“断言"的使用是什么?在Python中? [英] What is the use of "assert" in Python?

问题描述

我一直在阅读一些源代码，并且在几个地方看到了assert的用法.

I have been reading some source code and in several places I have seen the usage of assert.

这到底是什么意思?它的用途是什么?

What does it mean exactly? What is its usage?

推荐答案

assert语句几乎存在于每种编程语言中.它有助于在程序中尽早发现问题，找出原因所在，而不是在其他操作后再发现问题.

The assert statement exists in almost every programming language. It helps detect problems early in your program, where the cause is clear, rather than later as a side-effect of some other operation.

当你做...

assert condition

...您要告诉程序测试该条件，如果条件为假，则立即触发错误.

... you're telling the program to test that condition, and immediately trigger an error if the condition is false.

在Python中，它大致等效于此:

In Python, it's roughly equivalent to this:

if not condition:
    raise AssertionError()

在Python shell中尝试一下:

Try it in the Python shell:

>>> assert True # nothing happens
>>> assert False
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

断言可以包含可选消息，您可以在运行解释器时将其禁用.

Assertions can include an optional message, and you can disable them when running the interpreter.

要在断言失败时打印一条消息:

To print a message if the assertion fails:

assert False, "Oh no! This assertion failed!"

不要不要像函数一样使用括号来调用assert.这是一个声明.如果您执行assert(condition, message)，则将以(condition, message)元组作为第一个参数运行assert.

Do not use parenthesis to call assert like a function. It is a statement. If you do assert(condition, message) you'll be running the assert with a (condition, message) tuple as first parameter.

关于禁用它们，当在优化模式下运行python时，其中__debug__为False时，assert语句将被忽略.只需传递-O标志:

As for disabling them, when running python in optimized mode, where __debug__ is False, assert statements will be ignored. Just pass the -O flag:

python -O script.py

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