Python Custom Error
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Python Custom Exception Best Practices
posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss python exception class 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 python exception class methods takes a minute: Sign up Proper way to declare custom exceptions in modern Python? up vote 595 down vote favorite 158 What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever
Python Exception Message
standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception. By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to
Python Error Vs Exception
the exception. I was tripped up by the following deprecation warning in Python 2.6.2: >>> class MyError(Exception): ... def __init__(self, message): ... self.message = message ... >>> MyError("foo") _sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6 It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh. I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3. Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary? python exception share|improve this question edited Jun 5 '12 at 20:37 Eitan T 27.9k113978 asked Aug 23 '09 at 21:29 Nelson 6,43532027 2 *args (or *foo, or *whatever, all that matters is that it has the star in front) is for functions that have an indefinite number of positional arguments. So if you have def myfunction(*args), you can call it like myfunction("foo") or myfunction("foo", "bar") and the arguments will be accessible in the body of the function as the tuple args. See docs.python.org/tutorial/… for more
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 syntax for generic except clause in python the most common kind of complaint you get while you are still learning
Python Print Exception
Python: >>> while True print 'Hello world' File "
output an error when something in it goes wrong. However, sometimes you may need to create custom exceptions that serves your purpose. In Python, users can http://www.programiz.com/python-programming/user-defined-exception define such exceptions by creating a new class. This exception class has to be derived, either directly or indirectly, from Exception class. Most of the built-in exceptions are also derived form this class. http://www.dummies.com/programming/python/how-to-create-and-use-custom-exceptions-in-python/ >>> class CustomError(Exception): ... pass ... >>> raise CustomError Traceback (most recent call last): ... __main__.CustomError >>> raise CustomError("An error occurred") Traceback (most recent call last): ... __main__.CustomError: An error occurred Here, we have python exception created a user-defined exception called CustomError which is derived from the Exception class. This new exception can be raised, like other exceptions, using the raise statement with an optional error message. When we are developing a large Python program, it is a good practice to place all the user-defined exceptions that our program raises in a separate file. Many standard modules do this. They define their exceptions separately python exception class as exceptions.py or errors.py (generally but not always). User-defined exception class can implement everything a normal class can do, but we generally make them simple and concise. Most implementations declare a custom base class and derive others exception classes from this base class. This concept is made clearer in the following example. Example: User-Defined Exception in Python In this example, we will illustrate how user-defined exceptions can be used in a program to raise and catch errors. This program will ask the user to enter a number until they guess a stored number correctly. To help them figure it out, hint is provided whether their guess is greater than or less than the stored number. # define Python user-defined exceptions class Error(Exception): """Base class for other exceptions""" pass class ValueTooSmallError(Error): """Raised when the input value is too small""" pass class ValueTooLargeError(Error): """Raised when the input value is too large""" pass # our main program # user guesses a number until he/she gets it right # you need to guess this number number = 10 while True: try: i_num = int(input("Enter a number: ")) if i_num < number: raise ValueTooSmallError elif i_num > number: raise ValueTooLargeE
WorkSocial MediaSoftwareProgrammingWeb Design & DevelopmentBusinessCareersComputers Online Courses B2B Solutions Shop for Books San Francisco, CA Brr, it´s cold outside Search Submit RELATED ARTICLES How to Create and Use Custom Exceptions in Python Python For Kids For Dummies Beginning Programming with Python For Dummies Beginning Programming with Python For Dummies Cheat Sheet Python for Data Science For Dummies Cheat Sheet Load more ProgrammingPythonHow to Create and Use Custom Exceptions in Python How to Create and Use Custom Exceptions in Python Related Book Beginning Programming with Python For Dummies By John Paul Mueller Python provides a wealth of standard exceptions that you should use whenever possible. These exceptions are incredibly flexible, and you can even modify them as needed (within reason) to meet specific needs. However, sometimes you simply must create a custom exception because none of the standard exceptions will work. Perhaps the exception name just doesn't tell the viewer the purpose that the exception serves. You may need a custom exception for specialized database work or when working with a service. This example shows a quick method for creating your own exceptions. To perform this task, you must create a class that uses an existing exception as a starting point. To make things a little easier, this example creates an exception that builds upon the functionality provided by the ValueError exception. The advantage of using this approach is that this approach tells anyone who follows you precisely what the addition to the ValueError exception is; additionally, it makes the modified exception easier to use. Open a Python File window. You see an editor in which you can type the example code. Type the following code into the window -- pressing Enter after each line: class CustomValueError(ValueError): def __init__(self, arg): self.strerror = arg self.args = {arg} try: raise CustomValueError("Value must be within 1 and 10.") except CustomValueError as e: print("CustomValueError Exception!", e.strerror) This example places the same error in both strerror and args so that the developer has access to either (as would normally happen). The code begins by creating the CustomValueError class that uses the ValueError exception class as a starting point. The __init__() function provides the means for creating a new instance of that class. Think of the class as a blueprint and the instance as the building created from the blueprint. Notice that the strerror attribute has the value assigned directly to it, but args receives it as an array. The args member normally contains an array of al