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Python Try...Except · Examples
Core Python

Python Try...Except Examples

Short, runnable programs that show python exceptions: try, except, else and finally in action. Read the explanation, press Run, then change the code and try your own ideas. New to the topic? Start with the Python Exceptions: try, except, else and finally lesson.

1Handling several exception types at once

A tuple of exception types catches any of them. type(err).__name__ tells you which one actually happened.

Handling several exception types at once 
values = ["12", "zero", "0", None]

for v in values:
    try:
        print(f"100 / {v} =", 100 / int(v))
    except (ValueError, TypeError, ZeroDivisionError) as err:
        print(f"{v!r} -> {type(err).__name__}: {err}")
Expected output100 / 12 = 8.333333333333334
'zero' -> ValueError: invalid literal for int() with base 10: 'zero'
'0' -> ZeroDivisionError: division by zero
None -> TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

2finally runs even after return

finally runs whether the function returns normally or an exception escapes, which makes it the safe place for clean-up code.

finally runs even after return 
def load_settings(name):
    print("Opening", repr(name))
    try:
        if name == "":
            raise ValueError("file name is empty")
        return {"theme": "dark"}
    finally:
        print("Closing", repr(name))

print(load_settings("app.cfg"))

try:
    load_settings("")
except ValueError as err:
    print("Error:", err)
Expected outputOpening 'app.cfg'
Closing 'app.cfg'
{'theme': 'dark'}
Opening ''
Closing ''
Error: file name is empty

3Asking again until the input is valid

A while True loop with try/except keeps asking until the answer is usable; else: break leaves the loop once it is. The Input box supplies three answers, one per input() call.

Asking again until the input is valid 
while True:
    text = input("How many tickets? ")
    try:
        tickets = int(text)
        if tickets < 1:
            raise ValueError("you need at least 1 ticket")
    except ValueError as err:
        print("Try again:", err)
    else:
        break

print("Booked", tickets, "tickets")
Expected outputHow many tickets? abc
Try again: invalid literal for int() with base 10: 'abc'
How many tickets? 0
Try again: you need at least 1 ticket
How many tickets? 3
Booked 3 tickets

4A custom exception for your own rules

A custom exception class gives your error a meaningful name, so callers can catch exactly that problem and nothing else.

A custom exception for your own rules 
class OutOfStockError(Exception):
    pass

stock = {"pen": 5, "notebook": 0}

def buy(item, qty):
    available = stock.get(item, 0)
    if qty > available:
        raise OutOfStockError(f"only {available} {item}(s) left")
    stock[item] = available - qty
    return f"Bought {qty} {item}(s)"

for item, qty in [("pen", 2), ("notebook", 1), ("pen", 4)]:
    try:
        print(buy(item, qty))
    except OutOfStockError as err:
        print("Sorry:", err)
Expected outputBought 2 pen(s)
Sorry: only 0 notebook(s) left
Sorry: only 3 pen(s) left

Practise next

Python Exceptions: try, except, else and finally exercises → Back to the lesson

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