Python Return Error Message
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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, are perhaps the most common kind python error types of complaint you get while you are still learning Python: >>> while True print
Python Exception Message
'Hello world' File "
Python Custom Exception
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 "
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 to examine the exception python print exception from code, you could have: 1 (x,y) = (5,0) 2 try: 3 z = x/y python try without except 4 except ZeroDivisionError as e: 5 z = e # representation: "
Python Try Except Else
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 as possible (CatchWhatYouCanHandle). In the first example above, if https://docs.python.org/2.7/tutorial/errors.html 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 to continue to run, if at all possible. But you have no idea https://wiki.python.org/moin/HandlingExceptions 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/exceptions.html Look to class documentation to find out what exceptions a given class can raise. See Also: On this wiki: WritingExceptionClasses, TracebackModule. For general (non-Python sCleaner Python: Use Exceptions Many programmers have had it drilled into their head that exceptions, in any language, should only https://jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/ 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 python exception 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 python return error 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