[Python] Variable scopes

Rex Chiang

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Apr 30, 2023

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• The Python scope concept is generally presented using a rule known as the LEGB rule.
• LEGB stand for Local, Enclosing, Global, and Built-in scopes.

Local Scope

def func():
  s = "Hello"
  print(s)

if __name__ == "__main__":
  func() # Hello

def func():
  s = "Hello"
  print(s)

if __name__ == "__main__":
  print(s) # NameError : name "s" is not defined

• Local scope refers to the names which are defined inside the function, and become local variables in this function.
• These names will only be visible from the block of the function.

Enclosing Scope (Nonlocal Scope)

def outer():
  s = "Hello from outer"
  
  def inner():
    print(s)
  
  inner() 
  print(s)

if __name__ == "__main__":
  outer() # Hello from outter
          # Hello from outter

def outer():
  s = "Hello from outer"
  
  def inner():
    s = "Hello from inner"
    print(s)
  
  inner() 
  print(s)

if __name__ == "__main__":
  outer() # Hello from inner
          # Hello from outter

def outer():
  s = "Hello from outer"
  
  def inner():
    nonlocal s
    s = "Hello from inner"
    print(s)
  
  inner() 
  print(s)

if __name__ == "__main__":
  outer() # Hello from inner
          # Hello from inner

• Enclosing scope refers to the names which are defined in the nested function, and become enclosing variables in inner and enclosing functions.
• These names are visible from the code of the inner and enclosing functions.
• Use nonlocal statement to ask Python to use the enclosing variables instead of creating a local variables, and modify variables defined in the enclosing.

Global Scope

s = "Hello from global"

def func():
  print(s)

if __name__ == "__main__":
  func() # Hello from global

s = "Hello from global"

def func():
  s = "Hello from local"
  print(s)

if __name__ == "__main__":
  print(s) # Hello from global
  func()   # Hello from local
  print(s) # Hello from global

s = "Hello from global"

def func():
  global s
  s = "Hello from local"
  print(s)

if __name__ == "__main__":
  print(s) # Hello from global
  func()   # Hello from local
  print(s) # Hello from local

• Global scope refers to the names which are defined in the main body of a program, and become global variables in all functions.
• These names are visible and accessed throughout the program.
• Use global statement to ask Python to use the globally defined variables instead of creating a local variables, and modify variables defined in the global.

Built-in Scope

def func():
  print(True)

if __name__ == "__main__":
  func() # True

• Built-in scopes are the widest scopes that cover all reserved keywords.
• These names are visible anywhere in the program, without the need to define them.