The catch code block then catches that exception into variable e. And uses the console. That way you can see what happened. Sometimes on your computer, you might get an error about an un-caught exception right before your program closes. Those are cases like this one where the system. They really do the same thing but have different names.
In the above script, we start the try statement and we attempt to execute the code in that section. If it succeeds, it will print the text trying. It will then jump down to the finally section and print Finally! If the above script is not able to open the test. The next logical question about these statements are what are the roles of each section?
You may find that simply writing fiddlesticks when you have an error is not all that helpful. You need to capture the error so you can write it to a log file, or maybe you want to display the error on the screen. The above script will read test. Unfortunately, if test. Notice the type of error is IOError.
That is helpful because we can create an exception block of code specifically around IOErrors. Lets look at how we would write that code:. The above code will attempt to run what is in the try block.
If it failed with an IOError, it will run the code in the except block. In this case, it will print out the error saying something about how it could not open or close a file. It will then run the finally code block when everything is finished. If we want to have different logic for different kinds of exceptions, we could keep adding similar code like the code below. Notice we call out each type of exception.
Then we have the option to have a different remediation step for each exception type. Raised when the input function hits an end-of-file condition EOF without reading any data. Raised when a generator or coroutine is closed; see generator. It directly inherits from BaseException instead of Exception since it is technically not an error. Raised when the import statement has troubles trying to load a module.
The name and path attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the module that was attempted to be imported and the path to any file which triggered the exception, respectively.
A subclass of ImportError which is raised by import when a module could not be located. It is also raised when None is found in sys. Raised when a sequence subscript is out of range. Slice indices are silently truncated to fall in the allowed range; if an index is not an integer, TypeError is raised. Raised when the user hits the interrupt key normally Control - C or Delete. During execution, a check for interrupts is made regularly. The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.
Raised when an operation runs out of memory but the situation may still be rescued by deleting some objects. The associated value is a string indicating what kind of internal operation ran out of memory. Raised when a local or global name is not found. This applies only to unqualified names. The associated value is an error message that includes the name that could not be found. The name attribute can be set using a keyword-only argument to the constructor. When set it represent the name of the variable that was attempted to be accessed.
This exception is derived from RuntimeError. In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added. NotImplementedError and NotImplemented are not interchangeable, even though they have similar names and purposes.
See NotImplemented for details on when to use it. The second form of the constructor sets the corresponding attributes, described below.
The attributes default to None if not specified. For backwards compatibility, if three arguments are passed, the args attribute contains only a 2-tuple of the first two constructor arguments. The particular subclass depends on the final errno value. This behaviour only occurs when constructing OSError directly or via an alias, and is not inherited when subclassing.
A numeric error code from the C variable errno. Under Windows, this gives you the native Windows error code. Under Windows, if the winerror constructor argument is an integer, the errno attribute is determined from the Windows error code, and the errno argument is ignored. On other platforms, the winerror argument is ignored, and the winerror attribute does not exist. The corresponding error message, as provided by the operating system.
For exceptions that involve a file system path such as open or os. For functions that involve two file system paths such as os. Also, the filename2 constructor argument and attribute was added. Raised when the result of an arithmetic operation is too large to be represented.
This cannot occur for integers which would rather raise MemoryError than give up. However, for historical reasons, OverflowError is sometimes raised for integers that are outside a required range. Because of the lack of standardization of floating point exception handling in C, most floating point operations are not checked. It is raised when the interpreter detects that the maximum recursion depth see sys.
New in version 3. This exception is raised when a weak reference proxy, created by the weakref. For more information on weak references, see the weakref module. The associated value is a string indicating what precisely went wrong. The exception object has a single attribute value , which is given as an argument when constructing the exception, and defaults to None.
When a generator or coroutine function returns, a new StopIteration instance is raised, and the value returned by the function is used as the value parameter to the constructor of the exception. Raised when the parser encounters a syntax error. This may occur in an import statement, in a call to the built-in functions compile , exec , or eval , or when reading the initial script or standard input also interactively. The str of the exception instance returns only the error message. Details is a tuple whose members are also available as separate attributes.
Which line number in the file the error occurred in. This is 1-indexed: the first line in the file has a lineno of 1. The column in the line where the error occurred.
After the except clause s , you can include an else-clause. The code in the else-block executes if the code in the try: block does not raise an exception.
This kind of a try-except statement catches all the exceptions that occur. Using this kind of try-except statement is not considered a good programming practice though, because it catches all exceptions but does not make the programmer identify the root cause of the problem that may occur. You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block raised an exception or not.
When an exception is thrown in the try block, the execution immediately passes to the finally block. After all the statements in the finally block are executed, the exception is raised again and is handled in the except statements if present in the next higher layer of the try-except statement. An exception can have an argument , which is a value that gives additional information about the problem. The contents of the argument vary by exception. If you write the code to handle a single exception, you can have a variable follow the name of the exception in the except statement.
If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception. This variable receives the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location. You can raise exceptions in several ways by using the raise statement.
The general syntax for the raise statement is as follows. Here, Exception is the type of exception for example, NameError and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None. The final argument, traceback, is also optional and rarely used in practice , and if present, is the traceback object used for the exception.
An exception can be a string, a class or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Note: In order to catch an exception, an "except" clause must refer to the same exception thrown either class object or simple string. Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions.
Here is an example related to RuntimeError. Here, a class is created that is subclassed from RuntimeError. This is useful when you need to display more specific information when an exception is caught.
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