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Nikita Ignatiev
Nikita Ignatiev

Except Handler 4 Common 12

The if, while and for statements implementtraditional control flow constructs. try specifies exceptionhandlers and/or cleanup code for a group of statements, while thewith statement allows the execution of initialization andfinalization code around a block of code. Function and class definitions arealso syntactically compound statements.

Except Handler 4 Common 12


If the evaluation of an expressionin the header of an except clause raises an exception,the original search for a handler is canceled and a search starts forthe new exception in the surrounding code and on the call stack (it is treatedas if the entire try statement raised the exception).

This means the exception must be assigned to a different name to be able torefer to it after the except clause.Exceptions are cleared because with thetraceback attached to them, they form a reference cycle with the stack frame,keeping all locals in that frame alive until the next garbage collection occurs.

The except* clause(s) are used for handlingExceptionGroups. The exception type for matching is interpreted as inthe case of except, but in the case of exception groups we can havepartial matches when the type matches some of the exceptions in the group.This means that multiple except* clauses can execute,each handling part of the exception group.Each clause executes at most once and handles an exception groupof all matching exceptions. Each exception in the group is handled by at mostone except* clause, the first that matches it.

An except* clause must have a matching type,and this type cannot be a subclass of BaseExceptionGroup.It is not possible to mix except and except*in the same try.break, continue and returncannot appear in an except* clause.

The optional else clause is executed if the control flow leaves thetry suite, no exception was raised, and no return,continue, or break statement was executed. Exceptions inthe else clause are not handled by the preceding exceptclauses.

If the suite was exited due to an exception, and the return value from the__exit__() method was false, the exception is reraised. If the returnvalue was true, the exception is suppressed, and execution continues with thestatement following the with statement.

An exception is a PL/SQL error that is raised during program execution, either implicitly by TimesTen or explicitly by your program. Handle an exception by trapping it with a handler or propagating it to the calling environment.

For example, if your SELECT statement returns multiple rows, TimesTen returns an error (exception) at runtime. As the following example shows, you would see TimesTen error 8507, then the associated ORA error message. (ORA messages, originally defined for Oracle Database, are similarly implemented by TimesTen.)

The RAISE statement stops normal execution of a PL/SQL block or subprogram and transfers control to an exception handler. RAISE statements can raise predefined exceptions, or user-defined exceptions whose names you decide.

In this example, the department number 500 does not exist, so no rows are updated in the departments table. The RAISE statement is used to explicitly raise an exception and display an error message, returned by the SQLERRM built-in function, and an error code, returned by the SQLCODE built-in function. Use the RAISE statement by itself within an exception handler to raise the same exception again and propagate it back to the calling environment.

Use the RAISE_APPLICATION_ERROR procedure in the executable section or exception section (or both) of your PL/SQL program. TimesTen reports errors to your application so you can avoid returning unhandled exceptions.

TimesTen PL/SQL differs from Oracle Database PL/SQL in a scenario where an application executes PL/SQL in the middle of a transaction, and an unhandled exception occurs during execution of the PL/SQL. Oracle Database rolls back to the beginning of the anonymous block. TimesTen does not roll back.

The built-in exception classes can be subclassed to define new exceptions;programmers are encouraged to derive new exceptions from the Exceptionclass or one of its subclasses, and not from BaseException. Moreinformation on defining exceptions is available in the Python Tutorial underUser-defined Exceptions.

The expression following from must be an exception or None. Itwill be set as __cause__ on the raised exception. Setting__cause__ also implicitly sets the __suppress_context__attribute to True, so that using raise new_exc from Noneeffectively replaces the old exception with the new one for displaypurposes (e.g. converting KeyError to AttributeError), whileleaving the old exception available in __context__ for introspectionwhen debugging.

The default traceback display code shows these chained exceptions inaddition to the traceback for the exception itself. An explicitly chainedexception in __cause__ is always shown when present. An implicitlychained exception in __context__ is shown only if __cause__is None and __suppress_context__ is false.

