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Lexical Scoping == Tyranny

5 2020-10-30 21:38

Python nonlocal allows us to explicitly choose scope behavior in nested function definitions.

def outer():
    x = 1
    def inner():
        x = 2
        print(x)
    inner()
    print(x)

def outer_nl():
    x = 1
    def inner():
        nonlocal x
        x = 2
        print(x)
    inner()
    print(x)

outer()    # 2 1
outer_nl() # 2 2

We can achieve the same behavior without nonlocal.

def outer():
    context = {}
    context['x'] = 1
    def inner():
        context['x'] = 2
        print(context['x'])
    inner()
    print(context['x'])

Basic lexical vs. dynamic scoping...

MIT Scheme(lexical):

(define (definer)
  (define x 1))

(define (printer)
  (display x))

(define (letter)
  (let ((x 2))
    (printer)))

(definer)
(printer) ; error undefined variable
(letter)  ; error undefined variable

Common Lisp(dynamic):

(defun definer ()
  (defvar x 1))

(defun printer ()
  (print x))

(defun letter ()
  (let ((x 2))
    (printer)))

(definer)
(printer) ; 1
(letter)  ; 2

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