Exceptions and try/except

2 min read 5 runnable examples Quiz included

When something goes wrong while a program runs, Python raises an exception. Unhandled, it stops the program with an error message. With try and except, you can catch it and decide what happens instead.

try and except

Python

text = input("Enter a number: ")
try:
    number = int(text)
    print("Double is", number * 2)
except ValueError:
    print(f"'{text}' isn't a whole number.")
print("The program carries on.")

Python runs the try block. If a ValueError happens inside it, Python jumps to the except block instead of crashing.

Catch specific exceptions

Name the exceptions you expect, and handle each one appropriately:

Python

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        print("Can't divide by zero.")
    except TypeError:
        print("Both values must be numbers.")

print(safe_divide(10, 2))
safe_divide(10, 0)
safe_divide(10, "two")

Common built-in exceptions

Exception	Raised when
`ValueError`	a value has the right type but a wrong value: `int("abc")`
`TypeError`	an operation gets the wrong type: `"a" + 1`
`ZeroDivisionError`	dividing by zero
`KeyError`	a dictionary key doesn't exist
`IndexError`	a list index is out of range
`FileNotFoundError`	opening a file that isn't there
`NameError`	using a name that doesn't exist

Getting the error message

as gives you the exception object, whose message you can print or log:

Python

prices = {"tea": 20}
try:
    print(prices["coffee"])
except KeyError as error:
    print("Missing key:", error)

else and finally

else runs only if no exception happened.
finally runs always, whether there was an error or not. It's the place for clean-up.

except runs only after an error and else only without one; finally runs either way, which makes it the place to clean up.

Python

def parse_age(text):
    try:
        age = int(text)
    except ValueError:
        print(f"  {text!r}: not a number")
    else:
        print(f"  {text!r}: age {age}")
    finally:
        print("  (checked)")

parse_age("42")
parse_age("forty")

Keep asking until the input is valid

Python

while True:
    try:
        n = int(input("Positive number: "))
        if n <= 0:
            print("It must be positive.")
            continue
        break
    except ValueError:
        print("That's not a number.")
print("You chose", n)

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