Python exception handling try...except, raise

First, try...except

Sometimes when we write a program, there are some errors or exceptions that cause the program to terminate. For example, when division is done, the divisor is 0, which causes a zerodivisionerror

Example:

1234	a=10b=0c=a/bprint"done"

Operation Result:

Traceback (most recent):
File "c:/users/lirong/pycharmprojects/untitled/openfile.py", line 3, <module>
C=a/b
Zerodivisionerror:integer division or modulo by zero

We found that the program was interrupted because of Zerodivisionerror, and the statement print "done" was not running. To handle the exception, we use Try...except to change the code:

12345678

a=10b=0try:    c=a/b    printcexceptZeroDivisionError,e:    printe.messageprint"done"

　　

Operation Result:

Integer division or modulo by zero
Done

This way, the program does not break because of an exception, and the print "done" statement executes normally.

We put statements that could have errors in the Try module and use except to handle exceptions. Except can handle a specialized exception, or it can handle an exception in a set of parentheses, and if no exception is specified after except, all exceptions are handled by default. Every try must have at least one except

Handling a set of exceptions can be written like this (where E represents an instance of the exception):

1234	try：    passexcept(IOError ,ZeroDivisionError),e:    printe

Try .... except...else statement, when no exception occurs, the statement in else is executed.

Example:

12345678910

a=10b=0try:    c =b/a    printcexcept(IOError ,ZeroDivisionError),x:    print xelse:    print"no error"print"done"

Operation Result:

0
No error
Done

Second, raise throws an exception

Example: If the input data is not an integer, a valueerror is raised

12345

inputvalue = input ( " please input a int Data: " " if  type (inputvalue)! = type ( 1 Code class= "python spaces" >     raise  valueerror else :      print  inputvalue

Suppose you enter 1.2 and run the result as:

Please input a int data:1.2
Traceback (most recent):
File "c:/users/lirong/pycharmprojects/untitled/openfile.py", line 3, <module>
Raise ValueError
ValueError

If you enter 1, the result of the operation is:

Please input a int data:1
1

Third, try ... finally

The statements in finally are executed, regardless of whether the exception occurred or not, until the end of the program.

123456

a=10b=0try:    printa/bfinally:    print"always excute"

Operation Result:

Traceback (most recent):
Always Excute
File "c:/users/lirong/pycharmprojects/untitled/openfile.py", line 4, <module>
Print A/b
Zerodivisionerror:integer division or modulo by zero

Although an exception occurs, the statements in finally can be executed normally before the program terminates.

The finally statement can also be used with the except statement.

12345678

a = b = 0 try :      print  a / b Except :      Code class= "Python functions" >print   finally :      print   "always Excute"

Operation Result:

Error

Always Excute

Iv. Customizing an Exception class

Customize a MyException class, inheriting exception.

1234	class   myexception (Exception):      def   __init__ ( self          exception.__init__ ( self          self .message = message

If the number entered is less than 10, a MyException exception is thrown:

123456

a=input("please input a num:")ifa<10:    try:        raise MyException("my excepition is raised ")    exceptMyException,e:        printe.message

　　

Operation Result:

Please input a num:1
My excepition is raised

Five, Python all the standard exception class:

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
Systemexit	Python Interpreter Request 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
Keyboardinterrupt	User interrupt execution (usually input ^c)
Lookuperror	base class for invalid data queries
Indexerror	There is no such index (index) in the sequence
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

Python exception handling try...except, raise