try except example in Python

When Python cannot do something, it raises an exception and the program stops. A try/except block catches the exception so the program can print a helpful message and continue.

The program

def divide():
    try:
        a = float(input("First number: "))
        b = float(input("Second number: "))
        print(f"{a} / {b} = {a / b}")
    except ValueError:
        print("That is not a number.")
    except ZeroDivisionError:
        print("You cannot divide by zero.")


# run it three times to show all three results
for _ in range(3):
    divide()
    print()

Typed in this example: 10, 4, 10, 0, ten. Press Run to type your own.

First number: 10
Second number: 4
10.0 / 4.0 = 2.5

First number: 10
Second number: 0
You cannot divide by zero.

First number: ten
That is not a number.

How it works

• Python runs the lines inside try one by one. If one of them raises an exception, the rest of the try block is skipped and Python looks for a matching except.
• float("ten") raises a ValueError. In the third run this happens on the first question, so the second question is never asked.
• 10 / 0 raises a ZeroDivisionError.
• Each except names one exception type, so each mistake gets its own message. An exception that matches no except (for example a KeyboardInterrupt from Ctrl+C) is not caught, and it still stops the program.

Without the try, the program would stop with a traceback such as ZeroDivisionError: float division by zero.

One except for both, with the message

To handle several types the same way, put them in a tuple. as error gives you the exception object, and printing it shows Python's own message:

for a, b in [("8", "2"), ("8", "0"), ("8", "two")]:
    try:
        print(float(a) / float(b))
    except (ValueError, ZeroDivisionError) as error:
        print(f"Error: {error}")

4.0
Error: float division by zero
Error: could not convert string to float: 'two'

Avoid a bare except: with no type. It catches every error, including mistakes in your own code, and hides them. The input validation loop uses try/except to keep asking until the answer is valid.

Related programs: try except else finally.