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

User · Answer

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

User · Answer

raise causes an exception to be raised  Some other languages use the verb  throw  instead   It s intended to signal an error situation  it flags that the situation is exceptional to the normal flow   Raised exceptions can be caught again by code  upstream   a surrounding block  or a function earlier on the stack  to handle it  using a try  except combination

User · Answer

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

User · Answer

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

User · Answer

It s used for raising errors   if something      raise Exception  My error      Some examples here

User · Answer

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

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

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