Python 简明教程

Python - Exceptions Handling

Exception Handling in Python

Python 中的异常处理是指在程序执行期间可能发生的运行时错误的管理。在 Python 中，当程序执行期间出现错误或意外情况（例如除以零、尝试访问不存在的文件或尝试针对不兼容的数据类型执行操作）时，将会引发异常。

Exception handling in Python refers to managing runtime errors that may occur during the execution of a program. In Python, exceptions are raised when errors or unexpected situations arise during program execution, such as division by zero, trying to access a file that does not exist, or attempting to perform an operation on incompatible data types.

Python 提供了两个非常重要的功能来处理 Python 程序中的任何意外错误，并添加调试功能−

Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −

Exception Handling − This would be covered in this tutorial. Here is a list of standard Exceptions available in Python: Standard Exceptions.
Assertions − This would be covered in Assertions in Python tutorial.

Assertions in Python

断言是一种健全检查，当您完成对程序的测试后，您可以将其开启或关闭。

An assertion is a sanity-check that you can turn on or turn off when you are done with your testing of the program.

最简单的考虑断言的方法是将其比作 raise-if 语句（或更准确地说，比作 raise-if-not 语句）。对某个表达式进行测试，如果结果为假，则引发一个异常。

The easiest way to think of an assertion is to liken it to a raise-if statement (or to be more accurate, a raise-if-not statement). An expression is tested, and if the result comes up false, an exception is raised.

程序员经常在函数开始时放置断言来检查有效输入，并在函数调用后检查有效输出。

Programmers often place assertions at the start of a function to check for valid input, and after a function call to check for valid output.

The assert Statement

当遇到 assert 语句时，Python 会计算随附的表达式，希望它为真。如果表达式为假，Python 会引发 AssertionError 异常。

When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises an AssertionError exception.

assert 的 syntax 是 −

The syntax for assert is −

assert Expression[, Arguments]

如果断言失败，Python 会使用 ArgumentExpression 作为 AssertionError 的参数。可以使用 try-except 语句捕获和处理 AssertionError 异常，就像处理任何其他异常一样，但如果没有处理，它们将终止程序并生成回溯。

If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a trace back.

这是一个将开尔文温度转换为华氏温度的函数。由于绝对零度有多冷，因此如果函数看到负温度，则会中止 −

Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since zero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature −

def KelvinToFahrenheit(Temperature):
   assert (Temperature >= 0),"Colder than absolute zero!"
   return ((Temperature-273)*1.8)+32
print (KelvinToFahrenheit(273))
print (int(KelvinToFahrenheit(505.78)))
print (KelvinToFahrenheit(-5))

执行上述代码后，将生成以下结果 −

When the above code is executed, it produces the following result −

32.0
451
Traceback (most recent call last):
File "test.py", line 9, in <module>
print (KelvinToFahrenheit(-5))
File "test.py", line 4, in KelvinToFahrenheit
assert (Temperature >= 0),"Colder than absolute zero!"
AssertionError: Colder than absolute zero!

What is Exception?

异常是一种事件，它在程序执行期间发生，会中断程序指令的正常流。通常情况下，当 Python 脚本遇到无法处理的情况时，它会引发异常。异常是表示错误的 Python 对象。

An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program’s instructions. In general, when a Python script encounters a situation that it cannot cope with, it raises an exception. An exception is a Python object that represents an error.

当 Python 脚本引发异常时，它必须立即处理该异常，否则它将终止并退出。

When a Python script raises an exception, it must either handle the exception immediately otherwise it terminates and quits.

Handling an Exception in Python

如果您有一些可能引发异常的可疑代码，您可以通过将可疑代码放置在 try ：块中来防御您的程序。在 try ：块后面，包含一个 except ：语句，后面是一个以尽可能优雅的方式处理问题的代码块。

If you have some suspicious code that may raise an exception, you can defend your program by placing the suspicious code in a try: block. After the try: block, include an except: statement, followed by a block of code which handles the problem as elegantly as possible.

The try: block contains statements which are susceptible for exception
If exception occurs, the program jumps to the except: block.
If no exception in the try: block, the except: block is skipped.

Syntax

以下是 try…except…else 块的简单语法 −

Here is the simple syntax of try…except…else blocks −

try:
   You do your operations here
   ......................
except ExceptionI:
   If there is ExceptionI, then execute this block.
except ExceptionII:
   If there is ExceptionII, then execute this block.
   ......................
else:
   If there is no exception then execute this block.

