Python Notes-Exception handling

Exception handling

Exception is the program error does not work properly, exception handling is through a number of methods to capture the error occurred, friendly display or appropriate processing, so that the program can run longer.

1. type of exception

Common:

SyntaxError Syntax error

Indentationerror Indent Error

TypeError Object type does not conform to requirements

Importerror Module or package import error; generic path or name error

Keyerror keys that do not exist in the dictionary

Nameerror variable does not exist

Indexerror subscript out of sequence range

IOError input / output exception, usually cannot open file

There are no attributes in the Attributeerror object

Keyboardinterrupt keyboard accepted to CTRL + C

Exception generic exception type; Generally catches all exceptions

See all of the exception types:

>>> import exceptions>>> dir (exceptions) [' Arithmeticerror ',  ' Assertionerror ',  ' attributeerror ',  ' baseexception ',  "Buffererror ',  ' BytesWarning ',  ' Deprecationwarning ',  ' eoferror ',  ' environmenterror ',  ' Exception ',,  ' floatingpointerror ',   ' futurewarning ',  ' generatorexit ',  ' IOError ',  ' importerror ',  ' importwarning ',  ' Indentationerror ',  ' indexerror ',  ' keyerror ',  "Keyboardinterrupt ',  ' LookupError ',  ' Memoryerror ',  ' nameerror ',  ' notimplementederror ',  "OSError ',  ' overflowerror ',  ' Pendingdeprecationwarning ',  ' referenceerror ',  ' runtimeerror ',  ' runtimewarning ',  ' StandardError ',  ' stopiteration ',  ' syntaxerror ',  "syntaxwarning ',  ' SystemError ',  ' Systemexit ',  ' taberror ',  ' TypeError ',  "Unboundlocalerror ',  ' unicodedecodeerror ',  ' Unicodeencodeerror ',  ' unicodeerror ',  ' UniCodetranslateerror ',  ' unicodewarning ',  ' userwarning ',  "ValueError ',  ' Warning ',  ' Windowserror ',  ' zerodivisionerror ',  ' __doc__ ',  ' __name__ ',  ' [__package__ ']

2. Catching exception Syntax

Try:pass #被检查的语句except except Type,e: #except type indicates the kind of exception to catch, E is a variable, and the exception information is saved. Pass # ", E" can be written as "E"

For example:

>>> Try:print a #a是一个未定义的变量, directly execute "print a" will report Nameerror exception except Nameerror,e:print "Error:", e error:name ' a ' is n OT defined

Attention:

Multiple except can be written if the exception can occur in more than one category. For example:

>>> a = ' Hello ' >>> try:int (a) except Indexerror,e:print eexcept keyerror,e:print eexcept Valu Eerror,e:print E

If you do not know what is going to happen, you can use exception to catch any exception, for example:

>>> a= ' Hello ' >>> try:int (a) except Exception,e:print E

If the exception is not caught and the exception type does not match, the program directly error, for example:

>>> a= ' Hello ' >>> try:b=int (a) except Indexerror,e:print e Traceback (most recent call last): File "< Pyshell#27> ", line 2, in <module> B=int (a) valueerror:invalid literal for int. () with base: ' Hello '

Other structures to catch exceptions:

#else语句

Try:pass #主代码块except Keyerror,e:passelse:pass # The main code block executes without exception, executing the block.

#finally语句

Try:pass #主代码块except Keyerror,e:passfinally:pass # no matter the exception or not, the block is eventually executed.

#try/except/else/finally Combined statements, it is important to note that the else and finally are placed after except , and finally is placed at the end.

Try:passexcept Keyerror,e:passelse:passfinally:pass

4. proactively triggering exceptions

Raise can actively throw an exception, for example:

>>> Raise Nameerror (' This is a Nameerror ') Traceback (most recent call last): File "<pyshell#2>", line 1, In <module> raise Nameerror (' a nameerror ') Nameerror:this is an nameerror

#捕捉主动触发的异常

>>> try:raise Exception (' ERROR!!! ') Except Exception,e:print e error!!!

5. Custom Exceptions

>>> class myexception (Exception):      #继承Exception类     def __init__ (self,msg):            #使用Exception类的__init__方法         self.message= msg              #添加一个 "message" Property for storing error messages     def __str__ (self):        return  Self.message >>> try:    raise myexception ("myerror!")      #主动引发自定义异常except  MyException,e:         print e myerror! 

6. Assert statement assert

Assert is used to determine whether a conditional judgment statement is true or not, and it is not true to punish an exception, for example:

>>> assert 2>1>>> assert 2<1traceback (most recent): File ' <pyshell#22> ', line 1, In <module> assert 2<1assertionerror

#添加异常描述信息:

>>> assert 2<1, "flase" Traceback (most recent call last): File "<pyshell#23>", line 1, in <module>    ; Assert 2<1, "flase" assertionerror:flase

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Python Notes-Exception handling