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205 lines
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205 lines
5.7 KiB
Markdown
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[#]: subject: (3 features that debuted in Python 3.0 you should use now)
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[#]: via: (https://opensource.com/article/21/5/python-30-features)
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[#]: author: (Moshe Zadka https://opensource.com/users/moshez)
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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你现在应该使用的在 Python 3.0 中首次亮相的 3 个特性
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======
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探索一些未被充分利用但仍然有用的 Python 特性。
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![Hands on a keyboard with a Python book ][1]
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这是关于首次出现在 Python 3.x 版本中的特性的系列文章的第一篇。Python 3.0 于 2008 年首次发布，尽管它已经发布了一段时间，但它引入的许多特性都没有被充分利用，而且相当酷。这里有三个你应该知道的。
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### 仅限关键字参数
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Python 3.0 首次引入了**仅限关键字参数**参数的概念。在这之前，不可能指定一个 API，其中一些参数只能通过关键字传入。这在有许多参数，其中一些参数可能是可选的函数中很有用。
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请看一个特意设计的例子：
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```
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def show_arguments(base, extended=None, improved=None, augmented=None):
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print("base is", base)
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if extended is not None:
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print("extended is", extended)
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if improved is not None:
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print("improved is", improved)
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if augmented is not None:
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print("augmented is", augmented)
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```
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当阅读调用该函数的代码时，有时很难理解发生了什么：
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```
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`show_arguments("hello", "extra")`[/code] [code]
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base is hello
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extended is extra
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[/code] [code]`show_arguments("hello", None, "extra")`[/code] [code]
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base is hello
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improved is extra
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```
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虽然可以用关键字参数来调用这个函数，但这明显不是最好的方法。相反，你可以将这些参数标记为仅限关键字：
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```
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def show_arguments(base, *, extended=None, improved=None, augmented=None):
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print("base is", base)
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if extended is not None:
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print("extended is", extended)
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if improved is not None:
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print("improved is", improved)
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if augmented is not None:
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print("augmented is", augmented)
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```
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现在，你不能用位置参数传入额外的参数：
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```
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`show_arguments("hello", "extra")`[/code] [code]
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---------------------------------------------------------------------------
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TypeError Traceback (most recent call last)
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<ipython-input-7-6000400c4441> in <module>
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----> 1 show_arguments("hello", "extra")
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TypeError: show_arguments() takes 1 positional argument but 2 were given
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```
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对该函数的有效调用更容易预测：
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```
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`show_arguments("hello", improved="extra")`[/code] [code]
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base is hello
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improved is extra
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```
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### nonlocal
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有时，函数式编程的人根据编写累加器的容易程度来判断一种语言。 累加器是一个函数，当它被调用时，返回目前为止发给它的所有参数的总和。
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在 3.0 之前，Python 的标准答案是：
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```
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class _Accumulator:
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def __init__(self):
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self._so_far = 0
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def __call__(self, arg):
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self._so_far += arg
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return self._so_far
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def make_accumulator():
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return _Accumulator()
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```
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虽然承认有些啰嗦，但这确实有效：
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```
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acc = make_accumulator()
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print("1", acc(1))
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print("5", acc(5))
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print("3", acc(3))
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```
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这样做的输出结果将是：
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```
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1 1
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5 6
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3 9
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```
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在Python 3.x中，**nonlocal** 可以用少得多的代码实现同样的行为。
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```
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def make_accumulator():
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so_far = 0
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def accumulate(arg):
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nonlocal so_far
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so_far += arg
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return so_far
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return accumulate
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```
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虽然累加器是经过设计的例子，但使用 `nonlocal` 关键字来拥有具有状态的内部函数的能力是一个强大的工具。
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### 扩展析构
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想象一下，你有一个 CSV 文件，每一行由几个元素组成：
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* 第一个元素是年份
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* 第二个元素是月
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* 其他元素是该月发表的全部文章，每天一个条目
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请注意，最后一个元素是_文章总数_，而不是_每天发表的文章_。例如，一行的开头可以是：
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```
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`2021,1,5,8,10`
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```
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这意味着在 2021 年 1 月，第一天发表了 5 篇文章。第二天，又多发表了三篇文章，使总数达到 8 篇。第三天，又多发表了两篇文章。
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一个月可以有 28 天、30 天或 31 天。提取月份、日期和文章总数有多难？
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在 3.0 之前的 Python 版本中，你可能会这样写：
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```
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`year, month, total = row[0], row[1], row[-1]`
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```
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这是正确的，但它掩盖了格式。使用**扩展析构**，同样可以这样表达：
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```
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`year, month, *rest, total = row`
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```
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这意味着如果格式加了前缀描述，你可以把代码改成：
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```
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`_, year, month, *rest, total = row`
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```
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而不需要在每个索引中添加 `1`。
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### 接下来是什么？
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Python 3.0 和它的后期版本已经推出了 12 年多，但是它的一些功能还没有被充分利用。在本系列的下一篇文章中，我将会写另外三个。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/5/python-30-features
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作者：[Moshe Zadka][a]
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选题：[lujun9972][b]
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译者：[geekpi](https://github.com/geekpi)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/moshez
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/python-programming-code-keyboard.png?itok=fxiSpmnd (Hands on a keyboard with a Python book )
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