Default parameters, keyword arguments — Hinglish notes

1.2.28 · Coding › Introduction to Programming (Python)

1. Vocabulary (WHAT)

def greet(name, greeting="Hello"):   # greeting has a default
    return greeting + ", " + name
 
greet("Asha")                # 'Hello, Asha'   -> default used
greet("Asha", "Namaste")     # 'Namaste, Asha' -> positional override
greet("Asha", greeting="Hi") # 'Hi, Asha'      -> keyword override

2. Defaults KYUN exist karte hain (need derive karo)

Maano aap ek "connect to database" function likhte ho. Almost hamesha port 5432 hota hai. Defaults ke bina aap likhoge:

connect("localhost", 5432, "utf8", 30)   # everyone repeats the same junk

Har caller wahi 5432, "utf8", 30 copy karta hai. Repetition = bugs jab ek copy change ho. Toh hum common value ko definition mein bake kar dete hain:

def connect(host, port=5432, encoding="utf8", timeout=30):
    ...
connect("localhost")                 # the easy, common case
connect("localhost", timeout=60)     # change ONLY what's unusual

3. Keyword arguments KYUN exist karte hain (HOW they help)

Kai parameters ke saath, position fragile hoti hai. box(2, 5, 0, 1) padh ke — kaun sa width hai? height? Koi nahi jaanta. Keyword arguments calls ko self-documenting aur order-independent banate hain:

def box(width, height, x=0, y=0):
    ...
box(width=2, height=5)      # crystal clear
box(height=5, width=2)      # order doesn't matter with keywords
box(2, 5, y=1)              # mix: positional then keyword

Figure — Default parameters, keyword arguments

4. Ordering RULES (aur KYUN)

Rule 1 KYUN: Python parameters ko left-to-right position se fill karta hai. Agar pehle default aata, toh usse diya gaya positional argument ambiguous ho jaata — kya woh default wale ko fill karta hai ya skip karta hai? Ise forbid karne se ambiguity hat jaati hai.

Rule 2 KYUN: Jab aap d=9 bol dete ho, Python ko aage ka bare 2 kisi slot mein daalna nahi pata. Positionals pehle resolve hone chahiye.

5. Famous mutable-default trap

Common mistake

def f(x, lst=[]) almost hamesha ek bug hai Kyun sahi lagta hai: "Har call jisme lst na ho, woh ek fresh empty list se start karni chahiye." Obvious lagta hai. Asal mein kya hota hai: Default value ek baar create hoti hai, jab def line run hoti hai — har call par NAHI. Toh saari calls usi list object ko share karti hain.

def add(item, bag=[]):
    bag.append(item)
    return bag
add(1)   # [1]
add(2)   # [1, 2]   <-- surprise! same list reused

Fix: None ko sentinel ki tarah use karo aur list andar banao:

def add(item, bag=None):
    if bag is None:
        bag = []        # fresh list every call
    bag.append(item)
    return bag

6. Worked examples

Worked example Example A — sirf middle parameter override karo

def power(base, exp=2, mod=None):
    r = base ** exp
    return r if mod is None else r % mod
power(5)              # 25
power(5, mod=7)       # 25 % 7 = 4

power(5) = 25 KYUN? exp default 2 hai, mod default None hai → 5**2 = 25, koi modulo nahi.
power(5, mod=7) bina exp diye KYUN kaam karta hai? Keyword mod=7 exp ko skip karta hai, jo apna default 2 rakhta hai. Aap yeh positionally nahi kar sakte — yahi reason hai ki keywords matter karte hain.

Worked example Example B — label banana

def label(text, color="black", bold=False):
    s = text
    if bold: s = "**" + s + "**"
    return f"[{color}] {s}"
 
label("Hi")                       # [black] Hi
label("Hi", bold=True)            # [black] **Hi**
label("Hi", "red", True)          # [red] **Hi**

Yeh step (label("Hi", bold=True)) KYUN? Bold chahiye par normal color; keyword color ko skip karne deta hai aur uska default rakhta hai.

Worked example Example C — trap pakda aur fix kiya

def collect(x, seen=None):
    seen = [] if seen is None else seen
    seen.append(x)
    return seen
print(collect(1))   # [1]
print(collect(2))   # [2]  -> correct, independent

is None kyun, == [] kyun nahi? Ek caller legitimately apni khud ki empty list pass kar sakta hai; is None check karta hai "kya kuch pass nahi kiya gaya", na ki "kya yeh empty hai".

Recall Feynman: 12-saal ke bachche ko explain karo

Socho pizza order karna. Shop already medium size aur cheese assume karti hai jab tak tum kuch aur na kaho — yeh defaults hain. Size tabhi bolte ho jab large chahiye. Aur toppings ek fixed secret order mein chillaane ki bajay, tum bas bolte ho "topping = mushroom, size = large" — yeh keyword arguments hain, naam lete ho taaki koi confuse na ho. Ek problem: cook ko kabhi ek shared topping bowl sabke liye mat do, warna agli customer ko tumhara bachaa hua milega — yahi mutable-default bug hai!

7. Active recall

Parameter aur argument mein kya fark hai?

Parameter = definition mein naam; argument = call mein pass ki gayi actual value.

Default parameter kya hota hai?

Woh parameter jisme def mein ek fallback value ho (def f(x=10)); agar caller omit kare toh use hota hai.

Function definition mein default parameters kahan jaane chahiye?

Saare non-default parameters ke baad (defaults drift right).

Kya function call mein keyword argument positional se pehle aa sakta hai?

Nahi — saare positional args pehle aane chahiye kisi bhi keyword arg se.

Default value kab evaluate hoti hai — definition par ya har call par?

Ek baar, definition time par (isse mutable-default trap hota hai).

def f(x, lst=[]) dangerous KYUN hai?

Same list saari calls mein share hoti hai kyunki default def time par ek baar create hota hai.

Mutable-default trap ko kaise fix karte hain?

None ko default use karo, phir andar fresh object banao: if lst is None: lst = [].

greet("Asha", name="Asha") se kaun sa error aata hai?

TypeError: got multiple values for argument 'name'.

Kai parameters ke saath keyword arguments KYUN use karte hain?

Yeh self-documenting aur order-independent hote hain, fragile positional mistakes se bachate hain.

2nd parameter ko uske default par rakhte hue 3rd parameter kaise set karoge?

3rd ko keyword se pass karo, jaise f(a, third=9).

Connections

Functions - def and return
Variable scope - local vs global
*args and **kwargs (variadic functions)
Mutable vs immutable objects
DRY principle - Don't Repeat Yourself