4.13 Scope Rules

• Each identifier has a scope that determines where you can use it in your program.
• Complete scope details.

Local Scope

• A local variable’s identifier has local scope.

Global Scope

• Identifiers defined outside any function (or class) have global scope—these may include functions, variables and classes.

Accessing a Global Variable from a Function

x = 7

def access_global():
    print('x printed from access_global:', x)

access_global()

x printed from access_global: 7

• By default, you cannot modify a global variable in a function
• Python creates a new local variable when you first assign a value to a variable in a function’s block.
• In function try_to_modify_global’s block, the local x shadows the global x, making it inaccessible in the scope of the function’s block.

def try_to_modify_global():
    x = 3.5
    print('x printed from try_to_modify_global:', x)

try_to_modify_global()

x printed from try_to_modify_global: 3.5

x

7

• To modify a global variable in a function’s block, you must use a global statement to declare that the variable is defined in the global scope:

def modify_global():
    global x;
    x = 'hello'
    print('x printed from modify_global:', x)
    

modify_global()

x printed from modify_global: hello

x

'hello'

Blocks vs. Suites

• When you create a variable in a block, it’s local to that block.
• When you create a variable in a control statement’s suite, the variable’s scope depends on where the control statement is defined:
  • If it's in the global scope, any variables defined in the control statement have global scope.
  • If it's in a function’s block, any variables defined in the control statement have local scope.

Shadowing Functions

• In the preceding chapters, when summing values, we stored the sum in a variable named total.
• If you define a variable named sum, it shadows the built-in function sum, making it inaccessible in your code.

sum = 10 + 5

sum

15

sum([10, 5])

------------------------------------------------------------------------
TypeError                              Traceback (most recent call last)
<ipython-input-12-1237d97a65fb> in <module>
----> 1 sum([10, 5])

TypeError: 'int' object is not callable

Statements at Global Scope

• Script statements at global scope execute as soon as they’re encountered by the interpreter, whereas statements in a block execute only when the function is called.

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©1992–2020 by Pearson Education, Inc. All Rights Reserved. This content is based on Chapter 4 of the book Intro to Python for Computer Science and Data Science: Learning to Program with AI, Big Data and the Cloud.

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