[Python] Why are variables not available to a sub-function on run? [duplicate]

This script defines a variable inside main(), but the variable isn't available to func(), which runs inside main(). Why is that?

#!/usr/bin/env python3
# vars_in_func.py
# Test script for variables within a function.

def func():
print(greeting)

def main():
greeting = "Hello world"
func()

main()


Error:

Traceback (most recent call last):
File "./vars_in_func.py", line 11, in <module>
main()
File "./vars_in_func.py", line 9, in main
func()
File "./vars_in_func.py", line 5, in func
print(greeting)
NameError: name 'greeting' is not defined


If I convert the script to Python2, the error is the same, except it says global name instead of name.

I assume I'm just missing a key concept. I just started learning Python after learning Bash.

Edit: After reading the answers, I realized my mistake: I'm still thinking in terms of Bash, where functions either run in the same shell as the caller (with the same variables), or a subshell of the caller (inherited variables).

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