Variable Visibility and Scope in Python Functions, Study notes of Computer Science

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Today ...

• Functions (cont.)

Homework

• HW 2 due
• HW 3 out

Function Variable Visibility

Variables defined inside a function are local to the function

• i.e., they only exist inside the function

For example

def add_and_print(x, y): sum = x + y print(sum)

Q: What does this function do?

arg1 = 3 arg2 = 4 add_and_print(arg1, arg2) 7

When a function terminates, local variables are destroyed

• If we try to print them, we’ll get an error ...

print(sum) NameError: name ’sum’ is not defined

The same is true for function parameters!

• e.g., trying to print x gives the same error

What about now?

sum = 10 x = 5 y = 4 z = 1 def add_and_print(x, y): sum = x + y + z print(sum) add_and_print(3, 4) print(sum, x, y, z, sep=", ")

Q: What is printed by the add and print function call?

8

Q: What is printed by the last statement?

10, 5, 4, 1

Equivalent to:

sum = 10 x = 5 y = 4 z = 1 def add_and_print(x0, y0): sum = x0 + y0 + z print(sum) add_and_print(3, 4) print(sum, x, y, z, sep=", ")

• here z is a “global variable” ... it is defined outside of the function
• sum, x, and y are also global variables

What about this one?

sum = 10 x = 5 y = 4 z = 1 def add_and_print(x, y): sum = x + y + z print(sum) z = 2 add_and_print(3, 4) print(sum, x, y, z, sep=", ")

• This confuses Python!

UnboundLocalError: local variable ’z’ referenced before assignment

• Python thinks z is local (because of the assignment) ...
• But then is used before it is assigned a value