8. Errors and Exceptions 错误和异常
Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions.
到目前为止，错误消息还没有提到，但是如果你已经尝试过这些例子，你可能已经看过了一些。存在（至少）两种可区分的错误：语法错误和异常。
8.1. Syntax Errors 语法错误
Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:
语法错误（也称为解析错误）可能是您在学习Python时最常见的抱怨：
>>> while True print('Hello world')
File "<stdin>", line 1
while True print('Hello world')
^
SyntaxError: invalid syntax
The parser repeats the offending line and displays a little ‘arrow’ pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow: in the example, the error is detected at the function print(), since a colon (':') is missing before it. File name and line number are printed so you know where to look in case the input came from a script.
解析器重复出现违规行，并显示一个指向检测到错误的行中最早点的“箭头”。该错误是由箭头前面的令牌（或至少在其处检测到）引起的：在该示例中，在函数print()处检测到错误，因为在它之前缺少冒号''）。打印文件名和行号，以便当输入来自脚本时知道查找位置。
8.2. Exceptions 异常
Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptionsand are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:
即使语句或表达式在语法上是正确的，但在尝试执行它时可能会导致错误。在执行期间检测到的错误被称为* exceptions *并且不是无条件致命的：您将很快学会如何在Python程序中处理它们。虽然大多数异常都不是由程序处理的，但是会导致错误消息，如下所示:
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly
The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are ZeroDivisionError, NameError and TypeError. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).
错误消息的最后一行表明发生了什么。异常有不同的类型，类型作为消息的一部分打印出来：示例中的类型是ZeroDivisionError，NameError和TypeError。作为异常类型打印的字符串是发生的内置异常的名称。对于所有内置异常都是如此，但对于用户定义的异常不一定是这样（尽管它是一个有用的约定）。标准异常名称是内置标识符（不是保留关键字）。
The rest of the line provides detail based on the type of exception and what caused it.
该行的其余部分根据异常类型及其原因提供详细信息。
The preceding part of the error message shows the context where the exception happened, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.
错误消息的前一部分以堆栈回溯的形式显示发生异常的上下文。通常它包含列出源代码行的堆栈回溯;但是，它不会显示从标准输入读取的行。
Built-in Exceptions lists the built-in exceptions and their meanings.
内置异常列出了内置异常及其含义。
8.3. Handling Exceptions 异常处理
It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using Control-C or whatever the operating system supports); note that a user-generated interruption is signalled by raising the KeyboardInterrupt exception.
可以编写处理所选异常的程序。请查看以下示例，该示例要求用户输入，直到输入有效整数，但允许用户中断程序（使用Control-C或操作系统支持的任何内容）;请注意，通过引发KeyboardInterrupt异常来指示用户生成的中断。
>>> while True:
... try:
... x = int(input("Please enter a number: "))
... break
... except ValueError:
... print("Oops! That was no valid number. Try again...")
...
The try statement works as follows.
try语句的工作原理如下:
- First, the try clause (the statement(s) between the
tryandexceptkeywords) is executed.
首先，执行try子句（try和except关键字之间的语句）。 - If no exception occurs, the except clause is skipped and execution of the
trystatement is finished.
如果没有异常发生，则跳过except子句并完成try语句的执行。 - If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the
exceptkeyword, the except clause is executed, and then execution continues after thetrystatement.
如果在执行try子句期间发生异常，则跳过该子句的其余部分。然后，如果其类型匹配except关键字后面的异常，则执行except子句，然后在try语句之后继续执行。 - If an exception occurs which does not match the exception named in the except clause, it is passed on to outer
trystatements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.
如果发生的异常与except子句中指定的异常不匹配，则将其传递给外部try语句;如果没有找到处理程序，则它是一个未处理的异常，执行将停止并显示如上所示的消息。
A try statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same try statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:
try语句可能有多个except子句，以指定不同异常的处理程序。最多将执行一个处理程序。处理程序仅处理相应try子句中发生的异常，而不处理同一try语句的其他处理程序中的异常。 except子句可以将多个异常命名为带括号的元组，例如：
... except (RuntimeError, TypeError, NameError):
... pass
A class in an except clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around — an except clause listing a derived class is not compatible with a base class). For example, the following code will print B, C, D in that order:
如果一个类是同一个类或它的基类，那么except子句中的类与异常兼容（但不是相反 -- 列出派生类的except子句与基类不兼容）。例如，以下代码将按以下顺序打印B，C，D:
class B(Exception):
pass
class C(B):
pass
class D(C):
pass
for cls in [B, C, D]:
try:
raise cls()
except D:
print("D")
except C:
print("C")
except B:
print("B")
Note that if the except clauses were reversed (with except B first), it would have printed B, B, B — the first matching except clause is triggered.
