NameError：未定义名称“值”

Question

(background first: I am NEW to programming and currently in my very first "intro to programming class" in my college. This is our second assignment dealing with Functions. So far functions have been a pain in the ass for me because they don't really make any sense. ex: you can use miles_gas(gas) but then don't use "miles_gas" anywhere else, but the program still runs??, anyways)

Okay, I've looked EVERYWHERE online for this and can't find an answer. Everything is using "Exceptions" and "try" and all that advanced stuff. I'm NEW so I have no idea what exceptions are, or try, nor do I care to use them considering my teacher hasn't assigned anything like that yet.

My project is to make a program that gives you the assessment value, and the property tax upon entering your property price. Here is the code I came up with (following the video from my class, as well as in the book)

ASSESSMENT_VALUE = .60
TAX = 0.64

def main():
    price = float(input('Enter the property value: '))
    show_value(value)
    show_tax(tax)

def show_value():
    value = price * ASSESSMENT_VALUE
    print('Your properties assessment value is $', \
        format(value, ',.2f'), \
        sep='')

def show_tax(value,TAX):
    tax = value * TAX
    print('Your property tax will be $', \
        format(tax, ',.2f'), \
        sep='')    

main()

Upon running it, I get it to ask "blah blah enter price:" so I enter price then I get a huge red error saying

Traceback (most recent call last):
      File "C:/Users/Gret/Desktop/chapter3/exercise6.py", line 41, in <module>
      main()
File "C:/Users/Gret/Desktop/chapter3/exercise6.py", line 24, in main
show_value(value)
NameError: name 'value' is not defined

But I DID define 'value'... so why is it giving me an error??

Answer 1

Python is lexically scoped. A variable defined in a function isn't visible outside the function. You need to return values from functions and assign them to variables in the scopes where you want to use the values. In your case, value is local to show_value .

Answer 2

When you define a function, it needs parameters to take in. You pass those parameters in the brackets of the function, and when you define your function, you name those parameters for the function. I'll show you an example momentarily.

Basically what's happened is you've passed the function a parameter when you call it, but in your definition you don't have one there, so it doesn't know what to do with it.

Change this line:

def show_value():

To this line:

def show_value(price):

And show_value to show_value(price)

For example:

In this type of error:

def addition(a,b):
    c = a + b
    return c

addition() # you're calling the function, 
           # but not telling it the values of a and b

With your error:

def addition():
    c = a + b
    return c

addition(1,2) # you're giving it values, but it 
              # has no idea to give those to a and b

The thing about functions, is that those variable only exist in the function, and also the name of the parameters doesn't matter, only the order. I understand that's frustrating, but if you carry on programming with a more open mind about it, I guarantee you'll appreciate it. If you want to keep those values, you just need to return them at the end. You can return multiple variables by writing return c, a, b and writing the call like this sum, number1, number2 = addition(1,2)

Another problem is that I could call my addition function like this:

b = 1
a = 2
addition(b,a)

and now inside the function, a = 1 and b = 2, because it's not about the variable names, it's about the order I passed them to the function in.

You also don't need to pass TAX into show_tax because TAX is already a global variable. It was defined outside a function so it can be used anywhere. Additionally, you don't want to pass tax to show_tax, you want to pass value to it. But because show_value hasn't returned value, you've lost it. So return value in show value to a variable like so value = show_value(price) .