global and nonlocal keywords — Hinglish notes

1.2.31 · Coding › Introduction to Programming (Python)

WHY do these keywords exist?

Ye rule HI poora reason hai kyun global/nonlocal exist karte hain. Aao dekhte hain kyun ye tumhe takleef deta hai.

Worked example "Ye crash kyun ho raha hai?" — classic example

count = 0
def bump():
    count = count + 1   # ERROR
bump()

Ye step kyun? Kyunki bump count ko assign karta hai, Python count ko poore function ke liye local mark kar deta hai. Toh right-hand side pe count + 1 mein, local count ki abhi koi value nahi → UnboundLocalError. Error pagal lagta hai ("count toh upar defined hai!") lekin Python ne locality compile time pe decide kar li thi, ek bhi line run hone se pehle. Fix: Python ko batao ki tumhara matlab bahar wala hai → global count.

The scope ladder (LEGB)

Reading ke liye naam lookup jaata hai: Local → Enclosing → Global → Built-in (LEGB).

global = "seedha G level pe jaao aur wahan bind karo."
nonlocal = "sabse kareeb wale E (enclosing function) level pe bind karo — na local, na global."

global — WHAT, WHY, HOW

Worked example Counter jo sach mein count karta hai

count = 0
def bump():
    global count
    count = count + 1
bump(); bump()
print(count)   # 2

Ye step kyun? global count "ise local banao" wala decision hata deta hai, toh dono sides pe count module variable hai. Har call global ko rebind karta hai.

nonlocal — WHAT, WHY, HOW

Worked example Ek closure jo yaad rakhta hai

def make_counter():
    n = 0
    def step():
        nonlocal n
        n += 1
        return n
    return step
 
c = make_counter()
print(c(), c(), c())   # 1 2 3

Ye step kyun? n make_counter (enclosing scope) mein rehta hai. step ko n ko rebind karna hai, toh bina nonlocal ke ye ek naya local n bana deta aur n += 1 pe crash ho jaata. nonlocal n assignment ko enclosing n ki taraf point karta hai, jo calls ke beech survive karta hai kyunki closure ise alive rakhta hai.

Forecast-then-Verify

Recall Answer padhne se PEHLE output predict karo

x = "global"
def outer():
    x = "enclosing"
    def inner():
        nonlocal x
        x = "changed"
    inner()
    print(x)
outer()
print(x)

Answer: pehle changed phir global print hoga. Kyun? nonlocal x inner ki assignment ko outer ke x se bind karta hai, ise "changed" kar deta hai. Module-level x untouched rehta hai, toh wo "global" hi rehta hai.

Flashcards

Function ke andar assign kiye gaye naam ka default scope kya hota hai?

Us function ka local, poore function body ke liye (compile time pe decide hota hai).

Usi function mein assign karne se pehle naam read karne pe kaun sa error aata hai?

UnboundLocalError.

global x kya karta hai?

x ko module-level variable ko refer karne (aur rebind karne) deta hai, na ki local ko.

nonlocal x kya karta hai?

x ko nearest enclosing function scope (global nahi) se bind karta hai aur use rebind karne deta hai.

Kya outer variable sirf READ karne ke liye global/nonlocal chahiye?

Nahi — reading LEGB automatically climb karta hai; inhe sirf rebind/assign karne ke liye chahiye.

Kya global list pe mylist.append(5) karne ke liye global chahiye?

Nahi — ye object ko mutate karta hai, naam ko rebind nahi karta.

LEGB ka full form kya hai?

Local, Enclosing, Global, Built-in (naam lookup ka order).

Jab nonlocal variable nahi milta, toh kya hota hai?

SyntaxError — enclosing function scope mein pehle se ek binding honi chahiye.

global aur nonlocal targets mein kya fark hai?

global → module top level; nonlocal → nearest enclosing function.

Recall Feynman: 12-saal ke bachche ko samjhao

Ek ghar imagine karo jisme kamre ke andar kamre hain. Har kamre ka apna toy box ho sakta hai. Normally, agar tum kisi kamre mein khade hokar ek box mein toy rakhte ho, tum us kamre mein naya box banate ho. global kehta hai: "Naya box mat banao — front hall mein wala BADA box use karo, jo sabka shared hai." nonlocal kehta hai: "Is kamre ke thoda bahar wale kamre ka box use karo, front hall wala nahi." Agar tum sirf bahar ke box se toy dekhna chahte ho, kuch bolne ki zaroorat nahi — tum hamesha bahar jhank sakte ho.

Connections

Scope and LEGB rule
Functions in Python
Closures
Mutable vs Immutable objects
UnboundLocalError
Pure functions and side effects