"Python"--try except (Exception capture)

Try except (Exception capture)

When the program is wrong, but we do not want to let the user see this error, and I write the program has been expected that it can make such a mistake, the occurrence of such an error represents what, we can catch these errors ahead of time

1, exception processing flowchart:

2. Common anomalies

Attributeerror attempts to access a tree that does not have an object, such as foo.x, but Foo does not have a property xioerror input/output exception; it is basically impossible to open the file Importerror the module or package cannot be introduced is basically a path problem or name error Indentationerror syntax error (subclass); The code is not aligned correctly indexerror the subscript index exceeds the sequence boundary, for example, when X has only three elements, it attempts to access the X[5]keyerror Attempting to access a key that does not exist in the dictionary Keyboardinterrupt CTRL + C is pressed Nameerror use a variable that has not been assigned to the object SyntaxError Python code is illegal, the code cannot compile (personally think this is a syntax error, Wrong) TypeError the incoming object type is not compliant with the requirements Unboundlocalerror attempts to access a local variable that is not yet set, basically because another global variable with the same name causes you to think that you are accessing it valueerror a value that the caller does not expect , even if the type of the value is correct

More Exceptions:

Arithmeticerrorassertionerrorattributeerrorbaseexceptionbuffererrorbyteswarningdeprecationwarningenvironmenterroreoferror Exceptionfloatingpointerrorfuturewarninggeneratorexitimporterrorimportwarningindentationerrorindexerrorioerrorkeyboardint Erruptkeyerrorlookuperrormemoryerrornameerrornotimplementederroroserroroverflowerrorpendingdeprecationwarningreferenceerr Orruntimeerrorruntimewarningstandarderrorstopiterationsyntaxerrorsyntaxwarningsystemerrorsystemexittaberrortypeerrorunbou Ndlocalerrorunicodedecodeerrorunicodeencodeerrorunicodeerrorunicodetranslateerrorunicodewarninguserwarningvalueerrorwarni Ngzerodivisionerror

3. Handling a single exception

The syntax is as follows:

Try:    code   #处理的语句except  Error1 as E:   #遇到Error1执行下面的语句, written in Python2 except  error1,e    print (e)

The code is as follows:

name = [1,2,3]try:    name[3]  #不存在3这个下标值except indexerror as E:   #抓取 indexerror This exception    print (e) #e是错误的详细信息 # Output list index out of range

4. Handling multiple exceptions

① write multiple except with the following syntax:

Try:    codeexcept Error1 as E:  #处理Error1异常    print (e) except Error2 as E:   #处理Error2异常    print (e)

The code is as follows:

name = [1,2,3]data = {"A": "B"}try:    data["C"]   #这边已经出现异常KeyError, so jump right out of code and skip to Keyerror to process    name[3] Except Indexerror as E:    print (e) except Keyerror as E:    print (e) #输出 ' C '

② writes 1 except with the following syntax:

Try:    codeexcept (Error1,error2,...) as E:    print (e)

The code is as follows:

Try:    data["C"]    name[3]except (indexerror,keyerror) as E:    print (e) #输出 ' C '

Note: The second way of writing is useful: all errors in parentheses, regardless of the occurrence of any one of the errors are used in a unified processing method.

5, exception abnormal

The syntax is as follows:

Try:    codeexcept (Error1,error2,...) as E:    print (e) except Exception as E:   #用Exception表示一下子抓住所有异常, This is generally recommended in the last face of the exception, with the last catch of the unknown exception    print (e)

The code is as follows:

Try:    open ("Qigao.text", "R", encoding= "Utf-8") except (Indexerror,keyerror) as E:   #没有IndexError, Keyerror These two exceptions    print (e) except Exception as E:  #只能通过这个异常处理, Exception catch all exceptions    print (e) #输出 [Errno 2] No such File or directory: ' Qigao.text '

6. Else role

Function: No exception, then go to the else part of the logic code

Try:    print ("Qigao,handson")    #代码没有异常except (Indexerror,keyerror) as E:    print (e) except Exception as E:    Print (e) Else:             #没有异常出错, go to Else's logic code    print ("no exception") #输出qigao, HandsOn no exception

7. Finnally Effect

Function: Executes code in finnally, regardless of error

The syntax is as follows:

Try:    codeexcept (Error1,error2,...) as E:    print (e) except Exception as E:    print (E) Else:    print ("No error, Execute ") finnally:    print (" Execute finnally whether or not it is wrong ")

　　

① no abnormal conditions

Try:    print ("Qigao,handson")  #没有异常except (Indexerror,keyerror) as E:    print (e) except Exception as E:    Print (e) Else:    print ("no exception") Finally:    print ("No matter what is wrong, this line finnally") #输出qigao, HandsOn no exception, whether or not it is wrong, this line finnally  #没有报错, execute finnally

② abnormal conditions occur

Try:    data = {"A": "B"}    data["C"]   #data字典中没有 ' C ' This key value except (Indexerror,keyerror) as E:    print (e) Except Exception as E:    print (e) Else: print (    "no exception") Finally:    print ("Whatever is wrong, this line finnally") #输出 ' C ' whether or not it's wrong , all this line finnally   #出错了也执行了finnally语句

　　

8. Custom Exceptions

Class Gaoerror (Exception):  #定义一个异常类, inheriting Exception     def __init__ (self,message):        self.message = message     def __str__ (self):        return self.message #给对象取一个名

To trigger a custom exception:

Try:    raise Gaoerror ("Database connection not on")  #触发自定义异常, Gaoerror ("Database connection is not on") this object except Gaoerror as E:    print (e) # The output database is not connected to the

Custom Usage Summary:

1. Database connection not on the information
2. Permission problem, parsing is no permission, give exception prompt
3. Error in business logic

"Python"--try except (Exception capture)