Python exception handling try...except, raise

Source: Internet
Author: User
Tags integer division

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


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

Operation Result:

Traceback (most recent):
File "c:/users/lirong/pycharmprojects/untitled/", line 3, <module>
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

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.


12345678910 a=10b=0try:    =b/a    printcexcept(IOError ,ZeroDivisionError),x:    print xelse:    print"no error"print"done"

Operation Result:

No error

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/", line 3, <module>
Raise ValueError

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

Please input a int data: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/", 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:


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

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