8.2 异常 Exceptions

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 exceptions and 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:

即使是在语法上完全正确的语句，尝试执行它的时候，也有可能会发生错误。在程序运行中检测出的错误称之为异常，它通常不会导致致命的问题，你很快就会学到如何在 Python 程序中控制它们。大多数异常不会由程序处理，而是显示一个错误信息：

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

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 name for the 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).

错误信息的最后一行指出发生了什么错误。异常也有不同的类型，异常类型做为错误信息的一部分显示出来：示例中的异常分别为 零除错误（ ZeroDivisionError ） ， 命名错误（ NameError） 和 类型错误（ TypeError ）。打印错误信息时，异常的类型作为异常的内置名显示。对于所有的内置异常都是如此，不过用户自定义异常就不一定了（尽管这是一个很有用的约定）。标准异常名是内置的标识（没有保留关键字）。

The rest of the line is a detail whose interpretation depends on the exception type; its meaning is dependent on the exception type.

这一行后一部分是关于该异常类型的详细说明，这意味着它的内容依赖于异常类型。

The preceding part of the error message shows the context where the exception happened, in the form of a stack backtrace. In general it contains a stack backtrace listing source lines; however, it will not display lines read from standard input.

错误信息的前半部分以堆栈的形式列出异常发生的位置。通常在堆栈中列出了源代码行，然而，来自标准输入的源码不会显示出来。