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04 语法错误和异常处理（Syntax errors and exceptions）
《Python数据分析技术栈》第01章 04 语法错误和异常处理（Syntax errors and exceptions）
Syntax errors are errors that may be committed inadvertently by the user while writing the code, for example, spelling a keyword wrong, not indenting the code, and so on. An exception, on the other hand, is an error that occurs during program execution. A user may enter incorrect data while running the program. If you want to divide a number (say, ‘a’) by another number (say, ‘b’), but give a value of 0 to the denominator (‘b’), this will generate an exception. The exceptions, which are autogenerated in Python and displayed to the user, may not lucidly convey the problem. Using exception handling with the try-except construct, we can frame a user-friendly message to enable the user to better correct the error.
语法错误是指用户在编写代码时可能无意中犯下的错误，例如拼错关键字、没有缩进代码等。异常则是在程序执行过程中发生的错误。用户在运行程序时可能会输入错误的数据。如果你想用一个数（如 “a”）除以另一个数（如 “b”），但给分母（“b”）的值是 0，这就会产生异常。在 Python 中自动生成并显示给用户的异常可能无法清楚地表达问题。通过使用 try-except 结构的异常处理，我们可以编写出用户友好的信息，使用户能够更好地纠正错误。
There are two parts to exception handling. First, we put the code that is likely to cause an error under a try clause. Then, in the except clause, we try to deal with whatever caused an error in the try block. We mention the name of the exception class in the except clause, followed by a code block where we handle the error. A straightforward method for handling the error is printing a message that gives the user more details on what they need to correct.
异常处理分为两个部分。首先，我们将可能导致错误的代码放在 try 子句中。然后，在 except 子句中，我们尝试处理 try 代码块中导致错误的代码。我们在 except 子句中提及异常类的名称，然后在代码块中处理错误。处理错误的一种直接方法是打印一条消息，向用户提供需要更正的更多细节。
Note that all exceptions are objects that are derived from the class BaseException, and follow a hierarchy.
请注意，所有异常都是从 BaseException 类派生出来的对象，并遵循一个层次结构。
A simple example of a program, with and without exception handling, is shown below.
下面是一个简单的程序示例，包括异常处理和无异常处理两种情况。
while True:try:n = int(input('Enter your score:'))print('You obtained a score of ', n)breakexcept ValueError:print('Enter only an integer value')
Same program (Without exception handling):
n=int(input('Enter your score:'))
print('You obtained a score of ',n)
The statement that is likely to cause an error in the preceding code is: int(input(‘Enter your score:’)). The int function requires an integer as an argument. If the user enters a floating-point or string value, a ValueError exception is generated. When we use the tryexcept construct, the except clause prints a message asking the user to correct the input, making it much more explicit.
在前面的代码中，可能导致错误的语句是：int(input(‘输入您的分数：’))。int 函数需要一个整数作为参数。如果用户输入的是浮点数或字符串值，就会产生 ValueError 异常。当我们使用 tryexcept 结构时，except 子句会打印一条信息，要求用户更正输入内容，这样就更加明确了。
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