Functions — def, parameters, return, docstrings

1.2.27 · Coding › Introduction to Programming (Python)

WHY do functions exist?

Teen kaam jo ek function karta hai:

Abstraction — kaise hota hai usse ek naam ke peeche chhupao (area(r) instead of 3.14159*r*r).
Reuse — kahin bhi, kitni bhi baar call karo.
Isolation — iske andar ke variables apne scope mein rehte hain, toh woh baaki program ko pollute nahi karte.

WHAT is the anatomy? (Derivation from scratch)

Chalte hain ek function piece by piece banate hain, har step pe "yeh part kyun?" poochte hue.

Definition Definition syntax

def name(parameters):
    """docstring"""
    body
    return value

def ::: keyword jo Python ko batata hai "yahan se ek function definition shuru hoti hai".
name — woh identifier jise tum baad mein call karoge.
parameters — inputs ke liye placeholder variable names.
return — caller ko ek value wapas bhejta hai aur function khatam karta hai.

Kyun? Python ko ek marker chahiye taaki woh samjhe "is block ko store karo, abhi run mat karo." def ek function object create karta hai aur usse name se bind karta hai. Define karna ≠ run karna.

def greet():
    print("Hi")

Abhi kuch print nahi hoga. Sirf greet() (parentheses ke saath) usse call karta hai.

Step 2 — parameters `(x, y)`

Kyun? Bina inputs ke ek machine hamesha same kaam karti hai. Parameters ek definition ko infinitely many inputs ke saath kaam karne dete hain.

def add(a, b):     # a, b PARAMETERS hain (placeholders)
    return a + b
 
add(2, 3)          # 2 aur 3 ARGUMENTS hain (actual values)

Step 3 — `return`

Kyun? Bina return ke, kaam hota hai lekin jawab shoonya mein ghum jaata hai. return result wapas bhejta hai taaki caller usse use kar sake.

def square(n):
    return n * n
 
y = square(5)      # y ab 25 hai

Step 4 — docstring

Kyun? Chhe mahine baad tum (ya koi teammate) poochte ho "yeh kya karta hai?" Docstring woh documentation hai jo function ke andar rehti hai aur help() se padhi ja sakti hai.

def area(r):
    """Return the area of a circle of radius r."""
    return 3.141592653589793 * r * r

help(area) woh line print karta hai. Yeh def ke neeche pehla string literal hai.

Figure — Functions — def, parameters, return, docstrings

HOW does a call actually work? (Execution trace)

Worked example Tracing

add(2, 3)

def add(a, b):
    return a + b
result = add(2, 3)

Python add(2, 3) dekhta hai → add naami function object dhundhta hai. Kyun? Usse woh machine milni chahiye.
a = 2, b = 3 bind karta hai (arguments → parameters). Kyun? Slots fill hote hain.
Body run karta hai: 2 + 3 = 5 compute karta hai. Kyun? Yahi toh kaam hai.
return 5 → call expression 5 ban jaata hai aur function exit karta hai. Kyun? return value bhi bhejta hai aur rokta bhi hai.
result = 5. Kyun? Assignment capture karta hai ki call kis value pe evaluate hua.

Parameter flavours

Worked example Default + keyword arguments

def power(base, exp=2):
    """Return base raised to exp (squares by default)."""
    return base ** exp
 
power(5)          # 25  -> exp defaults to 2.   Kyun? exp omit hua.
power(2, 3)       # 8   -> exp=3 by position.   Kyun? 3 exp slot fill karta hai.
power(exp=3, base=2)  # 8 -> by keyword.        Kyun? names order override karte hain.

Common mistake Mutable default argument trap

Galat feeling: def f(items=[]) har call pe ek nayi empty list banata hai. Yeh sahi lagta hai kyunki [] "naya dikhta hai." Fix: Default ek baar definition time pe banaya jaata hai aur calls ke beech share hota hai — yeh yaad rakhta hai! None sentinel use karo:

def f(items=None):
    if items is None:
        items = []

`return` details that bite

Worked example Returning two things

def divmod2(a, b):
    """Return (quotient, remainder)."""
    return a // b, a % b
 
q, r = divmod2(17, 5)   # q=3, r=2

Yeh step kyun? a // b integer division hai, a % b remainder hai; dono ko tuple mein pack karne se caller ek hi call mein dono le sakta hai.

Flashcards

Python mein function definition kaunse keyword se shuru hoti hai?

def

Parameter aur argument mein kya fark hai?

Parameter woh placeholder naam hai jo def line mein hota hai; argument woh actual value hai jo call karte waqt pass ki jaati hai.

Agar function mein koi return statement nahi hai toh kya return hoga?

None

print aur return mein kya fark hai?

print screen pe text display karta hai; return caller ko value wapas bhejta hai aage use ke liye.

Docstring kya hoti hai aur kaise likhte hain?

Ek string literal jo function body ke andar pehli line hoti hai, jo document karti hai function kya karta hai; help() se padhi ja sakti hai.

return a, b kya return karta hai?

Ek tuple (a, b).

def f(items=[]) kyun dangerous hai?

Default list ek baar banayi jaati hai aur calls ke beech share hoti hai, toh calls ke beech state accumulate karti rehti hai.

Parameter ko default value kaise dete hain?

def line mein assign karo, jaise def power(base, exp=2):.

return ke baad same path pe likhe code ka kya hota hai?

Woh kabhi execute nahi hota; return function se turant exit karta hai.

Function define karna vs call karna?

def name(): ... sirf function object banata hai; name() parentheses ke saath actually run karta hai.

Recall Feynman: explain to a 12-year-old

Ek function ek vending machine jaisi hoti hai. Tum usse ek baar banate ho aur label lagate ho (def). Tum slot mein coins daalo (the arguments), machine apna hidden kaam kare, aur ek snack bahar aaye (return). Slot ke naam parameters hain. Agar return bhool gaye, toh machine tumhare coins le leti hai aur kuch nahi deti (None). Machine pe woh chhota sticker jo kehta hai "yeh chips deta hai" woh docstring hai. Machine ek baar banao, hazaar baar use karo.

Connections

Variables and Scope — parameters aur locals function ke apne scope mein rehte hain.
Control Flow — if statements — function bodies ke andar branching returns ke liye use hota hai.
Lists and Tuples — multiple values return karne mein tuple packing/unpacking use hoti hai.
Recursion — ek function jo khud ko call karta hai, seedha def/return par bana.
Lambda and Higher-order Functions — anonymous one-line functions.
DRY Principle — functions ka design motivation.