Lesson 17 � Intermediate

TRY/EXCEPT:
ERROR HANDLING.

Errors inevitable hain � user galat input dega, file nahi milegi, API fail hogi. Error handling se program crash nahi hota gracefully handle hota hai. Ye lesson sikhaega ki Python mein errors ko kaise catch, handle aur custom errors kaise banayein.

WHY: Error handling kyun zaroori hai?

Socho aap ek program likh rahe ho jisme user se number lena hai. User ne "abc" type kar diya � program crash! Ya file read kar rahe ho aur file exist nahi karti � crash! Error handling se aap in situations ko gracefully handle kar sakte ho. Program rokna nahi, smart tarike se aage badhana hai.

WHEN: kab error handling use hota hai

Error handling tab use hota hai jab aapko pata hai ki kuch galat ho sakta hai � user input lene mein, files padhane mein, network calls karne mein, database operations mein. Har production-ready application mein error handling mandatory hai.

HOW: try/except se error handle karo

Basic try/except

try:
 num = int(input("Number daalo: "))
 print(100 / num)
except ValueError:
 print("Galat number!")
except ZeroDivisionError:
 print("Zero se divide mat karo!")
except Exception as e:
 print(f"Error: {e}")
else:
 print("Sab theek hai!")
finally:
 print("Cleanup done.")

try block mein wo code daalo jo error de sakta hai. except mein specific error handle karo. else tab chalega jab koi error na ho. finally hamesha chalega � chahe error ho ya na ho.

Multiple except blocks

try:
 data = {"name": "Aman"}
 print(data["age"])
except KeyError as e:
 print(f"Key nahi mili: {e}")
except TypeError as e:
 print(f"Type error: {e}")
except Exception as e:
 print(f"Kuch aur gaya wrong: {e}")

Har error ka apna except block rakho � debugging mein help milti hai. Exception sabse last mein rakho kyunki ye sab errors catch karta hai.

Custom errors raise karna

class AgeError(Exception):
 pass

def check_age(age):
 if age < 0:
 raise AgeError("Age negative nahi ho sakta!")
 if age > 150:
 raise AgeError("Ye realistic age nahi hai!")
 return True

try:
 check_age(-5)
except AgeError as e:
 print(f"Validation error: {e}")

raise se khud error throw kar sakte ho. Custom error class banake apne validation rules define karo � code clean aur readable rahega.

Try/Except flow

# Flow: try ? (error?) ✓ except OR else ✓ finally

try:
 print("Try: Koshish kar rahe hain")
 result = 10 / 0
except ZeroDivisionError:
 print("Except: Error mila, handle kar rahe hain")
else:
 print("Else: Koi error nahi tha!")
finally:
 print("Finally: Hamesha chalega!")

# Output:
# Try: Koshish kar rahe hain
# Except: Error mila, handle kar rahe hain
# Finally: Hamesha chalega!

Agar try mein error aata hai toh except chalega, else skip hoga. Agar koi error nahi toh else chalega. finally hamesha last mein chalega � cleanup ke liye best.

Common Python errors

• ValueError � Sahi type nahi hai, jaise int("abc")
• ZeroDivisionError � Zero se divide karne ki koshish
• FileNotFoundError � File exist nahi karti
• KeyError � Dictionary mein key nahi hai
• IndexError � List mein index exist nahi karta
• TypeError � Sahi data type nahi hai
• AttributeError � Object mein wo attribute nahi hai
• ImportError � Module import nahi ho raha

Common mistakes

• Bare except: lagana bina error type ke � hamesha specific error catch karo.
• finally block bhool jaana � cleanup code wahan likho jo hamesha chale.
• Sirf except Exception lagana � specific errors pehle catch karo, generic baad mein.
• try block mein bahut zyada code daalna � sirf risky code rakho try mein.
• Error ko silently ignore karna � except: pass se debugging mushkil ho jaati hai.