f-strings — embedding expressions — Hinglish notes

1.2.13 · Coding › Introduction to Programming (Python)

f-strings KYUN exist karte hain?

F-strings se pehle, variables se string banana kaafi clumsy tha:

name = "Ada"
age = 36
# Old way 1: concatenation — must manually convert ints to str
msg = "Hi " + name + ", you are " + str(age)
# Old way 2: .format() — placeholders separated from values
msg = "Hi {}, you are {}".format(name, age)

Dono error-prone hain: aap str() bhool jaate ho, {} aur arguments ka order galat ho jaata hai, aur values waahan se door padhni padti hain jahan use ho rahi hain.

f-string KYA hota hai?

f"text {expression} more text"

f prefix magic on karta hai.
Braces ke bahar sab kuch literal text hai.
Braces ke andar sab kuch code hai jo run hota hai.

Evaluation KAISE kaam karta hai? (first principles se samjho)

Socho interpreter string ko ek baar, left se right scan kar raha hai:

Literal characters seedhe output mein copy karo.
{ milne par, matching } tak padhte jao. Woh chunk current scope mein ek Python expression ki tarah parse aur execute hota hai.
Result text mein convert hota hai (default: str(...) jaisa) aur append hota hai.
String khatam hone tak yahi chalta raho.

Toh braces mein koi bhi expression ho sakta hai — variables, arithmetic, function calls, method calls, indexing — sirf plain variable names nahi.

Figure — f-strings — embedding expressions

Worked examples

Worked example 2 — Braces ke andar arithmetic

a, b = 7, 5
print(f"{a} + {b} = {a + b}")   # 7 + 5 = 12

Yeh step kyun? {a + b} ek poora expression hai; Python pehle 12 compute karta hai, phir text mein convert karta hai. Pehle do braces ke beech ka literal + sirf ek character hai.

Worked example 3 — Method calls aur indexing

word = "python"
print(f"Shout: {word.upper()}")     # Shout: PYTHON
print(f"First letter: {word[0]}")   # First letter: p

Yeh step kyun? Koi bhi expression allowed hai, toh .upper() aur [0] braces ke andar normally run hote hain.

Worked example 4 — Format spec (

: ke baad wala part)

pi = 3.14159265
print(f"pi ≈ {pi:.2f}")        # pi ≈ 3.14
price = 1234567
print(f"{price:,}")            # 1,234,567
print(f"{42:05d}")             # 00042

Yeh step kyun? Colon ke baad aap ek format specification dete ho: .2f = 2 decimals float, , = thousands separator, 05d = integer ko width 5 tak zeros se pad karo. Hood ke neeche yeh format(value, spec) hai.

Worked example 5 — Self-documenting

= (Python 3.8+)

x = 9
print(f"{x=}")          # x=9
print(f"{x*2 = }")      # x*2 = 18

Yeh step kyun? {expr=} expression ka source text aur uski value dono print karta hai — debugging ke liye zabardast.

Common mistakes

Common mistake

f prefix bhool jaana

name = "Ada"
print("Hello, {name}")   # Hello, {name}   ← braces literally print ho gaye!

Mistake sahi kyun lagti hai: braces dekhne mein lagte hain jaise fill ho jayenge. Galat kyun hai: f ke bina, yeh ek ordinary string hai aur braces sirf characters hain. Fix: f add karo → f"Hello, {name}".

Common mistake Braces mein expression ki jagah statement daalna

# f"{x = 5}"   ← SyntaxError: assignment is not an expression

Sahi kyun lagta hai: = Python mein har jagah hai. Galat kyun hai: braces ko ek expression chahiye (kuch jo value deta ho). x = 5 ek statement hai. Fix: assignment bahar karo; sirf value-producing part andar rakho. (Note: {x=} bina value ke woh special debug form hai, jo alag hai.)

Common mistake Literal brace chahiye ho toh

print(f"Set notation: {{1, 2, 3}}")   # Set notation: {1, 2, 3}

Sahi kyun lagta hai: aapne ek brace type ki aur error ya empty result aaya. Kyun: ek akela brace expression start karta hai. Fix: braces double karo — {{ se literal { aur }} se literal } milta hai.

Common mistake Andar aur bahar same quote (pre-3.12)

d = {"k": 1}
# f"value is {d["k"]}"   ← breaks on older Python
f"value is {d['k']}"     # use the OTHER quote inside

Sahi kyun lagta hai: normal code mein quotes theek se nest hote hain. Kyun: purane parsers inner " par string band kar dete hain. Fix: braces ke andar alternate quote use karo.

Recall Feynman: 12-saal ke bacche ko samjhao

Ek fill-in-the-blank story socho: "Mera kutta ___ saal ka hai." Ek normal Python string woh story hai jisme blank khaali chhoot jaata hai — bas ___ print hoga. Ek f-string woh story hai jisme aap blank ke andar ek chhota instruction likhte ho, jaise {2 + 3}, aur computer secretly use solve karke sentence dikhane se pehle 5 likh deta hai. f woh magic stamp hai jo kehta hai "hey, blanks fill karo!"

Recall checkpoint

Kaunsa prefix ek string ko f-string banata hai?

Letter f (ya F) jo opening quote se bilkul pehle rakha jaata hai, jaise f"...".

Ek f-string mein { } ke beech kya ho sakta hai?

Koi bhi Python expression (variable, arithmetic, function/method call, indexing) — kuch bhi jo value evaluate kare.

7 + 5 ko f-string mein kaise embed karein aur result dikhayein?

f"{7 + 5}" → 12 (expression evaluate hota hai, phir text mein convert hota hai).

F-string ke andar literal { aur } kaise print karein?

Unhe double karo: {{ se { milta hai aur }} se } milta hai.

f"{pi:.2f}" mein colon ke baad wale part ka kya matlab hai?

Yeh ek format specification hai; .2f value ko 2 decimal places wale float ki tarah format karta hai.

f"{x=}" kya karta hai (Python 3.8+)?

Expression ka text aur uski value dono print karta hai, jaise x=9 — debugging ke liye bahut kaam ka.

F-string ka conceptual substitution rule kya hai?

Har {expr} ko format(expr, spec) se replace kiya jaata hai (default str(expr)), aur sab literal/converted pieces order mein join ho jaate hain.

print("Hi {name}") {name} literally kyun print karta hai?

f prefix ke bina yeh ek ordinary string hai, toh braces sirf characters hain, expression markers nahi.

f"{42:05d}" kya produce karta hai aur kyun?

00042 — 05d integer ko leading zeros ke saath width 5 tak pad karta hai.

Connections

Strings — literals and quotes — f-strings ek tarah ke string literal hain.
str() and type conversion — braces implicitly str()/format() call karte hain.
str.format() method — {} placeholders wala purana sibling.
Format specification mini-language — : ke baad kya aata hai.
Variables and scope — braces mein expressions current scope use karte hain.
print() function — f-strings ka sabse common consumer.