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138 lines
4.0 KiB
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[#]: subject: (Slice infinite generators with this Python 3.7 feature)
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[#]: via: (https://opensource.com/article/21/5/python-37-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: (wxy)
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[#]: publisher: (wxy)
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[#]: url: (https://linux.cn/article-13459-1.html)
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用这个 Python 3.7 的特性来切片无限生成器
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======
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> 了解更多关于这个和其他两个未被充分利用但仍然有用的 Python 特性。
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这是关于 Python 3.x 首发特性系列文章的第八篇。[Python 3.7][2] 于 2018 年首次发布，尽管它已经发布了几年，但它引入的许多特性都未被充分利用，而且相当酷。下面是其中的三个。
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### 注解推迟评估
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在 Python 3.7 中，只要激活了正确的 `__future__` 标志，注解在运行时就不会被评估：
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```
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from __future__ import annotations
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def another_brick(wall: List[Brick], brick: Brick) -> Education:
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pass
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```
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```
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another_brick.__annotations__
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```
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```
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{'wall': 'List[Brick]', 'brick': 'Brick', 'return': 'Education'}
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```
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它使递归类型（指向自己的类）和其他有趣的事情成为了可能。然而，这意味着如果你想做自己的类型分析，你需要明确地使用 `ast`。
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```
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import ast
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raw_type = another_brick.__annotations__['wall']
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[parsed_type] = ast.parse(raw_type).body
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```
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```
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subscript = parsed_type.value
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f"{subscript.value.id}[{subscript.slice.id}]"
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```
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```
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'List[Brick]'
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```
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### itertools.islice 支持 __index__
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Python 中的序列切片长期以来一直接受各种 _类 int 对象_（具有 `__index__()` 的对象）作为有效的切片部分。然而，直到 Python 3.7，`itertools.islice`，即核心 Python 中对无限生成器进行切片的唯一方法，才获得了这种支持。
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例如，现在可以用 `numpy.short` 大小的整数来切片无限生成器：
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```
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import numpy
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short_1 = numpy.short(1)
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short_3 = numpy.short(3)
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short_1, type(short_1)
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```
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```
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(1, numpy.int16)
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```
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```
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import itertools
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list(itertools.islice(itertools.count(), short_1, short_3))
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```
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```
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[1, 2]
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```
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### functools.singledispatch() 注解注册
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如果你认为 [singledispatch][3] 已经很酷了，你错了。现在可以根据注解来注册了：
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```
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import attr
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import math
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from functools import singledispatch
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@attr.s(auto_attribs=True, frozen=True)
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class Circle:
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radius: float
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@attr.s(auto_attribs=True, frozen=True)
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class Square:
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side: float
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@singledispatch
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def get_area(shape):
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raise NotImplementedError("cannot calculate area for unknown shape",
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shape)
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@get_area.register
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def _get_area_square(shape: Square):
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return shape.side ** 2
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@get_area.register
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def _get_area_circle(shape: Circle):
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return math.pi * (shape.radius ** 2)
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get_area(Circle(1)), get_area(Square(1))
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```
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```
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(3.141592653589793, 1)
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```
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### 欢迎来到 2017 年
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Python 3.7 大约是四年前发布的，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，那么将它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-37-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：[wxy](https://github.com/wxy)
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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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[2]: https://opensource.com/downloads/cheat-sheet-python-37-beginners
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[3]: https://opensource.com/article/19/5/python-singledispatch
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