Python Default Error Classes
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a try statement with an except clause that mentions a particular class, that clause also handles any exception classes derived from that class (but valueerror python not exception classes from which it is derived). Two exception classes that are
Python Custom Exception
not related via subclassing are never equivalent, even if they have the same name. The built-in exceptions listed below python valueerror example can be generated by the interpreter or built-in functions. Except where mentioned, they have an "associated value" indicating the detailed cause of the error. This may be a string or a python exception message tuple of several items of information (e.g., an error code and a string explaining the code). The associated value is usually passed as arguments to the exception class's constructor. User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition "just like" the situation in which the interpreter raises the same exception; but beware
Python Filenotfounderror
that there is nothing to prevent user code from raising an inappropriate error. The built-in exception classes can be subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions. When raising (or re-raising) an exception in an except or finally clause __context__ is automatically set to the last exception caught; if the new exception is not handled the traceback that is eventually displayed will include the originating exception(s) and the final exception. When raising a new exception (rather than using a bare raise to re-raise the exception currently being handled), the implicit exception context can be supplemented with an explicit cause by using from with raise: raise new_exc from original_exc The expression following from must be an exception or None. It will be set as __cause__ on the raised exception. Setting __cause__ also implicitly sets the __suppress_context__ attribute to True, so that using r
you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. 8.1. python exception class methods Syntax Errors¶ Syntax errors, also known as parsing errors, are perhaps python errno the most common kind of complaint you get while you are still learning Python: >>> while True
Python Attributeerror Object Has No Attribute
print('Hello world') File " Exception StandardError ArithmeticError LookupError EnvironmentError Raised Exceptions AssertionError AttributeError EOFError FloatingPointError GeneratorExit IOError https://pymotw.com/2/exceptions/ ImportError IndexError KeyError KeyboardInterrupt MemoryError NameError NotImplementedError OSError OverflowError ReferenceError RuntimeError StopIteration SyntaxError SystemError SystemExit TypeError UnboundLocalError UnicodeError ValueError ZeroDivisionError Warning Categories Navigation Table of https://julien.danjou.info/blog/2016/python-exceptions-guide Contents Previous: Built-in Objects Next: String Services This Page Show Source Examples The output from all the example programs from PyMOTW has been generated with Python python exception 2.7.8, unless otherwise noted. Some of the features described here may not be available in earlier versions of Python. If you are looking for examples that work under Python 3, please refer to the PyMOTW-3 section of the site. Navigation index modules | next | previous | PyMOTW » Built-in Objects python default error » exceptions - Built-in error classes¶ Purpose:The exceptions module defines the built-in errors used throughout the standard library and by the interpreter. Available In:1.5 and later Description¶ In the past, Python has supported simple string messages as exceptions as well as classes. Since 1.5, all of the standard library modules use classes for exceptions. Starting with Python 2.5, string exceptions result in a DeprecationWarning, and support for string exceptions will be removed in the future. Base Classes¶ The exception classes are defined in a hierarchy, described in the standard library documentation. In addition to the obvious organizational benefits, exception inheritance is useful because related exceptions can be caught by catching their base class. In most cases, these base classes are not intended to be raised directly. BaseException¶ Base class for all exceptions. Implements logic for creating a string representation of the exception using str() from the arguments passed to the constructo it seems to be time for a new one. Here, I would like to dissect and discuss Python exceptions. Dissecting the base exceptions In Python, the base exception class is named BaseException. Being rarely used in any program or library, it ought to be considered as an implementation detail. But to discover how it's implemented, you can go and read Objects/exceptions.c in the CPython source code. In that file, what is interesting is to see that the BaseException class defines all the basic methods and attribute of exceptions. The basic well-known Exception class is then simply defined as a subclass of BaseException, nothing more: /* * Exception extends BaseException */SimpleExtendsException(PyExc_BaseException, Exception, "Common base class for all non-exit exceptions."); The only other exceptions that inherits directly from BaseException are GeneratorExit, SystemExit and KeyboardInterrupt. All the other builtin exceptions inherits from Exception. The whole hierarchy can be seen by running pydoc2 exceptions or pydoc3 builtins. Here are the graph representing the builtin exceptions inheritance in Python 2 and Python 3 (generated using this script). Python 2 builtin exceptions inheritance graph Python 3 builtin exceptions inheritance graph The BaseException.__init__ signature is actually BaseException.__init__(*args). This initialization method stores any arguments that is passed in the args attribute of the exception. This can be seen in the exceptions.c source code – and is true for both Python 2 and Python 3: static intBaseException_init(PyBaseExceptionObject *self, PyObject *args, PyObject *kwds){ if (!_PyArg_NoKeywords(Py_TYPE(self)->tp_name, kwds)) return -1; Py_INCREF(args); Py_XSETREF(self->args, args); return 0;} The only place where this args attribute is used is in the BaseException.__str__ method. This method uses self.args to convert an exception to a string: static PyObject *BaseException_str(PyBaseExceptionObject *self){ switch (PyTuple_GET_SIZE(self->args)) { case 0: return PyUnicode_FromString(""); case 1: return PyObject_Str(PyTuple_GET_ITEM(self->args, 0)); default: return PyObject_Str(self->args); }} This can be translated in Python to: def __str__(self): if len(self.args) == 0: return "" if len(self.args) == 1: return str(self.args[0]) return str(self.args) Therefore, t