Python Exception Handling

Python Exception Handling

Python provides two very important features to handle the exceptions and errors that Python programs run in. You can use this feature to debug a python program.

• Exception handling
• Assertion (assertions)

Python Standard exception

Exception name	Description
Baseexception	base class for all exceptions
Systemexit	Interpreter Request exited
Keyboardinterrupt	User interrupt execution (usually input ^c)
Exception	base class for general errors
Stopiteration	There are no more values for iterators
Generatorexit	Generator (generator) exception occurred to notify exit
StandardError	Base class for all built-in standard exceptions
Arithmeticerror	base class for all numeric calculation errors
Floatingpointerror	Floating-point calculation error
Overflowerror	Numeric operation exceeds maximum limit
Zerodivisionerror	Except (or modulo) 0 (all data types)
Assertionerror	Assertion statement failed
Attributeerror	Object does not have this property
Eoferror	No built-in input, EOF Mark reached
EnvironmentError	Base class for operating system errors
IOError	Input/output operation failed
OSError	Operating system error
Windowserror	System call failed
Importerror	Failed to import module/object
Lookuperror	base class for invalid data queries
Indexerror	This index is not in the sequence (index)
Keyerror	This key is not in the map
Memoryerror	Memory overflow error (not fatal for Python interpreter)
Nameerror	Object not declared/initialized (no attributes)
Unboundlocalerror	To access uninitialized local variables
Referenceerror	Weak references (Weak reference) attempt to access objects that have been garbage collected
RuntimeError	General run-time errors
Notimplementederror	Methods that have not been implemented
SyntaxError	Python syntax error
Indentationerror	Indentation Error
Taberror	Tab and Space Mix
Systemerror	General Interpreter system error
TypeError	An operation that is not valid for type
ValueError	Invalid parameter passed in
Unicodeerror	Unicode-related errors
Unicodedecodeerror	Error in Unicode decoding
Unicodeencodeerror	Unicode encoding Error
Unicodetranslateerror	Unicode Conversion Error
Warning	Base class for warnings
Deprecationwarning	Warnings about deprecated features
Futurewarning	Warning about the change in the construction of future semantics
Overflowwarning	Old warning about auto-promotion to Long integer
Pendingdeprecationwarning	Warnings about attributes that will be discarded
Runtimewarning	Warning for suspicious run-time behavior (runtime behavior)
Syntaxwarning	Warning of suspicious syntax
Userwarning	Warnings generated by user code

What is an exception?

An exception is an event that occurs during program execution and affects the normal execution of the program.

In general, an exception occurs when Python does not handle the program properly.

The exception is a Python object that represents an error.

When a Python script exception occurs, we need to capture and process it, or the program terminates execution.

Exception handling

You can use the Try/except statement to catch an exception.

The try/except statement is used to detect errors in a try statement block, allowing the except statement to catch exception information and handle it.

If you do not want to end your program when an exception occurs, simply capture it in a try.

Grammar:

The following is a simple syntax for try....except...else :

Try :< statements >        # Run other code except < name >:< statement >        # if the ' name ' exception is thrown in the try section except < name >,< data >:< statement >        # if the ' name ' exception is thrown, get additional data Else :< statement >        # If no exception occurs

Try works by starting a try statement, and Python is tagged in the context of the current program so that when an exception occurs, it can go back here, the TRY clause executes first, and what happens next depends on whether an exception occurs at execution time.

• If an exception occurs when the statement after the try is executed, Python jumps back to the try and executes the first except clause that matches the exception, and the control flow passes through the entire try statement (unless a new exception is thrown when the exception is handled).
• If an exception occurs in the statement after the try, but there is no matching except clause, the exception will be submitted to the upper try, or to the top of the program (This will end the program and print the default error message).
• If no exception occurs when the TRY clause executes, Python executes the statement after the Else statement (if there is else), and then the control flow passes through the entire try statement.

Instance

Here is a simple example that opens a file where the contents of the file are written and the exception does not occur:

#!/usr/bin/python#-*-coding:utf-8-*-Try: FH= Open ("testfile","W") Fh.write ("This is a test file for testing exceptions !")exceptIOError:Print "Error: Failed to find file or read file"Else:    Print "content written to file succeeded"fh.close ()

The above program output results:

$ python test.py content written to file successfully $ cat       testfile # View what is written this is a test file that is used to test for exceptions!!

