Function is changing wrong variable

Question

I have got a problem that changes wrong variable in a function(s) shown below.

def no_updown() -> int:
 print(g.matrix)
 count = 0
 arr = g.matrix
 arr = g.move_elements(arr)
 for x in range(g.size):
 for y in range(g.size - 1):
 if arr[y, x] == arr[y + 1, x] and arr[y, x] > 0:
 arr[y, x] *= 2
 arr[y + 1, x] = 0
 count += 1
 return count

And here is g.move_elements:

def move_elements(self, arr) -> np.matrix:
 for x in range(self.size):
 temp = 0
 for y in range(self.size):
 if not arr[y, x] == 0:
 arr[temp, x] = arr[y, x]
 if not temp == y:
 arr[y, x] = 0
 temp += 1
 return arr

I want to operate only on arr from no_updown(). Instead what am I getting is g.matrix changed. I believe this has something to do with referencing, but I don't know quietly what.

Answer 1

arr = g.matrix

You have made arr and g.matrix refer to the same object. So naturally, changes made via either name are visible from the other.

I don't know what kind of object g.matrix is (although I can see it's not a Python list) but there's probably a way to copy it and avoid this problem. If it's a NumPy array, you can use numpy.copy.

• Marek: Right, didn't know that, such a basic... g.matrix is np.matrix, so I can use np.copy(). Thanks!