Python try / except Explained

try and except let your program deal with something going wrong instead of stopping dead.

try:
    age = int(input("Your age: "))
except ValueError:
    print("That was not a whole number.")

Python runs the try block. If the specific error you named happens, it runs the except block instead of crashing. If nothing goes wrong, the except block is skipped entirely.

Catch the specific error, not everything#

This is the whole discipline in one rule. Name the exception you are prepared for:

try:
    with open("scores.txt") as f:
        data = f.read()
except FileNotFoundError:
    data = ""          # first run, no file yet

Compare that with catching everything:

try:
    with open("scores.txt") as f:
        data = f.read()
except:                # catches literally anything
    data = ""

The second version also swallows your typo in the filename variable, a permissions problem, and a NameError from a mistake three lines up. The program keeps running and quietly does the wrong thing, and you get no message telling you why.

The errors worth catching#

Each of these is explained with examples in common Python errors and how to fix them.

Catching more than one#

try:
    value = int(data["count"])
except (KeyError, ValueError):
    value = 0

Or handle them differently:

try:
    value = int(data["count"])
except KeyError:
    print("No count in the data.")
    value = 0
except ValueError:
    print("Count was not a number.")
    value = 0

Reading the error itself#

try:
    number = int("abc")
except ValueError as err:
    print("Could not convert:", err)
    # Could not convert: invalid literal for int() with base 10: 'abc'

as err gives you the exception object, and printing it gives you Python’s own description — usually more precise than anything you would write yourself.

else and finally#

try:
    f = open("data.txt")
except FileNotFoundError:
    print("No file.")
else:
    print("Opened fine:", len(f.read()), "characters")
    f.close()
finally:
    print("This runs either way.")

• else runs only if nothing went wrong. Use it to keep the try block down to the single line that might fail — the smaller the try, the more precisely you know what you caught.
• finally runs whatever happens, including when an exception is on its way up. It is for cleanup: closing files, releasing a connection.

For files specifically, with already does the cleanup for you, so you rarely need finally:

with open("data.txt") as f:      # closes itself, even on an error
    print(f.read())

Raising your own#

Sometimes the right move is to stop, loudly, with a message that explains what was wrong.

def set_age(value):
    if not isinstance(value, int):
        raise TypeError("Age must be a whole number")
    if value < 0:
        raise ValueError("Age cannot be negative")
    return value

This is better than returning None and hoping the caller checks. An exception cannot be ignored by accident.

When not to use try/except#

If you can simply check first, checking is clearer:

try:
    print(person["email"])       # heavy-handed
except KeyError:
    print("no email")

print(person.get("email", "no email"))   # better

try:
    average = total / len(items)
except ZeroDivisionError:
    average = 0

average = total / len(items) if items else 0   # better

Rough rule: use try/except for things outside your control — files, networks, other people’s input. Use a plain if for things you can check yourself.

A worked example#

Reading a number from someone, safely, without crashing and without giving up:

def ask_number(prompt, low, high):
    while True:
        raw = input(prompt)

        try:
            value = int(raw)
        except ValueError:
            print("Please type a whole number.")
            continue

        if value < low or value > high:
            print(f"Please stay between {low} and {high}.")
            continue

        return value

age = ask_number("Your age: ", 1, 120)
print("Thanks:", age)

The try covers exactly one line — the conversion — and the range check is a plain if, because that is something we can check ourselves. The while loop keeps asking until both pass.

Where to go next#

• Common Python errors and how to fix them
• How to read a stack trace
• Build a to-do list that saves to a file — try/except in a real program
• Reading and writing files in Python — where FileNotFoundError comes from