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: This site Python tutorial will be described in detail.

Assertion (Assertions): This site Python tutorial will be described in detail.

Python Standard exception

Exception name

Describe

Baseexception base class for all exceptions

Systemexit Interpreter Request Exit

Keyboardinterrupt user interrupt execution (usually input ^c)

Exception base class for general errors

Stopiteration iterator with no more values

Generatorexit Generator (generator) exception occurred to notify exit

Systemexit Python interpreter request to 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 has no built-in input to reach the EOF tag

EnvironmentError base class for operating system errors

IOError Input/output operation failed

OSError Operating system error

Windowserror system call failed

Importerror Import Module/object failed

Keyboardinterrupt user interrupt execution (usually input ^c)

Lookuperror base class for invalid data queries

There is no this index (index) in the indexerror sequence

This key is not in the Keyerror map

Memoryerror Memory overflow error (not fatal for Python interpreter)

Nameerror object not declared/initialized (no attributes)

Unboundlocalerror access to uninitialized local variables

Referenceerror Weak reference (Weak reference) attempts to access objects that have been garbage collected

RuntimeError General run-time errors

Notimplementederror methods that have not yet been implemented

SyntaxError Python Syntax error

Indentationerror Indent Error

Taberror Tab and Space mix

Systemerror General Interpreter System error

TypeError operations that are not valid for types

ValueError invalid arguments passed in

Unicodeerror Unicode-related errors

Unicodedecodeerror Unicode decoding Error

Unicodeencodeerror Unicode Encoding Error

Unicodetranslateerror Unicode Conversion Error

Base class for Warning warnings

Deprecationwarning warning 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 runtime behavior (runtime behavior)

Syntaxwarning warning of suspicious syntax

Userwarning warning for user code generation

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 > #运行别的代码

Except < name:

< statement > #如果在try部份引发了 ' name ' exception

Except < name >,< data;:

< statement > #如果引发了 ' name ' exception to get additional data

Else

< statements > #如果没有异常发生

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

Try

FH = open ("Testfile", "W")

Fh.write ("This was my test file for exception handling!!")

Except IOError:

print "error:can\ ' t find file or read data"

Else

Print "Written content in the file successfully"

Fh.close ()

The above program output results:

Written content in the file successfully

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

Try

FH = open ("Testfile", "W")

Fh.write ("This was my test file for exception handling!!")

Except IOError:

print "error:can\ ' t find file or read data"

Else

Print "Written content in the file successfully"

The above program output results:

Error:can ' t find file or read data

Use except without any exception type

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

Try

your operations here;

......................

Except

If there is any exception and then execute this block.

......................

Else

If there is no exception then execute this block.

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

your operations here;

......................

Except (exception1[, exception2[,... Exceptionn]]):

If there is a exception from the given exception list,

Then execute the this block.

......................

Else

If there is no exception then execute this block.

try-finally statements

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

Try

< statements >

Finally

< statements > #退出try时总会执行

Raise

Note: You can use the except statement or the finally statement, but both cannot be used at the same time. Else statement cannot be used in conjunction with the finally statement

Instance

#!/usr/bin/python

Try

FH = open ("Testfile", "W")

Fh.write ("This was my test file for exception handling!!")

Finally

print "error:can\ ' t find file or read data"

If the open file does not have writable permissions, the output is as follows:

Error:can ' t find file or read data

The same example can be written in the following way:

#!/usr/bin/python

Try

FH = open ("Testfile", "W")

Try

Fh.write ("This was my test file for exception handling!!")

Finally

Print "Going to close the file"

Fh.close ()

Except IOError:

print "error:can\ ' t find file or read data"

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 raised 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

your operations here;

......................

Except Exceptiontype, Argument:

You can print value of Argument here ...

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

# Define a function here.

def Temp_convert (Var):

Try

return Int (VAR)

Except ValueError, Argument:

Print "The argument does not contain numbers\n", argument

# Call above function here.

Temp_convert ("xyz");

The results of the above program execution are as follows:

The argument does not contain numbers

Invalid literal for int. () with base: ' XYZ '

Triggering an exception

We can use the raise statement to trigger the exception ourselves

The raise syntax format is as follows:

raise [Exception [, Args [, Traceback]]

The type of exception in the statement is an exception (for example, the Nameerror) parameter is an exception parameter value. This parameter is optional and if not provided, the exception parameter is "None".

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 "Invalid level!", Level

# The code below to this would isn't be executed

# If we raise the exception

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

Business Logic-here ...

Except "Invalid level!":

Exception Handling here ...

Else

Rest of the code here ...

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):

Self.args = 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

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