Having trouble using user input between functions

Question

I'm trying to learn Python and I'm really struggling with getting my code into self-contained functions. Here is an example:

def get_inputs():
    sales_amount = float(input("Enter total sales amount: "))

def calculate_discount(sales_amount):
    discount_amount = sales_amount * 2 
    return discount_amount

def output():
    print ( discount_amount )

def main():
    get_inputs()
    calculate_discount(sales_amount)
    output()

main()

Running this returns

File "/Users/Desktop/assA3.py", line 17, in <module>
main()

File "/Users/Desktop/assA3.py", line 14, in main
calculate_discount(sales_amount)

NameError: name 'sales_amount' is not defined

I thought the variable sales_amount was defined by the user input before it is referenced later. I can't figure out what I'm missing.

I apologise for how basic this problem is but I'm clearly misunderstanding something fundamental and I'm just really struggling. Any help would be greatly appreciated. Thanks.

Answer 1

You are trying to use function scope variables in other functions. Nothing outside of function get_inputs can see variable sales_amount, its a variable local to that function. You will encounter the same issue with discount_amount, which is scoped to calculate_discount.

Instead return the values and pass them to other functions.

def get_inputs():
    return float(input("Enter total sales amount: "))

def calculate_discount(sales_amount):
    discount_amount = sales_amount * 2 
    return discount_amount

def output(discount_amount):
    print ( discount_amount )

def main():
    inputs = get_inputs()
    discounts = calculate_discount(inputs)
    output(discounts)

main()

The dirty option is to make them globals

def get_inputs():
    global sales_amount
    sales_amount = float(input("Enter total sales amount: "))

def calculate_discount(sales_amount):
    global discount_amount
    discount_amount = sales_amount * 2 
    return discount_amount

def output():
    print ( discount_amount )

def main():
    get_inputs()
    calculate_discount(sales_amount)
    output()

main()

Globals are slower, make your code more difficult to maintain and are bad for a whole raft of reasons.

Answer 2

The simple answer is to pass sales_amount out of gets_input():

def get_inputs():
    sales_amount = float(input("Enter total sales amount: "))
    return sales_amount

and to use it in main():

def main():
    sales_amount = get_inputs()
    calculate_discount(sales_amount)
    output()

If you want to make it "self-contained" then you want to use classes:

class Sales(object):
    def get_inputs(self):
        self.sales_amount = float(input("Enter total sales amount: "))

    def calculate_discount(self):
        self.discount_amount = self.sales_amount * 2 

    def output(self):
        print ( self.discount_amount )


def main():
    my_sale = Sales()
    my_sale.get_inputs()
    my_sale.calculate_discount()
    my_sale.output()

main()