UnboundLocalError: local variable var referenced before assignment – how to debug

I’m working through a tutorial based on this and have hit this error.

I am trying to work up my skill in Python and am not sure how to debug this because the error confuses me.

import random

MAX_LINES = 3
MAX_BET = 100
MIN_BET = 1

ROWS = 3
COLS = 3

symbol_count = {
    "A": 2,
    "B": 4,
    "C": 6,
    "D":8
}

symbol_value = {
    "A": 5,
    "B": 4,
    "C": 3,
    "D":2
}

def check_winnings(columns, lines, bet, values):
    winnings - 0
    winning_lines = []
    for line in range(lines):
        symbol = columns[0][line]
        for column in columns:
            symbol_to_check = column[line]
            if symbol != symbol_to_check:
                break
        else:
            winnings += values[symbol] * bet
            winning_lines.append(lines +1) # Add one because index starts at 0 and it needs to start at 1
    
    return winnings, winning_lines 
     

def get_slot_machine_spin(rows, cols, symbols):
    all_symbols = []
    for symbol, symbol_count in symbols.items():
        for _ in range(symbol_count): #The _ is an anonymous value
            all_symbols.append(symbol)
    #print(all_symbols)
  
    columns = []
    for _ in range(cols):
        column = []
        current_symbols = all_symbols[:]# This creates a copy of all symbols
        for _ in range(rows):
            value = random.choice(current_symbols)
            current_symbols.remove(value)
            column.append(value)
        columns.append(column)
    return columns

def print_slot_machine(columns):
    for row in range(len(columns[0])):
        for i,column in enumerate(columns):
            if i != len(columns) - 1:
                print(column[row], end = " | ")
            else:
                print(column[row], end="")
        print()
def deposit():
    while True:
        amount = input("What would you like to deposit?")
        if amount.isdigit():
            amount = int(amount)
            if amount > 0:
                break
            else:
                print("Amount must be greater than zero.")
        else:
            print("Please enter a number.")
    return amount

def get_number_of_lines():
    while True:
        lines = input("Enter the number of lines to bet on (1-" + str(MAX_LINES) + "? ")
        if lines.isdigit():
            lines = int(lines)
            if 1 <= lines <= MAX_LINES:
                break
            else:
                print("Enter a valid number of lines.")
        else:
            print("Please enter a number.")
    return lines

def get_bet():
    while True:
        amount = input("How much would you like to bet on each line?")
        if amount.isdigit():
            amount = int(amount)
            if MIN_BET <= amount <= MAX_BET:
                break
            else:
                print(f"Amount must be between {MIN_BET} - {MAX_BET}.")
        else:
            print("Please enter a number.")
    return amount
    


def main():
    balance = deposit()
    lines = get_number_of_lines()
    while True:
        bet = get_bet()
        total_bet = bet * lines
        if total_bet > balance:
            print (f"You do not have enough cash to pay the bet from your balance. Your current balance is {balance}")
        else:
            break
    slots = get_slot_machine_spin (ROWS, COLS, symbol_count)
    print_slot_machine(slots)
    winnings, winning_lines = check_winnings(slots, lines, bet,  symbol_value)
    print(f"You won {winnings}")
    print(f"You won on", *winning_lines) #splat or unpack operator
    return

main()

The output at runtime is:

What would you like to deposit?100
Enter the number of lines to bet on (1-3? 3
How much would you like to bet on each line?3
A | B | D
B | C | C
D | A | C

But then I get this traceback…

UnboundLocalError                         Traceback (most recent call last)
/var/folders/v_/yq26pm194xj5ckqy8p_njwc00000gn/T/ipykernel_54333/4159941939.py in <module>
    122     return
    123 
--> 124 main()

/var/folders/v_/yq26pm194xj5ckqy8p_njwc00000gn/T/ipykernel_54333/4159941939.py in main()
    117     slots = get_slot_machine_spin (ROWS, COLS, symbol_count)
    118     print_slot_machine(slots)
--> 119     winnings, winning_lines = check_winnings(slots, lines, bet,  symbol_value)
    120     print(f"You won {winnings}")
    121     print(f"You won on", *winning_lines) #splat or unpack operator

/var/folders/v_/yq26pm194xj5ckqy8p_njwc00000gn/T/ipykernel_54333/4159941939.py in check_winnings(columns, lines, bet, values)
     23 
     24 def check_winnings(columns, lines, bet, values):
---> 25     winnings - 0
     26     winning_lines = []
     27     for line in range(lines):

UnboundLocalError: local variable 'winnings' referenced before assignment

How do I unpack this?