以下关于上述语法是几个重要要点 −

Here are few important points about the above-mentioned syntax −

A single try statement can have multiple except statements. This is useful when the try block contains statements that may throw different types of exceptions.
You can also provide a generic except clause, which handles any exception.
After the except clause(s), you can include an else clause. The code in the else block executes if the code in the try: block does not raise an exception.
The else block is a good place for code that does not need the try: block’s protection.

Example

此示例将打开一个文件，在该文件中写内容，并优雅地退出，因为根本不存在任何问题。

This example opens a file, writes content in the file and comes out gracefully because there is no problem at all.

try:
   fh = open("testfile", "w")
   fh.write("This is 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()

它将生成以下 output −

It will produce the following output −

Written content in the file successfully

但是，将 open() 函数中的模式参数更改为“w”。如果 testfile 尚不存在，则程序在 except 代码块中会遇到 IOError，并打印以下错误消息：

However, change the mode parameter in open() function to "w". If the testfile is not already present, the program encounters IOError in except block, and prints following error message −

Error: can't find file or read data

Example

此示例尝试打开一个文件，而你没有写入权限，因此会引发异常：

This example tries to open a file where you do not have write permission, so it raises an exception −

try:
   fh = open("testfile", "r")
   fh.write("This is 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")

这会产生以下结果：

This produces the following result −

Error: can't find file or read data

The except Clause with No Exceptions

你还可以使用如下所示，不定义任何异常的 except 语句：

You can also use the except statement with no exceptions defined as follows −

try:
   You do your operations here;
   ......................
except:
   If there is any exception, then execute this block.
   ......................
else:
   If there is no exception then execute this block.

此类 try-except 语句会捕捉到发生的全部异常。然而，使用这种 try-except 语句不被视为良好的编程实践，因为它会捕捉到全部异常，但是并不会让程序员识别到可能发生的根本问题。

This kind of a try-except statement catches all the exceptions that occur. Using this kind of try-except statement is not considered a good programming practice though, because it catches all exceptions but does not make the programmer identify the root cause of the problem that may occur.

The except Clause with Multiple Exceptions

你还可以使用相同的 except 语句来处理多个异常，如下所示：

You can also use the same except statement to handle multiple exceptions as follows −

try:
   You do your operations here;
   ......................
except(Exception1[, Exception2[,...ExceptionN]]]):
   If there is any exception from the given exception list,
   then execute this block.
   ......................
else:
   If there is no exception then execute this block.

The try-finally Clause

你可以使用 finally: 代码块和 try: 代码块。finally 代码块是一个存放必须执行的任何代码的地方，无论 try-block 是否引发了异常。try-finally 语句的语法如下：

You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block raised an exception or not. The syntax of the try-finally statement is this −

try:
   You do your operations here;
   ......................
   Due to any exception, this may be skipped.
finally:
   This would always be executed.
   ......................

你无法同时使用 else 子句及 finally 子句。

You cannot use else clause as well along with a finally clause.

Example

try:
   fh = open("testfile", "w")
   fh.write("This is my test file for exception handling!!")
finally:
   print ("Error: can\'t find file or read data")

如果无权以写入模式打开文件，那么这将产生以下结果：

If you do not have permission to open the file in writing mode, then this will produce the following result −

Error: can't find file or read data

可以按如下方式更简洁地编写相同的示例：

Same example can be written more cleanly as follows −

try:
   fh = open("testfile", "w")
   try:
      fh.write("This is 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")

如果在 try 代码块中引发了异常，则执行将立即传递到 finally 代码块。在 finally 代码块中的全部语句执行完毕之后，异常会再次引发，并且在 try-except 语句的下一层中存在时，将在 except 语句中处理异常。

When an exception is thrown in the try block, the execution immediately passes to the finally block. After all the statements in the finally block are executed, the exception is raised again and is handled in the except statements if present in the next higher layer of the try-except statement.

Argument of an Exception

异常可以具有一个参数，它是一个关于问题提供附加信息的值。参数的内容根据异常而有所不同。你通过如下在 except 子句中提供一个变量来捕获异常的参数：

An exception can have an argument, which is a value that gives additional information about the problem. The contents of the argument vary by exception. You capture an exception’s argument by supplying a variable in the except clause as follows −

try:
   You do your operations here;
   ......................
except ExceptionType, Argument:
   You can print value of Argument here...

如果编写代码来处理单个异常，则可以在 except 语句中让一个变量跟在异常名称后面。如果正在捕捉多个异常，则可以让一个变量跟在异常元组的后面。

If you write the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.

此变量接收异常的值，该值大多包含异常的成因。此变量可以接收单个值，或以元组形式接收多个值。此元组通常包含错误字符串、错误号和错误位置。

This variable receives the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.

