How does this variable get into the function? (Python)

Question

One of my students brought this to my attention in his coursework. I've re-written what happened below (but is not the same as the coursework):

def tester(number):

    print(number)    
    numbertotal = num1 + num2
    print(numbertotal)

number = input("Input a number: ")
num1 = 2
num2 = 2
tester("number")

Output:

Input a number: 12

number

4

How does the num1 and num2 get up into the function? All I do is pass it the string "number" and it prints that. It does then correctly add 2 and 2, which doesn't make any sense since I don't pass that function either number. I do pass the "number" variable so it does work to pass to but is also just so happens that num1 and num2 are available up there anyway. Does Python now make all variables global now? Even so it shouldn't work... Please help!

Possible duplicate of [Short Description of the Scoping Rules?](https://stackoverflow.com/questions/291978/short-description-of-the-scoping-rules) — jpp, Jul 12 '18 at 09:20

score 2 · Answer 1 · answered Jul 12 '18 at 09:17

2

Python resolve variable name at runtime, so when your tester function is called, num1 and num2 have already been defined.

answered Jul 12 '18 at 09:17

Olivier Cazade

777
5
10

Thank you for your reply. I see what you mean. Why don't I have to pass it up there though? Shouldn't I have to say "tester(num1,num2)"? – Mr Arch Jul 12 '18 at 09:19

Daniel B · Answer 2 · 2018-07-12T09:27:12.507

0

The variables are global, so it makes no difference to the function, any part of the program can 'see' them.

I guess this showcases my point just right.

def func(GLOBAL):
    print (GLOBAL)

GLOBAL = 64

func(32)      # <-- localvar naming overrides global
func(GLOBAL)  # <-- this one does what you'd expect

edited Jul 12 '18 at 09:27

answered Jul 12 '18 at 09:20

Daniel B

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4
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Replying you @mr-archard above, because I don't have enough rep. There'd be a couple of problems with calling `tester(n1,n2)`: firstly, it expects only one variable. If, however, you fix the function to `def tester(n1, n2): numbertotal = n1 + n2`, then it does the expected thing. – Daniel B Jul 12 '18 at 09:23
If at the bottom you put "print(numbertotal)" it says that numbertotal is not defined as it is in the function. That variable isn't global so how can num1 and num2 be? Or is that because numbertotal is declared in the function and is therefore local? – Mr Arch Jul 12 '18 at 09:26
Yes of course. Good point. As you say then it would do the expected thing. If all variables are available globally then why does the option to pass a function a variable exist? Or is that for when you send variables from function to function? – Mr Arch Jul 12 '18 at 09:28
That's because `numbertotal` is only visible inside the function, and therefore local. Correct. – Daniel B Jul 12 '18 at 09:29
And, referring to your second comment, declaring everything as global is a waste of computing resources since it never gets dealocatted from memory. Not having functions being able to be passed or return variables would turn modern-day programming into line-by-line instructions and lots of copy-pasting. – Daniel B Jul 12 '18 at 09:31
Yeah true, that's a good point. But even in your code above it does what I'd expect. – Mr Arch Jul 12 '18 at 09:33

score 0 · Answer 3 · answered Jul 12 '18 at 09:22

0

Looking at the output of locals() and globals() will clarify this.

answered Jul 12 '18 at 09:22

Abhishek Jha

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How does this variable get into the function? (Python)

3 Answers3