The base class for all built-in exceptions. It is not meant to be directlyinherited by user-defined classes (for that, use Exception). Ifstr() is called on an instance of this class, the representation ofthe argument(s) to the instance are returned, or the empty string whenthere were no arguments.

The tuple of arguments given to the exception constructor. Some built-inexceptions (like OSError) expect a certain number of arguments andassign a special meaning to the elements of this tuple, while others areusually called only with a single string giving an error message.

This method sets tb as the new traceback for the exception and returnsthe exception object. It was more commonly used before the exceptionchaining features of PEP 3134 became available. The following exampleshows how we can convert an instance of SomeException into aninstance of OtherException while preserving the traceback. Onceraised, the current frame is pushed onto the traceback of theOtherException, as would have happened to the traceback of theoriginal SomeException had we allowed it to propagate to the caller.

The name and path attributes can be set using keyword-onlyarguments to the constructor. When set they represent the name of the modulethat was attempted to be imported and the path to any file which triggeredthe exception, respectively.

Raised when the user hits the interrupt key (normally Control-C orDelete). During execution, a check for interrupts is maderegularly. The exception inherits from BaseException so as to not beaccidentally caught by code that catches Exception and thus preventthe interpreter from exiting.

This exception is derived from RuntimeError. In user defined baseclasses, abstract methods should raise this exception when they requirederived classes to override the method, or while the class is beingdeveloped to indicate that the real implementation still needs to be added.

The constructor often actually returns a subclass of OSError, asdescribed in OS exceptions below. The particular subclass depends onthe final errno value. This behaviour only occurs whenconstructing OSError directly or via an alias, and is notinherited when subclassing.

For exceptions that involve a file system path (such as open() oros.unlink()), filename is the file name passed to the function.For functions that involve two file system paths (such asos.rename()), filename2 corresponds to the secondfile name passed to the function.

Changed in version 3.4: The filename attribute is now the original file name passed tothe function, instead of the name encoded to or decoded from thefilesystem encoding and error handler. Also, the filename2constructor argument and attribute was added.

Raised when the result of an arithmetic operation is too large to berepresented. This cannot occur for integers (which would rather raiseMemoryError than give up). However, for historical reasons,OverflowError is sometimes raised for integers that are outside a requiredrange. Because of the lack of standardization of floating point exceptionhandling in C, most floating point operations are not checked.

This exception is raised when a weak reference proxy, created by theweakref.proxy() function, is used to access an attribute of the referentafter it has been garbage collected. For more information on weak references,see the weakref module.

A call to sys.exit() is translated into an exception so that clean-uphandlers (finally clauses of try statements) can beexecuted, and so that a debugger can execute a script without running the riskof losing control. The os._exit() function can be used if it isabsolutely positively necessary to exit immediately (for example, in the childprocess after a call to os.fork()).

This exception may be raised by user code to indicate that an attemptedoperation on an object is not supported, and is not meant to be. If an objectis meant to support a given operation but has not yet provided animplementation, NotImplementedError is the proper exception to raise.

Changed in version 3.5: Python now retries system calls when a syscall is interrupted by asignal, except if the signal handler raises an exception (see PEP 475for the rationale), instead of raising InterruptedError.

The following are used when it is necessary to raise multiple unrelatedexceptions. They are part of the exception hierarchy so they can behandled with except like all other exceptions. In addition,they are recognised by except*, which matchestheir subgroups based on the types of the contained exceptions.

Both of these exception types wrap the exceptions in the sequence excs.The msg parameter must be a string. The difference between the twoclasses is that BaseExceptionGroup extends BaseException andit can wrap any exception, while ExceptionGroup extends Exceptionand it can only wrap subclasses of Exception. This design is so thatexcept Exception catches an ExceptionGroup but notBaseExceptionGroup.

The BaseExceptionGroup constructor returns an ExceptionGrouprather than a BaseExceptionGroup if all contained exceptions areException instances, so it can be used to make the selectionautomatic. The ExceptionGroup constructor, on the other hand,raises a TypeError if any contained exception is not anException subclass.


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