Exception handling — try, except (specific), else, finally

1.3.5 · Coding › Python Intermediate

YEH exist KYUN karta hai?

Ek exception ek object hai jise Python raise (throw) karta hai jab kuch abnormal hota hai. Agar koi ise catch nahi karta, toh yeh call stack upar propagate hoti hai jab tak program ek traceback ke saath crash nahi kar leta.

WHAT hain yeh chaar blocks?

try:
    risky()          # WHAT: dangerous hissa
except ValueError:   # WHAT: SPECIFIC error handle karo
    handle()
else:                # WHAT: sirf tab run hota hai jab try succeed kare
    on_success()
finally:             # WHAT: cleanup, hamesha
    cleanup()

Figure — Exception handling — try, except (specific), else, finally

HOW control flow actually move karta hai?

Specific vs bare `except`

Worked examples

Worked example 1 — Safe integer parsing

def to_int(s):
    try:
        n = int(s)            # ValueError raise ho sakta hai
    except ValueError:
        return None           # Yeh step kyun? predictable bad input handle karo
    else:
        return n              # Yeh step kyun? real n sirf tab return karo jab parse succeed kare

to_int("42") → 42; to_int("oops") → None. else kyun? int(s) ko ek hi guard karta hai; success path unambiguous hai.

Worked example 2 —

finally return ke saath bhi run hota hai

def f():
    try:
        return "from try"
    finally:
        print("cleanup!")     # Yeh step kyun? finally return complete hone SE PEHLE fire karta hai

Output: cleanup! print karta hai, phir "from try" return karta hai. Kyun? Rule 5: finally hamesha control jaane se pehle run hota hai — return pe bhi.

Worked example 3 — Multiple excepts ka order matter karta hai

try:
    d = {}; d["k"]            # KeyError raise karta hai
except LookupError:           # KeyError IS-A LookupError  -> pehle match karta hai
    print("lookup failed")
except KeyError:
    print("never reached")    # Kyun? broader parent ne pehle hi catch kar liya

Yeh step kyun? Specific (child) exceptions PEHLE rakhein, general (parent) wale baad mein — warna parent child ko shadow kar deta hai.

Worked example 4 —

try/except/else/finally saath mein

try:
    f = open("data.txt")      # FileNotFoundError raise ho sakta hai
except FileNotFoundError:
    print("no file")
else:
    print(f.read())           # Kyun? sirf tab read karo jab open() succeed kare
    f.close()
finally:
    print("done attempting")  # Kyun? hamesha log karo ki humne try kiya

Flashcards

Try statement ka else block kab run hota hai?

Sirf tab jab try block koi exception raise kiye bina complete hua ho.

finally kab run hota hai?

Hamesha — chahe exception aayi ho ya nahi, catch hui ho ya nahi, ya chahe return/break/continue use kiya ho.

Bare except: ke jagah except ValueError prefer kyun karein?

Bare except bugs (NameError), Ctrl-C (KeyboardInterrupt) aur SystemExit bhi swallow kar leta hai, real problems chhupa deta hai. Jo narrowest exception handle kar sako wahi catch karo.

Agar try body mein statement

S_{k}

pe exception raise ho, toh

S_{k + 1}

ka kya hota hai?

Woh skip ho jaata hai; Python seedha matching except clause pe jump karta hai.

Kya except ParentError ChildError (subclass) ko catch karega?

Haan — except listed class aur uski saari subclasses match karta hai. Isliye child-pehle, parent-baad mein rakho.

Sirf risky line try mein aur baaki else mein KYUN rakhein?

Taaki success-handling code mein error accidentally aapke except se catch na ho, jo ek alag bug mask kar deta.

Exception object tak kaise access karein?

except SomeError as e: se ise e se bind karo.

except (A, B): ka matlab kya hai?

Exception A ya B (types ka tuple) ko same handler se catch karo.

Recall Feynman: 12-saal ke bachhe ko explain karo

Socho tum ek recipe se khana bana rahe ho (try). Agar koi step galat ho jaye — anda gira diya — tum aankhein band karke recipe follow nahi karte. Tum "anda toot jaye toh kya karein" wale note pe jump karte ho (except). Agar khana perfectly bana, toh "ab khana plate karo" step karte ho (else). Aur chahe kuch bhi ho — jala ho ya perfect — tum hamesha end mein bartan zaroor dhoote ho (finally). except egg-broke specific hai: woh fire nahi karta agar oven mein aag lag gayi, woh alag emergency hai.

Connections

Raising exceptions — raise and custom exception classes
The exception class hierarchy — BaseException, Exception, LookupError
Context managers — with statement and __enter__/__exit__ (aksar try/finally ki jagah leta hai)
File handling in Python
Debugging and tracebacks
Logging vs printing errors