选题: 20190504 Add methods retroactively in Python with singledispatch

sources/tech/20190504 Add methods retroactively in Python with singledispatch.md

sources/tech/20190504 Add methods retroactively in Python with singledispatch.md

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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (Add methods retroactively in Python with singledispatch)
+[#]: via: (https://opensource.com/article/19/5/python-singledispatch)
+[#]: author: (Moshe Zadka  https://opensource.com/users/moshez)
+
+Add methods retroactively in Python with singledispatch
+======
+Learn more about solving common Python problems in our series covering
+seven PyPI libraries.
+![][1]
+
+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.
+
+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.
+
+### singledispatch
+
+Imagine you have a "shapes" library with a **Circle** class, a **Square** class, etc.
+
+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.
+
+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.
+
+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.
+
+Instead, the **singledispatch** function in **functools** can come to our rescue.
+
+
+```
+@singledispatch
+def get_area(shape):
+raise NotImplementedError("cannot calculate area for unknown shape",
+shape)
+```
+
+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.
+
+
+```
+@get_area.register(Square)
+def _get_area_square(shape):
+return shape.side ** 2
+@get_area.register(Circle)
+def _get_area_circle(shape):
+return math.pi * (shape.radius ** 2)
+```
+
+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.
+
+
+```
+from area_calculator import get_area
+
+@attr.s(auto_attribs=True, frozen=True)
+class Ellipse:
+horizontal_axis: float
+vertical_axis: float
+
+@get_area.register(Ellipse)
+def _get_area_ellipse(shape):
+return math.pi * shape.horizontal_axis * shape.vertical_axis
+```
+
+_Calling_ **get_area** is straightforward.
+
+
+```
+`print(get_area(shape))`
+```
+
+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**!
+
+In the next article in this series, we'll look at **tox** , a tool for automating tests on Python code.
+
+#### Review the previous articles in this series:
+
+  * [Cython][5]
+  * [Black][6]
+  * [attrs][7]
+
+
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/19/5/python-singledispatch
+
+作者：[Moshe Zadka ][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/moshez
+[b]: https://github.com/lujun9972
+[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/computer_code_programming_laptop.jpg?itok=ormv35tV
+[2]: https://opensource.com/article/18/5/numbers-python-community-trends
+[3]: https://pypi.org/
+[4]: https://pypi.org/project/singledispatch/
+[5]: https://opensource.com/article/19/4/7-python-problems-solved-cython
+[6]: https://opensource.com/article/19/4/python-problems-solved-black
+[7]: https://opensource.com/article/19/4/python-problems-solved-attrs