Creating a (very simple) computer opponent for connect4 - python-3.x

Question

I am currently having problem writing a function that should return a suggested move i.e. a simple computer opponent for a game of connect 4.
The following is some functions that I have written (and work correctly for the situations I have tested) but are used in the computeropponent function.
import random
#board: a list containing 6 lists with 7 elements each (a connect4 board)
#0 represents an empty space, and 1,2 represent player1 and player2
#turn: whose turn it is (i.e. player1 or player2)
#move: column they want to make a move in (i.e 0-6, but will be 1-7 to the player)
def validmoves(board):
"""This function iterates through the board to find all the valid moves
(i.e. it excludes the columns that are filled) and returns the list of
columns with valid moves"""
vm = []
for j in range(7):
i = 5
while vm.count(j) == 0 and i>=0:
if board[i][j] == 0:
vm.append(j)
i -= 1
return vm
def makemove(board,move,turn):
"""This function places turn's player piece in the move'th column and
returns the board"""
for i in reversed(board):
if i[move] == 0:
i[move] = turn
return board
def winner(board,turn):
"""This function checks if the player who just played a turn has won"""
#horizontals
for i in board:
for j in range(4):
if i[j]==turn and i[j+1]==turn and i[j+2]==turn and i[j+3]==turn:
return True
#verticals
for j in range(7):
for i in range(3):
if board[i][j]==turn and board[i+1][j]==turn and board[i+2][j]==turn and board[i+3][j]==turn:
return True
#positive diagonals
for j in range(4):
for i in range(3):
if board[i][j]==turn and board[i+1][j+1]==turn and board[i+2][j+2]==turn and board[i+3][j+3]==turn:
return True
#negative diagonals
for j in range(4):
for i in range(5,2,-1):
if board[i][j]==turn and board[i-1][j+1]==turn and board[i-2][j+2]==turn and board[i-3][j+3]==turn:
return True
return False
The following is the code I have a problem with
def computeropponent(board,turn):
#the following defines the other player in the context of this function
if turn == 1:
notturn = 2
elif turn == 2:
notturn = 1
"""for the following loops, it only computes the first loop and then skips straight to
the last line."""
for move in validmoves(board):
if winner(makemove(board,move,turn),turn)==True:
return move
for move in validmoves(board):
if winner(makemove(board,move,notturn),turn)==True:
return move
return random.choice(validmoves(board))
i.e. if i were to swap the order of the for loops (or set turn to 1 in
computer opponent), it will return the correct answer (which should be 0 in the test below), however in this case it just returns a random element from the list of valid moves.
board = [[0,0,0,0,0,0,0],
[0,0,0,0,0,0,0],
[0,0,0,0,0,0,0],
[0,0,0,0,1,2,0],
[0,0,0,1,2,2,0],
[0,1,1,1,2,1,0]]
print(validmoves(board))
print(computeropponent(board,2))
I apologise in advance for the messy code and length of my post. I have not been programming long and this is my first time posting on stackexchange, and thanks in advance for the help.