Python Raise Error Syntax
Contents |
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, are perhaps the most python error types common kind of complaint you get while you are still learning Python: >>> while python raise custom exception True print 'Hello world' File "
Python Raise Valueerror
offending line and displays a little ‘arrow' pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the
Syntax For Generic Except Clause In Python
arrow: in the example, the error is detected at the keyword print, since a colon (':') is missing before it. File name and line number are printed so you know where to look in case the input came from a script. 8.2. Exceptions¶ Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected syntax for raise clause in python 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 "
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the
Python Exception Message
workings and policies of this site About Us Learn more about is nested try block possible in python Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow Questions python print exception Jobs Documentation Tags 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. https://docs.python.org/2.7/tutorial/errors.html Join them; it only takes a minute: Sign up How to use “raise” keyword in Python up vote 92 down vote favorite 10 So I have read the official definition of "raise", but I still don't quite understand what it is doing. In simplest terms, what is "raise"? A small example of it's use would help too. python keyword http://stackoverflow.com/questions/13957829/how-to-use-raise-keyword-in-python raise share|improve this question edited Sep 9 '13 at 15:44 Nakilon 19.7k86288 asked Dec 19 '12 at 17:27 Capurnicus 1,1883810 12 It's known as throw in some other languages. –voithos Dec 19 '12 at 17:29 add a comment| 5 Answers 5 active oldest votes up vote 120 down vote accepted 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, e: if not can_handle(e): raise handle_exception(e) share|improve this answer edited Jan 9 '15 at 9:06 Phalgun 5971618 answered Dec 19 '12 at 17:32 Ignacio Vazquez-Abrams 447k64800956 12 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 –Curtis Yallop Feb 28 '14 at 16
program can't really proceed normally. For an overview, see Section 25, “Exceptions: Error signaling and https://infohost.nmt.edu/tcc/help/pubs/python/web/raise-statement.html handling”. There are three forms of the raise statement: raise https://jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/ 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. python raise The 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() clause in python class instance 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)
Cleaner Python: Use Exceptions Many programmers have had it drilled into their head that exceptions, in any language, should only be used in truly exceptional cases. They're wrong. The Python community's approach to exceptions leads to cleaner code that's easier to read. And that's without the monstrous hit to performance commonly associated with exceptions in other languages. EDIT: Updated with more useful exception idioms Using exceptions to write cleaner code? When I talk about "using exceptions", I'm specifically not referring to creating some crazy exception hierarchy for your package and raising exceptions at every possible opportunity. That will most certainly lead to unmaintainable and difficult to understand code. This notion has been widely discussed and is well summarized on Joel Spolsky's blog. Note: Python avoids much of the tension of the "error codes vs exceptions" argument. Between the ability to return multiple values from a function and the ability to return values of different types (e.g. None or something similar in the error case) the argument is moot. But this is besides the point. The style of exception usage I'm advocating is quite different. In short: take advantage of Python built-ins and standard library modules that already throw exceptions. Exceptions are built in to Python at the lowest levels. In fact, I guarantee your code is already using exceptions, even if not explicitly. Intermezzo: How the for statement works Any time you use for to iterate over an iterable (basically, all sequence types and anything that defines __iter__() or __getitem__()), it needs to know when to stop iterating. Take a look at the code below: words = ['exceptions', 'are', 'useful'] for word in words: print(word) How does for know when it's reached the last element in words and should stop trying to get more items? The answer may surprise you: the list raises a StopIteration except