raise
- Python course
- Beginner
- Lesson 102/116
- 4 min
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 ageThis 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.
Practice 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").
Starter code
# assign a negative age value
# if negative, raise ValueError
# catch with try/except and print messageSample solution
try:
age = -5
if age < 0:
raise ValueError("Geçersiz yaş")
except ValueError:
print("Geçersiz yaş")Expected output
Geçersiz yaş
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match raise concepts
| Term | Meaning |
|---|---|
raise |
Throws an error |
ValueError |
Error type |
if age < 0 |
Condition |
"Geçersiz yaş" |
Error message |
2 Which can be raised with raise?
- Only ValueError
- Only strings
- Only TypeError
- Exception subclasses Correct answer
Explanation: Any subclass of Exception can be raised with raise.
3 Complete the raise call
if age < 0:
____("Geçersiz yaş")if age < 0:
raise ValueError("Geçersiz yaş")Explanation: raise ValueError("message") raises a ValueError.