Variables — naming rules, assignment, reassignment

HOW assignment works

Why right-to-left? Because the value must exist before a name can point to it. You can't label nothing.

Worked example Basic assignment — step by step

price = 50 + 30

Why this step? Right side 50 + 30 is evaluated first → object 80 is created.
Then the name price is bound to 80.
Result: typing price now gives 80.

HOW reassignment works

Worked example Reassignment — the sticky note moves

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

Why x = x + 5 is legal: the right side is evaluated using the current value of x (10), producing 15, and then x is rebound. There's no contradiction because = is an action, not a math equation.

Worked example Two names, one object

a = 100
b = a       # b points to the SAME object as a
a = 200     # a now points to a NEW object; b is UNAFFECTED
print(a, b) # 200 100

Why this step? b = a copies the reference, not the value. Reassigning a only moves a's sticky note. b still labels 100.

Figure — Variables — naming rules, assignment, reassignment

Naming RULES (must obey — else `SyntaxError`)

Worked example Valid vs invalid

user_name = 1     # valid
_temp     = 1     # valid (underscore start OK)
age2      = 1     # valid (digit not first)
# 2age    = 1     # INVALID: starts with digit
# user-name = 1   # INVALID: '-' read as subtraction
# class   = 1     # INVALID: 'class' is a keyword

Why 2age fails: Python's parser sees 2 and expects a number, then hits letters → confusion → SyntaxError.

Flashcards

What does the = operator do in Python?

Evaluates the right-hand expression to an object, then binds the left-hand name to that object (assignment, not equality).

In what order is name = expression processed?

Right-to-left: the expression is evaluated first, then the name is bound.

Why is x = x + 5 valid in Python?

= is assignment; the old value of x is used to compute the result, then x is rebound to the new object.

After a=100; b=a; a=200, what is b?

100 — b still labels the original object; reassigning a only moved a's reference.

Can a variable name start with a digit?

No. It must start with a letter or underscore.

Is Age the same variable as age?

No — Python identifiers are case-sensitive.

Name three things forbidden in a variable name.

Starting with a digit, spaces/symbols like - @, and reserved keywords.

What error comes from using a name before it's assigned?

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

What does reassignment do to the OLD object?

Nothing — the name just stops pointing at it; if unreferenced it's garbage-collected.

What's the convention for naming a constant?

ALL_CAPS, e.g. MAX_SPEED = 120.

Recall Feynman: explain to a 12-year-old

Imagine names are sticky labels and values are toys. Saying score = 10 makes a toy "10" and sticks the label score on it. Saying score = 20 does NOT change the old toy — it peels the label off and sticks it on a new toy "20". If your friend put a label copy on the first toy, their label is still on toy "10" even after you moved yours. The = sign isn't saying "these are equal" — it's saying "go stick this label here."

Connections

1.2.01-Data-Types — the objects that variables point to.
1.2.02-Operators — = (assign) vs == (compare).
1.2.04-Mutable-vs-Immutable — why b=a is safe for ints but tricky for lists.
1.2.05-Namespaces-and-Scope — where names live.
1.1.00-Memory-and-References — heap objects and the garbage collector.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, Python me ek variable koi dabba (box) nahi hai jisme value rakhi ho — wo ek naam ka label hai jo memory me pade ek object ko point karta hai. Jab tum likhte ho x = 10, to pehle right side ka object 10 banta hai, phir naam x us object par chipak jaata hai. Isiliye = ko "barabar" mat padho — ise padho "right side run karo, phir uska label name ko de do". Yahi reason hai ki x = x + 5 bilkul valid hai: pehle purana x (10) use karke 15 banta hai, phir x ko naya label mil jaata hai.

Reassignment ka matlab — naam ka sticky note utha kar dusre object par chipka dena. Purana object change nahi hota; agar uspe koi aur label nahi bacha to garbage collector use saaf kar deta hai. Aur ek important baat: b = a value ki copy nahi, reference ki copy karta hai. To agar baad me a = 200 karo, to sirf a ka label hila, b abhi bhi purane 100 ko point kar raha hota hai.

Naming rules simple hain: sirf letters, digits aur underscore; pehla character digit nahi ho sakta; spaces ya -, @ jaise symbols nahi; aur keywords (if, class, for) use nahi kar sakte. Case matter karta hai — age aur Age alag hain. Convention me snake_case aur descriptive naam use karo, kyunki 80% bugs galat ya confusing naam se aate hmain. Yeh chhoti cheez lagti hai par poore programming ka foundation hai.

Variables — naming rules, assignment, reassignment

WHAT is a variable?

HOW assignment works

HOW reassignment works

Naming RULES (must obey — else `SyntaxError`)

Steel-man: common mistakes

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Test yourself — Introduction to Programming (Python)

Connections