How to use "raise" keyword in Python

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Top 5 Answer for How to use "raise" keyword in Python

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It has 2 purposes.

yentup 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) 
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It's used for raising errors.

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

Some examples here

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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.


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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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:

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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 

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