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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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[#]: subject: (Add methods retroactively in Python with singledispatch)
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[#]: via: (https://opensource.com/article/19/5/python-singledispatch)
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[#]: author: (Moshe Zadka https://opensource.com/users/moshez)
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Add methods retroactively in Python with singledispatch
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======
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Learn more about solving common Python problems in our series covering
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seven PyPI libraries.
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![][1]
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Python is one of the most [popular programming languages][2] in use today—and for good reasons: it's open source, it has a wide range of uses (such as web programming, business applications, games, scientific programming, and much more), and it has a vibrant and dedicated community supporting it. This community is the reason we have such a large, diverse range of software packages available in the [Python Package Index][3] (PyPI) to extend and improve Python and solve the inevitable glitches that crop up.
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In this series, we'll look at seven PyPI libraries that can help you solve common Python problems. Today, we'll examine [**singledispatch**][4], a library that allows you to add methods to Python libraries retroactively.
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### singledispatch
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Imagine you have a "shapes" library with a **Circle** class, a **Square** class, etc.
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A **Circle** has a **radius** , a **Square** has a **side** , and a **Rectangle** has **height** and **width**. Our library already exists; we do not want to change it.
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However, we do want to add an **area** calculation to our library. If we didn't share this library with anyone else, we could just add an **area** method so we could call **shape.area()** and not worry about what the shape is.
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While it is possible to reach into a class and add a method, this is a bad idea: nobody expects their class to grow new methods, and things might break in weird ways.
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Instead, the **singledispatch** function in **functools** can come to our rescue.
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```
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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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```
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The "base" implementation for the **get_area** function fails. This makes sure that if we get a new shape, we will fail cleanly instead of returning a nonsense result.
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```
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@get_area.register(Square)
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def _get_area_square(shape):
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return shape.side ** 2
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@get_area.register(Circle)
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def _get_area_circle(shape):
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return math.pi * (shape.radius ** 2)
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```
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One nice thing about doing things this way is that if someone writes a _new_ shape that is intended to play well with our code, they can implement **get_area** themselves.
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```
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from area_calculator import get_area
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@attr.s(auto_attribs=True, frozen=True)
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class Ellipse:
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horizontal_axis: float
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vertical_axis: float
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@get_area.register(Ellipse)
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def _get_area_ellipse(shape):
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return math.pi * shape.horizontal_axis * shape.vertical_axis
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```
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_Calling_ **get_area** is straightforward.
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```
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`print(get_area(shape))`
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```
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This means we can change a function that has a long **if isintance()/elif isinstance()** chain to work this way, without changing the interface. The next time you are tempted to check **if isinstance** , try using **singledispatch**!
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In the next article in this series, we'll look at **tox** , a tool for automating tests on Python code.
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#### Review the previous articles in this series:
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* [Cython][5]
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* [Black][6]
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* [attrs][7]
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/19/5/python-singledispatch
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作者：[Moshe Zadka ][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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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/computer_code_programming_laptop.jpg?itok=ormv35tV
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[2]: https://opensource.com/article/18/5/numbers-python-community-trends
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[3]: https://pypi.org/
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[4]: https://pypi.org/project/singledispatch/
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[5]: https://opensource.com/article/19/4/7-python-problems-solved-cython
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[6]: https://opensource.com/article/19/4/python-problems-solved-black
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[7]: https://opensource.com/article/19/4/python-problems-solved-attrs
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@ -0,0 +1,106 @@
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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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[#]: subject: (Add methods retroactively in Python with singledispatch)
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[#]: via: (https://opensource.com/article/19/5/python-singledispatch)
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[#]: author: (Moshe Zadka https://opensource.com/users/moshez)
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使用 singledispatch 在 Python 中追溯地添加方法
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======
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在我们覆盖 7 个 PyPI 库的系列文章中了解更多解决 Python 问题的信息。
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![][1]
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Python 是当今使用最多[流行的编程语言][2]之一，因为：它是开源的，它具有广泛的用途（例如 Web 编程、业务应用、游戏、科学编程等等），它有一个充满活力和专注的社区支持它。这个社区是我们在 [Python Package Index][3]（PyPI）中提供如此庞大、多样化的软件包的原因，用以扩展和改进 Python。并解决不可避免的问题。
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在本系列中，我们将介绍七个可以帮助你解决常见 Python 问题的 PyPI 库。今天，我们将研究 [**singledispatch**][4]，这是一个能让你追溯地向 Python 库添加方法的库。
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### singledispatch
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想象一下，你有一个有 **Circle**、**Square** 等类的“形状”库。
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**Circle** 类有**半径**、**Square** 有 **边**、**Rectangle**有**高**和**宽**。我们的库已经存在，我们不想改变它。
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然而，我们想给库添加一个**面积**计算。如果我们不会和其他人共享这个库，我们只需添加 **area** 方法，这样我们就能调用 **shape.area()** 而无需关心是什么形状。
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虽然可以进入类并添加一个方法，但这是一个坏主意：没有人希望他们的类会被添加新的方法，程序因奇怪的方式出错。
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相反，**functools** 中的 **singledispatch** 函数可以帮助我们。
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```
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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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```
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**get_area** 函数的“基类”实现会报错。这保证了如果我们出现一个新的形状时，我们会明确地报错而不是返回一个无意义的结果。
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```
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@get_area.register(Square)
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def _get_area_square(shape):
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return shape.side ** 2
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@get_area.register(Circle)
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def _get_area_circle(shape):
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return math.pi * (shape.radius ** 2)
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```
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这种方式的好处是如果某人写了一个匹配我们代码的_新_形状，它们可以自己实现 **get_area**。
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```
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from area_calculator import get_area
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@attr.s(auto_attribs=True, frozen=True)
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class Ellipse:
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horizontal_axis: float
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vertical_axis: float
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@get_area.register(Ellipse)
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def _get_area_ellipse(shape):
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return math.pi * shape.horizontal_axis * shape.vertical_axis
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```
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_调用_ **get_area** 很直接。
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```
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`print(get_area(shape))`
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```
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这意味着我们可以将有大量 **if isintance()/elif isinstance()** 的代码以这种方式修改，而无需修改接口。下一次你要修改 **if isinstance**，你试试 **singledispatch**！
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在本系列的下一篇文章中，我们将介绍 **tox**，一个用于自动化 Python 代码测试的工具。
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#### 回顾本系列的前几篇文章：
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* [Cython][5]
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* [Black][6]
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* [attrs][7]
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/19/5/python-singledispatch
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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/computer_code_programming_laptop.jpg?itok=ormv35tV
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[2]: https://opensource.com/article/18/5/numbers-python-community-trends
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[3]: https://pypi.org/
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[4]: https://pypi.org/project/singledispatch/
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[5]: https://opensource.com/article/19/4/7-python-problems-solved-cython
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[6]: https://opensource.com/article/19/4/python-problems-solved-black
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[7]: https://opensource.com/article/19/4/python-problems-solved-attrs
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