Instance

Here is a simple example that opens a file where the contents of the file are written, but the file does not have write permission, and an exception occurs:

#!/usr/bin/python#-*-coding:utf-8-*-Try: FH= Open ("testfile","W") Fh.write ("This is a test file for testing exceptions !")exceptIOError:Print "Error: Failed to find file or read file"Else:    Print "content written to file succeeded"fh.close ()

In order to test the convenience before executing the code, we can first remove the testfile file Write permission, the command is as follows:

-W testfile

Then execute the above code:

$ python test.  Error: failed to find file or read file   

Use except without any exception type

You can use except without any exception type, as in the following example:

Try :    normal operation    ......... ........ except : An    exception occurred, execution of this code   ... ...... ..... Else :    If there is no exception to execute this piece of code

The try-except statement above captures all occurrences of the exception. But this is not a good way to identify specific exception information through the program. Because it catches all the exceptions.

Using except with multiple exception types

You can also use the same except statement to handle multiple exception information, as follows:

Try :    normal operation    ......... ........ except (exception1[, exception2[,... Exceptionn]]): The   occurrence of one of the above multiple exceptions, the execution of   this piece of code ....... Else :    If there is no exception to execute this piece of code

try-finally statements

The try-finally statement executes the final code regardless of whether an exception occurs.

Try :< statement >finally:< statement >    # always executes raise when exiting a try 

Instance

#!/usr/bin/python#-*-coding:utf-8-*-Try: FH= Open ("testfile","W") Fh.write ("This is a test file for testing exceptions !")finally:    Print "Error: Failed to find file or read file"If the open file does not have writable permissions, the output is as follows: $ python test.py Error: No file found or read file failed the same example can be written as follows:#!/usr/bin/python#-*-coding:utf-8-*-Try: FH= Open ("testfile","W")    Try: Fh.write ("This is a test file for testing exceptions !")    finally:        Print "Close File"fh.close ()exceptIOError:Print "Error: Failed to find file or read file"

Executes the finally block code immediately when an exception is thrown in the try block.

After all the statements in the finally block are executed, the exception is triggered again, and the except block code is executed.

The contents of the parameter differ from the exception.

parameter of the exception

An exception can take a parameter that can be used as the output exception information parameter.

You can use the except statement to catch the parameters of the exception, as follows:

Try :    normal operation    ......... ........ except Exceptiontype, Argument:    You can output the value of Argument in this ...

The exception value that the variable receives is usually contained in the exception's statement. In a tuple's form, a variable can receive one or more values.

Tuples typically contain error strings, error numbers, and error locations.

Instance

The following is an instance of a single exception:

#!/usr/bin/python#-*-coding:utf-8-*-#Defining FunctionsdefTemp_convert (Var):Try:        returnInt (VAR)exceptValueError, Argument:Print "parameter does not contain a number \ n", Argument#calling FunctionsTemp_convert ("XYZ"); The above program execution results are as follows: The $ python test.py parameter does not contain a digital invalid literal forInt () with base 10:'XYZ'

Triggering an exception

We can use the raise statement to trigger the exception ourselves

The raise syntax format is as follows:

Raise[Exception[,[, traceback]      ]

The Exception statement is either of the exception type (for example, nameerror) parameter standard exception, and args is a self-supplied exception parameter.

The last parameter is optional (rarely used in practice) and, if present, is the tracking exception object.

Instance

An exception can be a string, a class, or an object. The Python kernel provides exceptions, most of which are instantiated classes, which are parameters of an instance of a class.

It is very simple to define an exception as follows:

def functionname (level):     if level < 1:        raise Exception ("Invalid level! " , level)         # after the exception is triggered, the following code will no longer execute

Note: to be able to catch exceptions, the "except" statement must have the same exception to throw the class object or string.

For example, we capture the above exception, and the "except" statement looks like this:

Try :    normal logic except  exception,err:    Trigger Custom    exception else:    The rest of the code

Instance

#!/usr/bin/python#-*-coding:utf-8-*-#Defining FunctionsdefMye (level):ifLevel < 1:        RaiseException,"Invalid level!"        #after the exception is triggered, the following code will no longer executeTry: Mye (0)#triggering an exceptionexceptException,err:Print1, ErrElse:    Print2execute the above code, the output is: $ python test.py1 Invalid level!

User-defined exceptions

By creating a new Exception class, programs can name their own exceptions. Exceptions should be typical of inheriting from the exception class, either directly or indirectly.

The following is an example of a runtimeerror-related instance in which a class is created and the base class is RuntimeError, which is used to output more information when the exception is triggered.

In the TRY statement block, after the user-defined exception executes the EXCEPT block statement, the variable e is used to create an instance of the Networkerror class.

class Networkerror (runtimeerror):     def __init__ (Self, arg):         = arg After you define the above class, you can trigger the exception as follows:try:    raise networkerror ("  Bad hostname")except  networkerror,e:    Print E.args

Python Exception Handling