Python function not finding global variable

I am creating a program to calculate b1 for a linear regression program in Python.

When trying to assign a value to my b1 global variable, my IDE doesn’t find it. You can find the variable assignment at the last line of my function.

x_data = [4, 8, 6, 3, 4, 6, 2, 1, 3]
y_data = [2, 4, 7, 3, 4, 8, 3, 2, 5]
b1 = 0

def b0_b1_calc():
    x_sum = 0
    y_sum = 0
    for i in x_data:
        x_sum += x_data[i]
        y_sum += y_data[i]
    x_mean = round(1, x_sum / len(x_data))
    y_mean = round(1, y_sum / len(y_data))
    x_list = []
    y_list = []
    for i in x_data:
        x_list.append(i - x_mean)
        y_list.append(i - y_mean)
    x_minus_x_squared = []
    for i in x_list:
        x_minus_x_squared.append((x_list[i] - x_mean) ** 2)
    x_sum = sum(x_minus_x_squared)
    x_y = []
    for i in x_data:
        x_y.append(y_list * x_list)
    x_y = sum(x_y)
    b1 = x_y / x_sum
    
print(str(b1))




    

>Solution :

You have to use global to access global variables. There were a few more errors in the code that I have fixed and annotated too:

def b0_b1_calc():
    # Use global to access global variables
    global b1

    # sum() summates a list
    x_sum = sum(x_data)
    y_sum = sum(y_data)

    # This was the wrong way around
    x_mean = round(x_sum / len(x_data))
    y_mean = round(y_sum / len(y_data))

    x_list = []  # could also be [i - x_mean for i in x_data]
    y_list = []  # could also be [i - y_mean for i in x_data]

    for i in x_data:
        x_list.append(i - x_mean)
        y_list.append(i - y_mean)

    # could also be [(x_list[i] - x_mean) ** 2 for i in x_list]
    x_minus_x_squared = []

    for i in x_list:
        x_minus_x_squared.append((x_list[i] - x_mean) ** 2)

    x_sum = sum(x_minus_x_squared)
    x_y = []

    for i in x_data:
        # could be simplified to [np.array(y_list) * np.array(x_list) for i in x_data]
        # use np.array for vectorial array multiplication
        x_y.append(np.array(y_list) * np.array(x_list))
    
    x_y = sum(x_y)
    b1 = x_y / x_sum


b0_b1_calc()

The preferred way to get around globals is to call and return:

def b0_b1_calc():
    # ... code here
    return b1

b1 = b0_b1_calc()