Example

下面是一个处理单个异常的示例：

Following is an example for a single exception −

# Define a function here.
def temp_convert(var):
   try:
      return int(var)
   except ValueError as Argument:
      print ("The argument does not contain numbers\n", Argument)

# Call above function here.
temp_convert("xyz")

这会产生以下结果：

This produces the following result −

The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'

Raising an Exceptions

你可以使用 raise 语句，通过几种方法引发异常。 raise 语句的通用语法如下。

You can raise exceptions in several ways by using the raise statement. The general syntax for the raise statement is as follows.

Syntax

raise [Exception [, args [, traceback]]]

在此处，Exception 是异常类型（例如，NameError），而 argument 是异常参数值。argument 是可选的；如果不提供，则异常参数为 None。

Here, Exception is the type of exception (for example, NameError) and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.

最后一个参数跟踪也是可选的（实际上很少会使用），如果存在，则它将成为异常所使用的追踪对象。

The final argument, trace back, is also optional (and rarely used in practice), and if present, is the traceback object used for the exception.

Example

异常可以是字符串、类或对象。Python 核心触发的异常大部分都是类，而参数则是类的实例。定义新异常非常简单，可以像下面这样操作：

An exception can be a string, a class or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Defining new exceptions is quite easy and can be done as follows −

def functionName( level ):
   if level < 1:
      raise "Invalid level!", level
      # The code below to this would not be executed
      # if we raise the exception

Note: 为了捕获异常，“except”子句必须引用相同异常所引发的类对象或简单字符串。例如，为了捕获上述异常，我们必须将 except 子句写成如下内容：

Note: In order to catch an exception, an "except" clause must refer to the same exception thrown either class object or simple string. For example, to capture above exception, we must write the except clause as follows −

try:
   Business Logic here...
except "Invalid level!":
   Exception handling here...
else:
   Rest of the code here...

User-Defined Exceptions

Python 还允许你通过从标准内置异常派生类来创建自己的异常。

Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions.

下面是一个与 RuntimeError 有关的示例。此处，创建了一个从 RuntimeError 继承的类。当异常被捕获时，这是在需要显示更多特定信息时有用的。

Here is an example related to RuntimeError. Here, a class is created that is subclassed from RuntimeError. This is useful when you need to display more specific information when an exception is caught.

在 try 块中，自定义异常被触发并在 except 块中被捕获。变量 e 用于创建 Networkerror 类的实例。

In the try block, the user-defined exception is raised and caught in the except block. The variable e is used to create an instance of the class Networkerror.

class Networkerror(RuntimeError):
   def __init__(self, arg):
      self.args = arg

因此，一旦定义了上述类，你就可以如下触发异常：

So once you defined above class, you can raise the exception as follows −

try:
   raise Networkerror("Bad hostname")
except Networkerror,e:
   print (e.args)

Standard Exceptions

下面是 Python 中提供的标准异常列表：

Here is a list of Standard Exceptions available in Python −

Sr.No.	Exception Name & Description
1	Exception Base class for all exceptions
2	StopIteration Raised when the next() method of an iterator does not point to any object.
3	SystemExit Raised by the sys.exit() function.
4	StandardError Base class for all built-in exceptions except StopIteration and SystemExit.
5	ArithmeticError Base class for all errors that occur for numeric calculation.
6	OverflowError Raised when a calculation exceeds maximum limit for a numeric type.
7	FloatingPointError Raised when a floating point calculation fails.
8	ZeroDivisionError Raised when division or modulo by zero takes place for all numeric types.
9	AssertionError Raised in case of failure of the Assert statement.
10	AttributeError Raised in case of failure of attribute reference or assignment.
11	EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached.
12	ImportError Raised when an import statement fails.
13	KeyboardInterrupt Raised when the user interrupts program execution, usually by pressing Ctrl+c.
14	LookupError Base class for all lookup errors.
15	IndexError Raised when an index is not found in a sequence.
16	KeyError Raised when the specified key is not found in the dictionary.
17	NameError Raised when an identifier is not found in the local or global namespace.
18	UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it.
19	EnvironmentError Base class for all exceptions that occur outside the Python environment.
20	IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist.
21	IOError Raised for operating system-related errors.
22	SyntaxError Raised when there is an error in Python syntax.
23	IndentationError Raised when indentation is not specified properly.
24	SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
25	SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.
26	TypeError Raised when an operation or function is attempted that is invalid for the specified data type.
27	ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
28	RuntimeError Raised when a generated error does not fall into any category.
29	NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.