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

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。

其他回答

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。

我补充说,我经常使用断言来指定属性,比如循环不变量或我的代码应该具有的逻辑属性,就像我在正式验证的软件中指定它们一样。

它们有两个目的,告诉读者,帮助我推理,并检查我在推理中没有犯错误。例如 :

k = 0
for i in range(n):
    assert k == i * (i + 1) // 2
    k += i 
    #do some things      

或者在更复杂的情况下:

def sorted(l):
   return all(l1 <= l2 for l1, l2 in zip(l, l[1:]))
 
def mergesort(l):
   if len(l) < 2: #python 3.10 will have match - case for this instead of checking length
      return l
   k = len(l // 2)
   l1 = mergesort(l[:k])
   l2 = mergesort(l[k:])
   assert sorted(l1) # here the asserts allow me to explicit what properties my code should have
   assert sorted(l2) # I expect them to be disabled in a production build
   return merge(l1, l2)

因为当python在优化模式下运行时,断言是禁用的,所以不要犹豫在它们中编写代价高昂的条件,特别是当它使您的代码更清晰,更不容易出现错误时

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。

"assert"语句在编译优化时被删除。所以,是的,它们在性能和功能上都有差异。

在编译时请求优化时,当前代码生成器不会为assert语句生成代码。Python 3 Docs

如果您使用assert来实现应用程序功能,然后优化部署到生产环境,那么您将受到“但它在开发中有效”缺陷的困扰。

参见PYTHONOPTIMIZE和-O -OO

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