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Python Raise Valueerror

hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join syntax for generic except clause in python 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 a minute: Sign up Manually raising (throwing) an

Python Exception Message

exception in Python up vote 800 down vote favorite 189 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 comment| 3 Answers 3 active oldest votes up vote 787 down vote syntax for raise clause in python 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 "", line 1, in File "", line 3, in demo_no_catch Exception: general exceptions not caught by specific handling Best Practice: Instead, use the most specific Exception constructor that semantically fits your issue. raise ValueError('A very speci

Pages Local Site Map ------------------------ Rename Page Delete Page ------------------------ ------------------------ Remove Spam Revert to this revision ------------------------ SlideShow User Login Handling Exceptions The simplest way to handle exceptions is with a "try-except" block: 1 (x,y) = (5,0) 2 try: 3 z = x/y 4 except ZeroDivisionError: 5 print "divide by zero" If you wanted

Python Exception Stack Trace

to examine the exception from code, you could have: 1 (x,y) = (5,0) 2

Python Print Exception

try: 3 z = x/y 4 except ZeroDivisionError as e: 5 z = e # representation: "" 6 print is nested try block possible in python z # output: "integer division or modulo by zero" General Error Catching Sometimes, you want to catch all errors that could possibly be generated, but usually you don't.In most cases, you want to be as specific http://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python as possible (CatchWhatYouCanHandle). In the first example above, if you were using a catch-all exception clause and a user presses Ctrl-C, generating a KeyboardInterrupt, you don't want the program to print "divide by zero". However, there are some situations where it's best to catch all errors. For example, suppose you are writing an extension module to a web service. You want the error information to output the output web page, and the server https://wiki.python.org/moin/HandlingExceptions to continue to run, if at all possible. But you have no idea what kind of errors you might have put in your code. In situations like these, you may want to code something like this: 1 import sys 2 try: 3 untrusted.execute() 4 except: # catch *all* exceptions 5 e = sys.exc_info()[0] 6 write_to_page( "

Error: %s

" % e ) MoinMoin software is a good example of where general error catching is good. If you write MoinMoin extension macros, and trigger an error, MoinMoin will give you a detailed report of your error and the chain of events leading up to it. Python software needs to be able to catch all errors, and deliver them to the recipient of the web page. Another case is when you want to do something when code fails: 1 try: 2 do_some_stuff() 3 except: 4 rollback() 5 raise 6 else: 7 commit() By using raise with no arguments, you will re-raise the last exception. A common place to use this would be to roll back a transaction, or undo operations. If it's a matter of cleanup that should be run regardless of success or failure, then you would do: 1 try: 2 do_some_stuff() 3 finally: 4 cleanup_stuff() Finding Specific Exception Names Standard exceptions that can be raised are detailed at: http://docs.python.org/library/

program can't really proceed normally. For an overview, see Section 25, “Exceptions: Error signaling and handling”. https://infohost.nmt.edu/tcc/help/pubs/python/web/raise-statement.html There are three forms of the raise statement: raise raise E1 raise E1, E2 The first form is equivalent to “raise None,None” and the second form is equivalent to “raise E1, None”. Each form raises an exception of a given type and with a given value. The python raise type and value depend on how many expressions you provide: E1E2Exception typeException valueNoneNone Re-raise the current exception, if any. This might be done, for example, inside an except, else, or finally block; see Section 23.8, “The try statement: Anticipate exceptions”. classNone E1 E1() class instance clause in python of E1 E1E2classtupleE1 E1(*E2) classnone of the aboveE1 E1(E2) instanceNone type(E1) E1 The current recommended practice is to use a raise statement of this form: raise E(...) where E is some class derived from the built-in Exception class: you can use one of the built-in exceptions, or you can create your own exception classes. For classes derived from Exception, the constructor takes one argument, an error message—that is, a string explaining why the exception was raised. The resulting instance makes that message available as an attribute named .message. Example: >>> try: ... raise ValueError('The day is too frabjous.') ... except ValueError as x: ... pass ... >>> type(x) >>> x.message 'The day is too frabjous.' To create your own exceptions, write a class that inherits from Exception and passes its argum

 

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