Handling Errors with try / except

Errors stop programs

age = int("twenty")
print("This line never runs")

That's an exception: Python couldn't continue, so it stopped. In real apps, users type unexpected things, files go missing and networks fail. Good code expects that.

try / except

text = "twenty"
try:
    age = int(text)
    print(f"In 10 years you'll be {age + 10}")
except ValueError:
    print("Please enter your age as a number, like 25")

Python runs the try block. If a ValueError happens, it jumps to except instead of crashing.

Catch specific errors

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return None

print(safe_divide(10, 2))
print(safe_divide(10, 0))

Avoid a bare except: that catches everything. It also hides real bugs, like a typo in a variable name, and makes them very hard to find.

Raising errors yourself

When your function gets input it can't handle, raise an error with a clear message:

def set_age(age):
    if age < 0:
        raise ValueError("Age can't be negative")
    return age

try:
    set_age(-5)
except ValueError as e:
    print("Problem:", e)

finally

finally runs no matter what. It's used for cleanup, like closing a file or a database connection:

try:
    print("Working…")
finally:
    print("Always runs")