Python Try...Except

try and except let your program handle errors calmly instead of crashing.

What is an Exception?

An exception is an error that happens while the program is running. Without handling, Python stops and prints an error message.

Example

python

print(10 / 0)

Output

ZeroDivisionError: division by zero

try and except

Put risky code inside try. If an error happens, Python jumps to except:

Example

python

try:
    print(10 / 0)
except:
    print("Something went wrong")
print("Program continues")

Output

Something went wrong
Program continues

Catching Specific Errors

It is better to name the error you expect. You can have several except blocks:

Example

python

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Cannot divide by zero"
    except TypeError:
        return "Please give numbers only"

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

Output

5.0
Cannot divide by zero
Please give numbers only

A bare except: catches every error, even ones you did not expect, which can hide bugs. Name the exceptions whenever you can.

Common Exceptions

Exception	When it happens
`ValueError`	Right type but wrong value, like `int("abc")`
`TypeError`	Wrong type, like `"5" + 5`
`ZeroDivisionError`	Dividing by zero
`IndexError`	List index that does not exist
`KeyError`	Dictionary key that does not exist
`FileNotFoundError`	Opening a file that is not there
`NameError`	Using a variable that was never created

Getting the Error Message

Use as to store the error and print its message:

Example

python

try:
    age = int("twenty")
except ValueError as e:
    print("Error:", e)

Output

Error: invalid literal for int() with base 10: 'twenty'

else and finally

else runs only if no error happened
finally runs always, error or not. It is good for clean-up work.

Example

python

try:
    marks = int("85")
except ValueError:
    print("Not a number")
else:
    print("Marks saved:", marks)
finally:
    print("Check complete")

Output

Marks saved: 85
Check complete

A Safe Input Loop

Keep asking until the user types a valid number:

Example

python

while True:
    try:
        qty = int(input("Quantity: "))
        break
    except ValueError:
        print("Please type a whole number")
print("You ordered", qty)

Output

Quantity: five
Please type a whole number
Quantity: 5
You ordered 5

Raising Your Own Errors

Use raise to stop the program on purpose when something is not allowed:

Example

python

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

try:
    set_age(-3)
except ValueError as e:
    print(e)

Output

Age cannot be negative

Exercise

Which block runs whether an error happened or not?

exceptelsefinallytry