How to use "raise" keyword in Python

Question

I have read the official definition of "raise", but I still don't quite understand what it does.

In simplest terms, what is "raise"?

Example usage would help.

Solution 1

It has 2 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)

Solution 2

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

Solution 3

It's used for raising errors.

if something:
    raise Exception('My error!')

Some examples here

Solution 4

Besides raise Exception("message") and raise Python 3 introduced a new form, raise Exception("message") from e. It's called exception chaining, it allows you to preserve the original exception (the root cause) with its traceback.

It's very similar to inner exceptions from C#.

More info: https://www.python.org/dev/peps/pep-3134/

Solution 5

You can use it to raise errors as part of error-checking:

if (a < b):
    raise ValueError()

Or handle some errors, and then pass them on as part of error-handling:

try:
    f = open('file.txt', 'r')
except IOError:
    # do some processing here
    # and then pass the error on
    raise

View more solutions

Comments

[voithos]

It's known as throw in some other languages.

[alisa]

I guess a relevant addition to this question: does raise exits the function automatically or does one need to return after raise?

[Curtis Yallop]

I typed that in and got "global name 'error' is not defined". To others new to Python, you need "raise Exception('My error!')". You replace "error" with your Exception name. A list of standard exceptions you can use is here: docs.python.org/2/library/exceptions.html

[P-Gn]

Note that in python 3.x the new syntax is except SomeException as e:

[alpha_989]

@user1735003, what does the as e do? How is it different from except SomeException?

[Ignacio Vazquez-Abrams]

@alpha_989: It lets you get the actual exception instance.

[alper]

During re-raising and catching the exception to the top level. should I print the trace on bottom call where error occurs or the most higher call? or it is recommend to do raise Exception("message") from e to carry the error tothe top level?

[alper]

When I follow this approach, is it recommended to print the trace on the bottom level where the error occurs, or at the highlest level to that catches the carried error?

[Martijn Pieters]

@alper: not sure what you are asking. If you are catching an exception in an except ...: handler, the traceback will include the whole stack, from the point where your Python program started up to the place where the exception was raised. So it doesn't matter where in the stack your handler is located, really. If you need to re-raise the exception after handling, use raise, nothing else.

[Martijn Pieters]

@alper: raise Exception("message") from e replaces the e exception with a new exception, but just a plain Exception instance carries no meaning. If e was a TypeError or ValueError or LibrarySpecificException you can't now catch those specific exceptions anymore, because you replaced it with Exception.