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How to use "raise" keyword in Python [duplicate]

stackoverflow.com › questions › 13957829 › how-to-use-raise-keyword-in-python

It has two purposes.

jackcogdill has given the first one:

It's used for raising your own errors.
if something:
   raise Exception('My error!')

The second is to reraise the current exception in an exception handler, so that it can be handled further up the call stack.

try:
  generate_exception()
except SomeException as e:
  if not can_handle(e):
    raise
  handle_exception(e)

Answer from Ignacio Vazquez-Abrams on Stack Overflow

w3schools.com › python › ref_keyword_raise.asp

Python raise Keyword

Bootcamp Python Certificate Python Training ... The raise keyword is used to raise an exception. You can define what kind of error to raise, and the text to print to the user. ... If you want to use W3Schools services as an educational ...

w3schools.com › python › gloss_python_raise.asp

Python Raise an Exception

Raise an error and stop the program if x is lower than 0: x = -1 if x < 0: raise Exception("Sorry, no numbers below zero") Try it Yourself » · The raise keyword is used to raise an exception.

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w3schools.com › python › python_try_except.asp

Python Try Except

Raise an error and stop the program if x is lower than 0: x = -1 if x < 0: raise Exception("Sorry, no numbers below zero") Try it Yourself » · The raise keyword is used to raise an exception.

w3schools.in › python › exception-handling

Python Exceptions Handling - W3Schools

try/except: catch the error and recover from exceptions hoist by programmers or Python itself. try/finally: Whether exception occurs or not, it automatically performs the clean-up action. assert: triggers an exception conditionally in the code. raise: manually triggers an exception in the code.

stackoverflow.com › questions › 13957829 › how-to-use-raise-keyword-in-python

How to use "raise" keyword in Python - Stack Overflow

It has two purposes.

jackcogdill has given the first one:

It's used for raising your own errors.
if something:
   raise Exception('My error!')

The second is to reraise the current exception in an exception handler, so that it can be handled further up the call stack.

try:
  generate_exception()
except SomeException as e:
  if not can_handle(e):
    raise
  handle_exception(e)

raise without any arguments is a special use of python syntax. It means get the exception and re-raise it. If this usage it could have been called reraise.

    raise

From The Python Language Reference:

If no expressions are present, raise re-raises the last exception that was active in the current scope.

If raise is used alone without any argument is strictly used for reraise-ing. If done in the situation that is not at a reraise of another exception, the following error is shown: RuntimeError: No active exception to reraise

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W3Schools online PYTHON editor

Run ❯ Get your own Python server · ❯Run Code Ctrl+Alt+R Change Orientation Ctrl+Alt+O Change Theme Ctrl+Alt+D Go to Spaces Ctrl+Alt+P · Traceback (most recent call last): File "demo_ref_keyword_raise.py", line 4, in <module> raise Exception("Sorry, no numbers below zero") Exception: Sorry, no numbers below zero

w3schools.com › python › python_ref_exceptions.asp

Python Built-in Exceptions

The table below shows built-in exceptions that are usually raised in Python: ❮ Previous Next ❯ · ★ +1 · Sign in to track progress · REMOVE ADS · PLUS · SPACES · GET CERTIFIED · FOR TEACHERS · BOOTCAMPS · CONTACT US · × · If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail: sales@w3schools.com ·

w3resource.com › python › python-raise-exceptions-with-examples.php

Python Raising Exceptions: Learn How and When to Raise Errors

August 26, 2024 - Master raising exceptions in Python with detailed examples, covering built-in and custom errors, loops, and functions for robust error handling.

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openstax.org › books › introduction-python-programming › pages › 14-5-raising-exceptions

14.5 Raising exceptions - Introduction to Python Programming | OpenStax

March 13, 2024 - The following class represents a pizza for sale. The SIZES dictionary maps an abbreviation to a size's name and price. Line 11 checks if the size received by the constructor is in the dictionary. Line 12 raises a ValueError with specific error message.

geeksforgeeks.org › python-raise-keyword

Python Raise Keyword - GeeksforGeeks

October 30, 2023 - In this article, we will learn how the Python Raise keyword works with the help of examples and its advantages.

