Python Exceptions: try, except, else and finally

1Python Try...Except: overview

An exception is Python's signal that something went wrong while the program was running. If nothing handles it the program stops, but a try/except block lets you catch it and recover.

Before this lesson: Python if, elif and else, Python Functions

2Python Try...Except: diagram

Quick reference: Python Try...Except

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4Python Try...Except: syntax

try:
    risky_code()
except SomeError as err:     # runs if SomeError happens
    handle(err)
except (ErrorA, ErrorB):     # several types at once
    handle_other()
else:                        # runs only if nothing was raised
    use_result()
finally:                     # always runs
    clean_up()

raise ValueError("message")  # signal a problem yourself

5Python Try...Except: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

def parse_quantity(text):
    qty = int(text)
    if qty < 0:
        raise ValueError("quantity cannot be negative")
    return qty

orders = ["3", "two", "5", "-1"]
total = 0

for text in orders:
    try:
        qty = parse_quantity(text)
    except ValueError as err:
        print(f"Skipped {text!r}: {err}")
    else:
        total += qty
        print(f"Added {qty}")

print("Total items:", total)

Expected outputAdded 3
Skipped 'two': invalid literal for int() with base 10: 'two'
Added 5
Skipped '-1': quantity cannot be negative
Total items: 8

7Python Try...Except: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1-5parse_quantity() turns text into a whole number. int() raises ValueError by itself for text like "two", and the function raises its own ValueError for negative numbers.
2. Line 7-8The orders arrive as strings, as if typed into a web form. total starts at 0.
3. Line 10-12The call that might fail sits inside try, so a bad order can't crash the whole loop.
4. Line 13-14except ValueError as err catches both kinds of problem. err holds the exception, and printing it shows its message. The loop then moves on to the next order.
5. Line 15-17else runs only when the try block succeeded, so only valid quantities are added to the total.
6. Line 19After the loop the total is 3 + 5 = 8. Two orders were rejected, but the program kept running to the end.

8Python Try...Except: common mistakes

def average(nums):
    try:
        return sum(nums) / len(numbers)
    except:
        return 0

print(average([4, 6]))

def average(nums):
    try:
        return sum(nums) / len(nums)
    except ZeroDivisionError:
        return 0

print(average([4, 6]))

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Something went wrong")

print(result)

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Something went wrong")
    result = None

print(result)

try:
    age = int("twelve")
except TypeError:
    print("Please type a number")

try:
    age = int("twelve")
except ValueError:
    print("Please type a number")

9Python Try...Except: exercises

Write your answer, then press Check. The checks run in your browser.

def safe_int(text):
    # return None if text is not a whole number
    return int(text)

print(safe_int("12"))

def safe_int(text):
    try:
        return int(text)
    except ValueError:
        return None

print(safe_int("12"))

def check(value):
    try:
        number = 100 / value
    except ZeroDivisionError:
        print("A")
    else:
        print("B", number)
    finally:
        print("C")

check(4)
check(0)

B 25.0
C
A
C

2 more exercises → 4 more examples

Further reading

• Errors and exceptions
• Built-in exceptions

10Next lesson

Related:
Python TypeError: What It Means and How to Fix ItPython KeyError: What It Means and How to Fix ItPython IndexError: list index out of rangePython File Handling

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