raise

Learn to raise exceptions with raise.

Raise errors with raise

The raise keyword throws an error. You use it to stop the program when you detect an invalid situation:

age = -5
if age < 0:
    raise ValueError("Geçersiz yaş")

Here a negative age is an error; raise ValueError stops the program and shows the message. The code that raises is raise; the code that catches is try/except.

This is the standard way for functions to report invalid input.

When to use raise

raise is used in two common situations. First, when a function receives an invalid argument. Second, when a precondition is not met:

def set_age(age):
    if age < 0:
        raise ValueError("Negatif yaş")
    return age

This function raises ValueError when given a negative age. The calling code can catch it with try/except. Otherwise the program stops.

This is a core tool for defensive programming.

Exercise Raise ValueError on negative age

Assign age = -5. Inside an if age < 0: block write raise ValueError("Geçersiz yaş"). Below it write print("yaş: " + str(age)). Wrap everything in try/except ValueError and in the except block write print("Geçersiz yaş").

Show example solution

try:
    age = -5
    if age < 0:
        raise ValueError("Geçersiz yaş")
except ValueError:
    print("Geçersiz yaş")

Test yourself

Match raise concepts

raise

Throws an error
ValueError

Error type
if age < 0

Condition
"Geçersiz yaş"

Error message
Which can be raised with raise?
- Only ValueError
- Only strings
- Only TypeError
- Exception subclasses
Show answer

Exception subclasses — Any subclass of Exception can be raised with raise.

Complete the raise call

if age < 0:
    ____("Geçersiz yaş")

Show answer

if age < 0:
    raise ValueError("Geçersiz yaş")

Next lesson: Exception hierarchy

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