GateShala
Playground

Handling Errors

Catch exceptions with try and except so your program does not crash.

When something goes wrong while a program runs, such as dividing by zero or converting "abc" to a number, Python raises an exception. If nothing handles it, the program stops with an error message. A try / except block lets you catch the exception and decide what to do.

Python
Open in playground
try:
    result = 10 / 0
    print("This line is skipped")
except ZeroDivisionError:
    print("You cannot divide by zero.")

print("The program carries on.")
Output
You cannot divide by zero.
The program carries on.

Catching different errors

You can have several except blocks, one for each kind of error. Add as e to get the error object and its message.

Python
Open in playground
def to_number(text):
    try:
        return int(text)
    except ValueError as e:
        print("Bad input:", e)
        return None

print(to_number("42"))
print(to_number("forty-two"))
Output
42
Bad input: invalid literal for int() with base 10: 'forty-two'
None

else and finally

The else block runs only if no exception happened. The finally block always runs, error or not, which makes it the right place for clean-up.

Python
Open in playground
for value in ["8", "0", "x"]:
    try:
        n = 100 / int(value)
    except ZeroDivisionError:
        print(value, "-> division by zero")
    except ValueError:
        print(value, "-> not a number")
    else:
        print(value, "->", n)
    finally:
        print("  checked", value)
Output
8 -> 12.5
  checked 8
0 -> division by zero
  checked 0
x -> not a number
  checked x

Raising your own errors

Use raise when your code receives something it cannot work with.

Python
Open in playground
def set_age(age):
    if age < 0:
        raise ValueError("age cannot be negative")
    return age

try:
    set_age(-5)
except ValueError as e:
    print("Error:", e)
Output
Error: age cannot be negative