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Learn Python Programming with W3Schools: Web Development

May 12, 2025 - Start learning Python programming for free on W3Schools. Understand basics, OOP, and more with hands-on tutorials for beginners.

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Python Iterators

class MyNumbers: def __iter__(self): self.a = 1 return self def __next__(self): if self.a <= 20: x = self.a self.a += 1 return x else: raise StopIteration myclass = MyNumbers() myiter = iter(myclass) for x in myiter: print(x) Try it Yourself » ... If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail: sales@w3schools.com · If you want to report an error, or if you want to make a suggestion, send us an e-mail: help@w3schools.com · HTML Tutorial CSS Tutorial JavaScript Tutorial How To Tutorial SQL Tutorial Python Tutorial W3.CSS Tutorial Bootstrap Tutorial PHP Tutorial Java Tutorial C++ Tutorial jQuery Tutorial

geeksforgeeks.org › python › user-defined-exceptions-python-examples

User-defined Exceptions in Python with Examples - GeeksforGeeks

February 12, 2026 - Follow these steps to create and ... or any of its subclasses. Raise the Exception: Use the raise statement to raise the user-defined exception when a specific condition occurs....

Python documentation

docs.python.org › 3 › tutorial › errors.html

8. Errors and Exceptions — Python 3.14.3 documentation

The sole argument to raise indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from BaseException, such as Exception or one of its subclasses).

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Python Getting Started

Python Examples Python Compiler Python Exercises Python Quiz Python Challenges Python Server Python Syllabus Python Study Plan Python Interview Q&A Python Bootcamp Python Certificate Python Training ... At W3Schools, you can try Python without installing anything.

Python Tutorial

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Python raise from - Python Tutorial

March 28, 2025 - Summary: in this tutorial, you will learn how to use the Python raise from statement to raise an exception with extra information.

realpython.com › ref › keywords › raise

raise | Python Keywords – Real Python

In this tutorial, you'll learn how to raise exceptions in Python, which will improve your ability to efficiently handle errors and exceptional situations in your code.

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W3Schools Python Exercise

I completed a Python exercise on w3schools.com

w3schools.com › python › gloss_python_iterator_stop.asp

Python Stop Iteration

class MyNumbers: def __iter__(self): self.a = 1 return self def __next__(self): if self.a <= 20: x = self.a self.a += 1 return x else: raise StopIteration myclass = MyNumbers() myiter = iter(myclass) for x in myiter: print(x) Try it Yourself » · Python Iterator Tutorial Iterators Iterator vs Iterable Loop Through an Iterator Create an Iterator ... If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail: sales@w3schools.com

reddit.com › r/learnpython › python exception handling: understanding raise statement

r/learnpython on Reddit: Python exception handling: Understanding raise statement

September 4, 2024 -

In comparison to try, except, else, and finally blocks, raise statement seems not always needed. I guess it is an optional tool in exception handling that may well be redundant in some scenarios.

For example, these are the two codes with and without use of raise:

Without raise

def check_input(val):
    if val < 0:
        print("Input must be non-negative")
    else:
        print(f"Processing value: {val}")

check_input(-10)

With raise:

def check_input(val):
    if val < 0:
        raise ValueError("Input must be non-negative")
    print(f"Processing value: {val}")

try:
    check_input(-10)
except ValueError as e:
    print(f"Error: {e}")

Given print function achieving the desired result, not sure if raise statement really needed.

In comparison to try, except, else, and finally blocks, raise statement seems not always needed. Consider that all of these exceptions you're getting in your code have to come from somewhere - and that you, a programmer eventually writing Python code that other people will use, may desire to implement your own system of exceptions so that the programmers using your code can respond to exceptional situations when they occur. Didn't you wonder how that works? How you create an exception, rather than just handling one that comes from somewhere else?

I think you’re getting a little ahead of yourself here. Since you functions don’t actually return anything they really don’t need a raise, but most of the time functions will return or mutate some object, if generally the next step after that function requires that certain aspect must be true/present, in this case positive, in order for the rest of the program to work…you may want to raise there so you get the error where the problem actually occurs and not further down in the operation when it became a problem.