断言应该用于测试不应该发生的情况。目的是在程序状态损坏的情况下尽早崩溃。

异常应该用于可能发生的错误，并且几乎总是应该创建自己的Exception类。

例如，如果您正在编写一个从配置文件读取到dict的函数，那么文件中的不当格式将引发ConfigurationSyntaxError，而您可以断言您不会返回None。

在您的示例中，如果x是通过用户界面或外部源设置的值，则最好使用异常。

如果x只是在同一个程序中由您自己的代码设置的，则使用断言。

这种方法唯一真正的错误是，很难使用断言语句产生非常描述性的异常。如果你正在寻找更简单的语法，记住你也可以这样做:

class XLessThanZeroException(Exception):
    pass

def CheckX(x):
    if x < 0:
        raise XLessThanZeroException()

def foo(x):
    CheckX(x)
    #do stuff here

另一个问题是，使用assert进行正常的条件检查会使使用-O标志禁用调试断言变得困难。

除了其他答案之外，断言本身也会抛出异常，但仅抛出AssertionErrors。从实用主义的角度来看，当您需要对捕获的异常进行精细控制时，断言并不适合。

是否存在性能问题?

Please remember to "make it work first before you make it work fast". Very few percent of any program are usually relevant for its speed. You can always kick out or simplify an assert if it ever proves to be a performance problem -- and most of them never will. Be pragmatic: Assume you have a method that processes a non-empty list of tuples and the program logic will break if those tuples are not immutable. You should write: def mymethod(listOfTuples): assert(all(type(tp)==tuple for tp in listOfTuples)) This is probably fine if your lists tend to be ten entries long, but it can become a problem if they have a million entries. But rather than discarding this valuable check entirely you could simply downgrade it to def mymethod(listOfTuples): assert(type(listOfTuples[0])==tuple) # in fact _all_ must be tuples! which is cheap but will likely catch most of the actual program errors anyway.

assert的使用和异常的引发都与通信有关。

Assertions are statements about the correctness of code addressed at developers: An assertion in the code informs readers of the code about conditions that have to be fulfilled for the code being correct. An assertion that fails at run-time informs developers that there is a defect in the code that needs fixing. Exceptions are indications about non-typical situations that can occur at run-time but can not be resolved by the code at hand, addressed at the calling code to be handled there. The occurence of an exception does not indicate that there is a bug in the code.

最佳实践

因此，如果您将运行时发生的特定情况视为您想要通知开发人员的错误(“嗨，开发人员，这种情况表明某处存在错误，请修复代码。”)，那么请使用断言。如果断言检查代码的输入参数，当输入参数违反条件时，您通常应该在文档中添加代码具有“未定义行为”。

相反，如果这种情况的出现并不是您眼中的错误，而是您认为应该由客户端代码处理的(可能很少，但可能)情况，则引发异常。引发哪种异常的情况应该是相应代码文档的一部分。

有没有演出[…]使用assert时的问题

断言的计算需要一些时间。但是，它们可以在编译时被删除。然而，这有一些后果，见下文。

有[…]使用assert的代码维护问题

Normally assertions improve the maintainability of the code, since they improve readability by making assumptions explicit and during run-time regularly verifying these assumptions. This will also help catching regressions. There is one issue, however, that needs to be kept in mind: Expressions used in assertions should have no side-effects. As mentioned above, assertions can be eliminated at compile time - which means that also the potential side-effects would disappear. This can - unintendedly - change the behaviour of the code.

英语单词assert在这里的意思是发誓、肯定、宣称。它的意思不是“检查”或“应该是”。这意味着你作为一个程序员要在这里做一个宣誓声明:

# I solemnly swear that here I will tell the truth, the whole truth, 
# and nothing but the truth, under pains and penalties of perjury, so help me FSM
assert answer == 42

如果代码是正确的，除了单事件中断、硬件故障等，任何断言都不会失败。这就是为什么程序对终端用户的行为不能受到影响。特别是，断言即使在异常的编程条件下也不能失败。这种事从来没有发生过。如果发生这种情况，程序员应该为此受到惩罚。