请注意，如果except子句被颠倒（除了B之外），它将打印B，B，B - 第一个匹配的except子句被触发。
The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):
最后一个except子句可以省略异常名称，以用作通配符。请谨慎使用，因为以这种方式很容易掩盖真正的编程错误！它还可以用于打印错误消息，然后重新引发异常（允许调用者处理异常）
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">import sys
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error: {0}".format(err))
except ValueError:
print("Could not convert data to an integer.")
except:
print("Unexpected error:", sys.exc_info()[0])
raise
The try … except statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except OSError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
</pre>
The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.
When an exception occurs, it may have an associated value, also known as the exception’s argument. The presence and type of the argument depend on the exception type.
The except clause may specify a variable after the exception name. The variable is bound to an exception instance with the arguments stored in instance.args. For convenience, the exception instance defines __str__() so the arguments can be printed directly without having to reference .args. One may also instantiate an exception first before raising it and add any attributes to it as desired.
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> try:
... raise Exception('spam', 'eggs')
... except Exception as inst:
... print(type(inst)) # the exception instance
... print(inst.args) # arguments stored in .args
... print(inst) # str allows args to be printed directly,
... # but may be overridden in exception subclasses
... x, y = inst.args # unpack args
... print('x =', x)
... print('y =', y)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
</pre>
If an exception has arguments, they are printed as the last part (‘detail’) of the message for unhandled exceptions.
Exception handlers don’t just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> def this_fails():
... x = 1/0
...
try:
... this_fails()
... except ZeroDivisionError as err:
... print('Handling run-time error:', err)
...
Handling run-time error: division by zero
</pre>
8.4. Raising Exceptions
The raise statement allows the programmer to force a specified exception to occur. For example:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> raise NameError('HiThere')
Traceback (most recent call last): File "<stdin>", line 1, in <module>
NameError: HiThere
</pre>
The sole argument to raise indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from Exception). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">raise ValueError # shorthand for 'raise ValueError()'
</pre>
If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the raise statement allows you to re-raise the exception:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> try:
... raise NameError('HiThere')
... except NameError:
... print('An exception flew by!')
... raise
...
An exception flew by!
Traceback (most recent call last): File "<stdin>", line 2, in <module>
NameError: HiThere
</pre>
8.5. User-defined Exceptions
Programs may name their own exceptions by creating a new exception class (seeClasses for more about Python classes). Exceptions should typically be derived from the Exception class, either directly or indirectly.
Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. When creating a module that can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">class Error(Exception):
"""Base class for exceptions in this module."""
pass
class InputError(Error):
"""Exception raised for errors in the input.
Attributes:
expression -- input expression in which the error occurred
message -- explanation of the error
"""
def __init__(self, expression, message):
self.expression = expression
self.message = message
class TransitionError(Error):
"""Raised when an operation attempts a state transition that's not
allowed.
Attributes:
previous -- state at beginning of transition
next -- attempted new state
message -- explanation of why the specific transition is not allowed
"""
def __init__(self, previous, next, message):
self.previous = previous
self.next = next
self.message = message
</pre>
Most exceptions are defined with names that end in “Error”, similar to the naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter Classes.
8.6. Defining Clean-up Actions
The try statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last): File "<stdin>", line 2, in <module>
</pre>
A finally clause is always executed before leaving the try statement, whether an exception has occurred or not. When an exception has occurred in the try clause and has not been handled by an except clause (or it has occurred in an except or elseclause), it is re-raised after the finally clause has been executed. The finallyclause is also executed “on the way out” when any other clause of the try statement is left via a break, continue or return statement. A more complicated example:
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">>>> def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print("division by zero!")
... else:
... print("result is", result)
... finally:
... print("executing finally clause")
...
divide(2, 1)
result is 2.0
executing finally clause
divide(2, 0)
division by zero!
executing finally clause
divide("2", "1")
executing finally clause
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'
</pre>
As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed.
In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
8.7. Predefined Clean-up Actions
Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">for line in open("myfile.txt"):
print(line, end="")
</pre>
The problem with this code is that it leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.
<pre style="overflow: auto hidden; padding: 5px; background-color: rgb(238, 255, 204); color: rgb(51, 51, 51); line-height: 18.528px; border: 1px solid rgb(170, 204, 153); font-family: monospace, sans-serif; font-size: 15.44px; border-radius: 3px;">with open("myfile.txt") as f:
for line in f:
print(line, end="")
</pre>
After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation.
