2021-05-21 08:09:27 +08:00
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[#]: subject: (Convenient matrices and other improvements Python 3.5 brought us)
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[#]: via: (https://opensource.com/article/21/5/python-35-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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2021-06-01 20:20:15 +08:00
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[#]: reviewer: (wxy)
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2021-06-01 20:21:01 +08:00
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[#]: publisher: (wxy)
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[#]: url: (https://linux.cn/article-13448-1.html)
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2021-05-21 08:09:27 +08:00
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Python 3.5 带给我们的方便的矩阵以及其他改进
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======
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2021-06-01 20:20:15 +08:00
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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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这是 Python 3.x 首发特性系列文章的第六篇。Python 3.5 在 2015 年首次发布，尽管它已经发布了很长时间，但它引入的许多特性都没有被充分利用，而且相当酷。下面是其中的三个。
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2021-05-21 08:09:27 +08:00
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### @ 操作符
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`@` 操作符在 Python 中是独一无二的，因为在标准库中没有任何对象可以实现它！它是为了在有矩阵的数学包中使用而添加的。
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矩阵有两个乘法的概念。元素积是用 `*` 运算符完成的。但是矩阵组合（也被认为是乘法）需要自己的符号。它是用 `@` 完成的。
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例如，将一个“八转”矩阵（将轴旋转 45 度）与自身合成，就会产生一个四转矩阵。
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```
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import numpy
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hrt2 = 2**0.5 / 2
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eighth_turn = numpy.array([
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[hrt2, hrt2],
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[-hrt2, hrt2]
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])
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eighth_turn @ eighth_turn
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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array([[ 4.26642159e-17, 1.00000000e+00],
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[-1.00000000e+00, -4.26642159e-17]])
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```
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2021-06-01 20:20:15 +08:00
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浮点数是不精确的，这比较难以看出。从结果中减去四转矩阵，将其平方相加，然后取其平方根，这样就比较容易检查。
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2021-05-21 08:09:27 +08:00
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这是新运算符的一个优点：特别是在复杂的公式中，代码看起来更像基础数学：
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```
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almost_zero = ((eighth_turn @ eighth_turn) - numpy.array([[0, 1], [-1, 0]]))**2
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round(numpy.sum(almost_zero) ** 0.5, 10)
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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2021-06-01 20:20:15 +08:00
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```
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0.0
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2021-05-21 08:09:27 +08:00
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```
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### 参数中的多个关键词字典
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Python 3.5 使得调用具有多个关键字-参数字典的函数成为可能。这意味着多个默认值的源可以与更清晰的代码”互操作“。
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例如，这里有个可笑的关键字参数的函数：
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```
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def show_status(
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*,
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the_good=None,
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the_bad=None,
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the_ugly=None,
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fistful=None,
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dollars=None,
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more=None
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):
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if the_good:
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print("Good", the_good)
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if the_bad:
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print("Bad", the_bad)
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if the_ugly:
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print("Ugly", the_ugly)
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if fistful:
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print("Fist", fistful)
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if dollars:
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print("Dollars", dollars)
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if more:
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print("More", more)
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```
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当你在应用中调用这个函数时，有些参数是硬编码的：
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```
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defaults = dict(
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the_good="You dig",
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the_bad="I have to have respect",
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the_ugly="Shoot, don't talk",
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)
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```
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从配置文件中读取更多参数：
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```
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import json
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others = json.loads("""
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{
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"fistful": "Get three coffins ready",
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"dollars": "Remember me?",
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"more": "It's a small world"
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}
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""")
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```
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你可以从两个源一起调用这个函数，而不必构建一个中间字典：
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```
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2021-06-01 20:20:15 +08:00
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show_status(**defaults, **others)
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```
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2021-05-21 08:09:27 +08:00
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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Good You dig
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Bad I have to have respect
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Ugly Shoot, don't talk
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Fist Get three coffins ready
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Dollars Remember me?
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More It's a small world
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```
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### os.scandir
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`os.scandir` 函数是一种新的方法来遍历目录内容。它返回一个生成器，产生关于每个对象的丰富数据。例如，这里有一种打印目录清单的方法，在目录的末尾跟着 `/`：
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```
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for entry in os.scandir(".git"):
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print(entry.name + ("/" if entry.is_dir() else ""))
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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2021-06-01 20:20:15 +08:00
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```
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2021-05-21 08:09:27 +08:00
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refs/
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HEAD
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logs/
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index
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branches/
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config
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objects/
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description
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COMMIT_EDITMSG
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info/
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hooks/
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```
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### 欢迎来到 2015 年
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Python 3.5 在六年前就已经发布了，但是在这个版本中首次出现的一些特性非常酷，而且没有得到充分利用。如果你还没使用，那么将他们添加到你的工具箱中。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/5/python-35-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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2021-06-01 20:20:15 +08:00
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校对：[wxy](https://github.com/wxy)
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2021-05-21 08:09:27 +08:00
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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/markus-spiske-iar-matrix-unsplash.jpg?itok=78u_4veR (Hacker code matrix)
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