Is there a performance or code maintenance issue with using assert as part of the standard code instead of using it just for debugging purposes? Is assert x >= 0, 'x is less than zero' better or worse than if x < 0: raise Exception('x is less than zero') Also, is there any way to set a business rule like if x < 0 raise error that is always checked without the try/except/finally so, if at anytime throughout the code x is less than 0 an error is raised, like if you set assert x < 0 at the start of a function, anywhere within the function where x becomes less then 0 an exception is raised?


当前回答

assert的四个目的

假设您与四位同事Alice、Bernd、Carl和Daphne一起处理20万行代码。 他们喊你的代码,你喊他们的代码。

那么assert有四个角色:

Inform Alice, Bernd, Carl, and Daphne what your code expects. Assume you have a method that processes a list of tuples and the program logic can break if those tuples are not immutable: def mymethod(listOfTuples): assert(all(type(tp)==tuple for tp in listOfTuples)) This is more trustworthy than equivalent information in the documentation and much easier to maintain. Inform the computer what your code expects. assert enforces proper behavior from the callers of your code. If your code calls Alices's and Bernd's code calls yours, then without the assert, if the program crashes in Alices code, Bernd might assume it was Alice's fault, Alice investigates and might assume it was your fault, you investigate and tell Bernd it was in fact his. Lots of work lost. With asserts, whoever gets a call wrong, they will quickly be able to see it was their fault, not yours. Alice, Bernd, and you all benefit. Saves immense amounts of time. Inform the readers of your code (including yourself) what your code has achieved at some point. Assume you have a list of entries and each of them can be clean (which is good) or it can be smorsh, trale, gullup, or twinkled (which are all not acceptable). If it's smorsh it must be unsmorshed; if it's trale it must be baludoed; if it's gullup it must be trotted (and then possibly paced, too); if it's twinkled it must be twinkled again except on Thursdays. You get the idea: It's complicated stuff. But the end result is (or ought to be) that all entries are clean. The Right Thing(TM) to do is to summarize the effect of your cleaning loop as assert(all(entry.isClean() for entry in mylist)) This statements saves a headache for everybody trying to understand what exactly it is that the wonderful loop is achieving. And the most frequent of these people will likely be yourself. Inform the computer what your code has achieved at some point. Should you ever forget to pace an entry needing it after trotting, the assert will save your day and avoid that your code breaks dear Daphne's much later.

在我看来,assert的两个文档目的(1和3)和 保障措施(2和4)同样有价值。 告知人民甚至可能比告知计算机更有价值 因为它可以防止assert要捕捉的错误(在情况1中) 无论如何,接下来还有很多错误。

其他回答

当x在整个函数中小于零时,能够自动抛出错误。您可以使用类描述符。这里有一个例子:

class LessThanZeroException(Exception):
    pass

class variable(object):
    def __init__(self, value=0):
        self.__x = value

    def __set__(self, obj, value):
        if value < 0:
            raise LessThanZeroException('x is less than zero')

        self.__x  = value

    def __get__(self, obj, objType):
        return self.__x

class MyClass(object):
    x = variable()

>>> m = MyClass()
>>> m.x = 10
>>> m.x -= 20
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "my.py", line 7, in __set__
    raise LessThanZeroException('x is less than zero')
LessThanZeroException: x is less than zero

在诸如PTVS、PyCharm、Wing assert isinstance()等IDE中,可以使用isinstance语句对一些不清楚的对象启用代码补全。

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。

这是一个悬而未决的问题,我有两个方面想涉及:何时添加断言以及如何编写错误消息。

目的

向初学者解释一下,断言是一种可能引发错误的语句,但您不会捕获它们。他们通常不应该被抚养,但在现实生活中,他们有时还是会被抚养。这是一个严重的情况,代码无法恢复,我们称之为“致命错误”。

其次,它是为了“调试目的”,虽然正确,但听起来非常轻蔑。我更喜欢“声明不变量,它永远不应该被违反”的提法,尽管它对不同的初学者有不同的作用……有些人“只是得到它”,而另一些人要么没有找到它的任何用途,要么取代正常的异常,甚至用它来控制流。

风格

在Python中,assert是语句,而不是函数!(记住assert(False, 'is true')不会引发。但是,先说一下:

何时以及如何编写可选的“错误消息”?

这实际上适用于单元测试框架,它通常有许多专用的方法来执行断言(assertTrue(条件),assertFalse(条件),assertEqual(实际的,预期的)等)。它们通常还提供了一种对断言进行评论的方法。

在一次性代码中,您可以不使用错误消息。

在某些情况下,没有什么可以添加到断言:

def垃圾场(东西): 屁 # ...

但除此之外,消息对于与其他程序员(有时是你代码的交互式用户,例如在Ipython/Jupyter等)的交流是有用的。

给他们信息,而不仅仅是泄露内部实现细节。

而不是:

assert meaningless_identifier <= MAGIC_NUMBER_XXX, 'meaningless_identifier is greater than MAGIC_NUMBER_XXX!!!'

写:

assert meaningless_identifier > MAGIC_NUMBER_XXX, 'reactor temperature above critical threshold'

或者甚至:

assert meaningless_identifier > MAGIC_NUMBER_XXX, f'reactor temperature({meaningless_identifier }) above critical threshold ({MAGIC_NUMBER_XXX})'

我知道,我知道——这不是静态断言的情况,但我想指出消息的信息值。

消极还是积极的信息?

这可能是有争议的,但读到这样的东西让我很受伤:

assert a == b, 'a is not equal to b'

这是两个相互矛盾的东西。因此,每当我对代码库产生影响时,我就会通过使用“必须”和“应该”等额外的动词来明确我们想要什么,而不是说我们不想要什么。 断言a == b, 'a必须等于b'

然后,获取AssertionError: a must equal to b也是可读的,并且语句在代码中看起来是合乎逻辑的。此外,您可以在不读取回溯(有时甚至不可用)的情况下从中获得一些信息。

如前所述,当您的代码should NOT到达某个点(意味着那里存在错误)时,应该使用断言。在我看来,使用断言最有用的原因可能是不变量/前置/后置条件。在循环或函数的每次迭代的开始或结束时,这些必须为真。

例如,一个递归函数(两个独立的函数,一个处理错误的输入,另一个处理错误的代码,因为它很难与递归区分开来)。如果我忘记写if语句,这将很明显地显示出哪里出了问题。

def SumToN(n):
    if n <= 0:
        raise ValueError, "N must be greater than or equal to 0"
    else:
        return RecursiveSum(n)

def RecursiveSum(n):
    #precondition: n >= 0
    assert(n >= 0)
    if n == 0:
        return 0
    return RecursiveSum(n - 1) + n
    #postcondition: returned sum of 1 to n

这些循环不变量通常可以用断言表示。