128 lines
4.5 KiB
Markdown
128 lines
4.5 KiB
Markdown
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# Python 中的 Assert 语句
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> 原文：<https://www.pythonforbeginners.com/basics/assert-statement-in-python>
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调试是软件开发人员旅程的重要部分之一。Python 编程语言还为调试程序提供了各种构造。一个这样的构造是 assert 语句。在本文中，我们将讨论什么是 assert 语句，它是如何工作的，以及我们如何在 python 中使用 assert 语句。
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## Python 中的 assert 语句是什么？
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在 Python 中，Assert 语句是一个用于在程序中强制执行某些条件的构造。python 中 assert 语句的语法如下。
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```py
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assert condition, message
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```
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* 这里**断言**是一个关键字。
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* **条件**包含需要为真的条件语句。
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* **消息**是当条件为假时将显示的语句。在断言语句中使用**消息**是可选的。
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## 断言语句是如何工作的？
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assert 语句通过使用 AssertionError 异常来工作。每当 assert 语句中给出的条件评估为真时，程序正常工作。
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```py
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name = "Bill"
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age = 20
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assert age >= 0
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print("{} is {} years old.".format(name,age))
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```
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输出:
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```py
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Bill is 20 years old.
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```
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相反，如果 assert 语句中的条件评估为 False，则 AssertionError 异常发生，程序终止。
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```py
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name = "Bill"
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age = -20
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assert age >= 0
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print("{} is {} years old.".format(name,age))
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```
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输出:
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```py
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Traceback (most recent call last):
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File "/home/aditya1117/PycharmProjects/pythonProject/string12.py", line 3, in <module>
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assert age >= 0
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AssertionError
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```
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如果我们在 assert 语句中传递一条消息，那么在 AssertionError 发生时也会打印这条消息。
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```py
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name = "Bill"
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age = -20
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assert age >= 0, "Age cannot be negative."
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print("{} is {} years old.".format(name,age))
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```
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输出:
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```py
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Traceback (most recent call last):
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File "/home/aditya1117/PycharmProjects/pythonProject/string12.py", line 3, in <module>
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assert age >= 0, "Age cannot be negative."
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AssertionError: Age cannot be negative.
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```
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因此，我们可以说，当 assert 语句中的条件为真时，assert 语句什么也不做。如果 assert 语句中的条件为 False，python 解释器将使用适当的消息引发 AssertionError。
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## python 中如何使用 assert 语句？
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不应该在 python 程序中使用 assert 语句来实现任何业务逻辑。assert 语句仅用于调试 python 程序。当程序没有产生预期的输出时，可以使用 assert 语句来检测错误发生的位置。
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例如，假设您的程序因为像“年龄”这样的变量有负值而遇到错误，这不是我们想要的情况。
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为了检测错误发生的位置，可以在使用“age”变量的每个语句之前放置一个 assert 语句。在每个 assert 语句中，您可以检查“age”的值是否大于零。您还可以添加一条消息来标识 IndexError 发生的位置，如下所示。
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```py
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name = "Bill"
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age = 10
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assert age >= 0, "Age takes negative before first print statement."
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print("{} is {} years old.".format(name, age))
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name = "Sam"
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age = -20
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assert age >= 0, "Age takes negative before second print statement."
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print("{} is {} years old.".format(name, age))
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```
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输出:
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```py
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Bill is 10 years old.
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Traceback (most recent call last):
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File "/home/aditya1117/PycharmProjects/pythonProject/string12.py", line 7, in <module>
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assert age >= 0, "Age takes negative before second print statement."
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AssertionError: Age takes negative before second print statement.
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```
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在上面的例子中，程序提前终止。为了避免这种情况，可以使用 [python try except](https://www.pythonforbeginners.com/error-handling/python-try-and-except) 块。在这里，您可以在 try 块中编写代码，并处理由 except 块中的 assert 语句引发的 AssertionError，如下所示。
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```py
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try:
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name = "Bill"
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age = 10
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assert age >= 0, "Age takes negative before first print statement."
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print("{} is {} years old.".format(name, age))
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name = "Sam"
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age = -20
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assert age >= 0, "Age takes negative before second print statement."
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print("{} is {} years old.".format(name, age))
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except AssertionError as e:
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print(e.args)
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```
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输出:
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```py
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Bill is 10 years old.
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('Age takes negative before second print statement.',)
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```
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## 结论
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在本文中，我们讨论了 python 中的 assert 语句。我们还讨论了 assert 语句是如何工作的，以及如何在调试程序时使用它们。要了解更多关于错误的信息，您可以阅读这篇关于 python 中的[异常的文章。](https://www.pythonforbeginners.com/error-handling/pythons-built-in-exceptions)
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