Raise Exception Error Python
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you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. 8.1. Syntax Errors¶ Syntax errors, also known as parsing errors, python exception types are perhaps the most common kind of complaint you get while you are
Python Custom Exception
still learning Python: >>> while True print 'Hello world' File "
Python Raise Valueerror
error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here: >>> 10 * (1/0) Traceback (most recent call last): File "
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Syntax For Raise Clause In Python
Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 6.2 million programmers, just like you, helping each other. Join them; it only takes https://docs.python.org/2.7/tutorial/errors.html a minute: Sign up Manually raising (throwing) an exception in Python up vote 802 down vote favorite 190 How can I raise an exception in Python so that it can later be caught via an except block? python exception exception-handling share|improve this question edited Feb 3 '15 at 14:37 DavidRR 5,20472747 asked Jan 12 '10 at 21:07 TIMEX 41.2k201525826 add a http://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python comment| 3 Answers 3 active oldest votes up vote 790 down vote accepted How do I manually throw/raise an exception in Python? Use the most specific Exception constructor that semantically fits your issue. Be specific in your message, e.g.: raise ValueError('A very specific bad thing happened') Don't do this: Avoid raising a generic Exception, to catch it, you'll have to catch all other more specific exceptions that subclass it. Hiding bugs raise Exception('I know Python!') # don't, if you catch, likely to hide bugs. For example: def demo_bad_catch(): try: raise ValueError('represents a hidden bug, do not catch this') raise Exception('This is the exception you expect to handle') except Exception as error: print('caught this error: ' + repr(error)) >>> demo_bad_catch() caught this error: ValueError('represents a hidden bug, do not catch this',) Won't catch and more specific catches won't catch the general exception: def demo_no_catch(): try: raise Exception('general exceptions not caught by specific handling') except ValueError as e: print('we will not catch e') >>> demo_no_catch() Traceback (most recent call last): File "
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a little mini-tutorial for Python programmers, aboutexceptions… First, this isbad: try: some_code() except: revert_stuff() raise Exception("some_code failed!") It is bad because all the information about how some_code() failed is lost. The traceback, the error message itself. Maybe it was an expected error, maybe itwasn't. Here's a modest improvement (but still not verygood): try: some_code() except: import traceback traceback.print_exc() revert_stuff() raise Exception("some_code failed!") traceback.print_exc() prints the original traceback to stderr. Sometimes that's the best you can do, because you really want to recover from an unexpected error. But if you aren't recovering, this is what you shoulddo: try: some_code() except: revert_stuff() raise Using raise with no arguments re-raises the last exception. Sometimes people give a blank never use "except:" statement, but this particular form (except: + raise) isokay. There's another form of raise that not many people know about, but can also be handy. Like raise with no arguments, it can be used to keep thetraceback: try: some_code() except: import sys exc_info = sys.exc_info() maybe_raise(exc_info) def maybe_raise(exc_info): if for some reason this seems like it should be raised: raise exc_info[0], exc_info[1], exc_info[2] This can be handy if you need to handle the exception in some different part of the code from where the exception happened. But usually it's not that handy; it's an obscure feature for areason. Another case when people often clobber the traceback is when they want to add information to it,e.g.: for lineno, line in enumerate(file): try: process_line(line) except Exception, exc: raise Exception("Error in line %s: %s" % (lineno, exc)) You keep the error message here, but lose the traceback. There's a couple ways to keep that traceback. One I sometimes use is to retain the exception, but change themessage: except Exception, exc: args = exc.args if not args: arg0 = '' else: arg0 = args[0] arg0 += ' at line %s' % lineno exc.args = arg0 + args[1:] raise It's a little awkward. Technically (though it's deprecated) you can raise anything as an exception. If you use except Exception: you won't catch things like string exceptions or other weird types. It's up to you to decide if you care about these cases; I generally ignore them. It's also possible that an exception won't have .args, or the string message for the exception won't be derived from those arguments, or that it will be formatted in a funny way (KeyError formats its message differently, for instance). So this isn't foolproof. To be a bit more robus