horizontal_South[i]
                Point=(North+West+East+South)/4
                Horizontal[i]=Point
            Field_new[j]=Horizontal
            print (Horizontal)
            print (Field[j])

So basically I used copies to work on. Python still overwrites the original and I get the same output for Horizontal and Field[j] even though it is only supposed to change the Field_new. Can anyone tell me how to keep the old list stored while changing the new one?

[Mariostg]

I am not sure I quite understand, but consider this.
When you say y=x[] you create a reference, not a copy. So if you alter y, you alter x.
To make a copy of x, use y=x[:]