try and except

try and except let a program say: attempt this, and if this particular thing goes wrong, do that instead. Used precisely, they turn a crash into a helpful message and let the program carry on. Used carelessly, they hide exactly the bugs you most need to see, so this page is as much about what not to catch as about what to catch.

Think of a safety net under one trapeze. It catches that performer's fall, and the show goes on. A net stretched under the whole circus would catch the audience too, and hide every other accident in the building.

The Shape

Put the risky code in a try block, and after it an except block that names the exception you expect. If that exception happens anywhere in the try block, Python jumps straight to the except block, runs it, and carries on after it. If nothing goes wrong, the except block is skipped. In the program below, the text forty cannot be converted, so the except block prints a message instead of the program crashing.

answer = "forty"
try:
    number = int(answer)
    print(f"Got {number}")
except ValueError:
    print(f"'{answer}' is not a whole number.")
print("The program carries on.")

'forty' is not a whole number.
The program carries on.

Notice that the line printing "Got" never ran. When the conversion failed, Python left the try block at once.

Catch the Error You Expect

Name the specific exception you know how to handle, and only that one. Here it is ValueError, the error int raises for text it cannot read. Any other kind of problem, such as a typo in a name, still stops the program with its own traceback, which is exactly what you want: it is a different problem that needs a different fix.

else and finally

Two optional blocks complete the shape. else runs only if the try block finished without an exception, which keeps the "it worked" code apart from the risky line. finally runs whatever happened, success or failure; it is for tidying up. In this book finally is rarely needed, because with already closes files for you.

for answer in ["15", "forty"]:
    try:
        percent = int(answer)
    except ValueError:
        print(f"'{answer}': not a number")
    else:
        print(f"'{answer}': tip of {percent} percent")

'15': tip of 15 percent
'forty': not a number

Never a Bare except

An except with no exception named catches everything, and that is the trap. The program below has a typo: answr, with the e missing, instead of answer. That is a NameError, a real bug. But the bare except catches it too and prints the friendly message about numbers, so the program looks as if it is handling bad input correctly while it is actually broken. Catching Exception, which covers nearly all errors, has the same problem on a smaller scale.

answer = "15"
try:
    percent = int(answr)
except:
    print("Please type a whole number.")

Please type a whole number.

The input was a perfectly good 15. With except ValueError, the typo would have shown itself at once as a NameError with a "Did you mean" suggestion.

The Ask-Until-Valid Loop, Finished

Chapter 4's loop used isdigit, which rejects minus signs and decimal points. The honest test of "can this be a number?" is to try converting it. The loop below asks for a temperature, tries float, and asks again only when the conversion raises a ValueError. It accepts minus eleven and rejects the word cold.

while True:
    answer = input("Temperature: ")
    try:
        temp = float(answer)
        break
    except ValueError:
        print("Please type a number, like 21.5 or -3.")

print(f"Recorded {temp} degrees")

Temperature: cold
Please type a number, like 21.5 or -3.
Temperature: -11
Recorded -11.0 degrees