Variables — naming rules, assignment, reassignment

1.2.3 · Coding › Introduction to Programming (Python)

Assignment KAISE kaam karta hai

Right-to-left kyun? Kyunki value ka pehle exist karna zaroori hai, tabhi koi naam use point kar sakta hai. Kuch nahi pe label nahi lagate.

Worked example Basic assignment — step by step

price = 50 + 30

Yeh step kyun? Right side 50 + 30 pehle evaluate hota hai → object 80 banta hai.
Phir price naam ko 80 se bind kiya jaata hai.
Result: ab price type karne pe 80 milega.

Reassignment KAISE kaam karta hai

Worked example Reassignment — sticky note move hoti hai

x = 10      # x → object 10
x = x + 5   # evaluate x+5 = 15 → naya object; rebind x → 15
print(x)    # 15

x = x + 5 legal kyun hai: right side x ki current value (10) use karke evaluate hoti hai, 15 produce hota hai, aur tab x rebind hota hai. Koi contradiction nahi kyunki = ek action hai, math equation nahi.

Worked example Do naam, ek object

a = 100
b = a       # b usi object ko point karta hai jise a karta hai
a = 200     # a ab ek NAYE object ko point karta hai; b UNAFFECTED hai
print(a, b) # 200 100

Yeh step kyun? b = a reference copy karta hai, value nahi. a ko reassign karna sirf a ki sticky note ko move karta hai. b abhi bhi 100 ko label karta hai.

Figure — Variables — naming rules, assignment, reassignment

Naming RULES (maanne zaroori hain — warna `SyntaxError`)

Worked example Valid vs invalid

user_name = 1     # valid
_temp     = 1     # valid (underscore start OK)
age2      = 1     # valid (digit pehla nahi)
# 2age    = 1     # INVALID: digit se shuru
# user-name = 1   # INVALID: '-' subtraction ki tarah padha jaata hai
# class   = 1     # INVALID: 'class' ek keyword hai

2age kyun fail hota hai: Python ka parser 2 dekhta hai aur number expect karta hai, phir letters aate hain → confusion → SyntaxError.

Flashcards

Python mein = operator kya karta hai?

Right-hand expression ko ek object mein evaluate karta hai, phir left-hand naam ko us object se bind karta hai (assignment, equality nahi).

name = expression kis order mein process hota hai?

Right-to-left: pehle expression evaluate hota hai, phir naam bind hota hai.

Python mein x = x + 5 valid kyun hai?

= assignment hai; x ki purani value result compute karne mein use hoti hai, phir x naye object se rebind hota hai.

a=100; b=a; a=200 ke baad b kya hai?

100 — b abhi bhi original object ko label karta hai; a ko reassign karne se sirf a ka reference move hua.

Kya variable naam digit se shuru ho sakta hai?

Nahi. Yeh letter ya underscore se shuru hona chahiye.

Kya Age aur age same variable hain?

Nahi — Python identifiers case-sensitive hote hain.

Variable naam mein teen forbidden cheezein batao.

Digit se shuru karna, spaces/symbols jaise - @, aur reserved keywords.

Naam ko assign karne se pehle use karne par kya error aata hai?

NameError: name '...' is not defined.

Reassignment PURANE object ke saath kya karta hai?

Kuch nahi — naam bas use point karna band kar deta hai; agar unreferenced ho toh garbage-collect ho jaata hai.

Constant naam rakhne ki convention kya hai?

ALL_CAPS, jaise MAX_SPEED = 120.

Recall Feynman: 12-saal ke bachche ko samjhao

Socho naam sticky labels hain aur values toys hain. score = 10 bolne se ek toy "10" banta hai aur label score use chipka deta hai. score = 20 bolne se purana toy nahi badlata — label ukhad ke ek naye toy "20" pe chipak jaata hai. Agar tumhare dost ne pehle toy pe ek label copy chipkaya tha, toh unka label abhi bhi toy "10" pe hai chahe tumne apna move kar liya ho. = sign yeh nahi kehta "yeh barabar hain" — yeh kehta hai "jao yeh label yahaan chipkao."

Connections

1.2.01-Data-Types — woh objects jinhe variables point karte hain.
1.2.02-Operators — = (assign) vs == (compare).
1.2.04-Mutable-vs-Immutable — kyun b=a ints ke liye safe hai lekin lists ke liye tricky.
1.2.05-Namespaces-and-Scope — naam kahaan rehte hain.
1.1.00-Memory-and-References — heap objects aur garbage collector.

Variables — naming rules, assignment, reassignment

Variable KYA hota hai?