mirror of
https://github.com/LCTT/TranslateProject.git
synced 2025-01-07 22:11:09 +08:00
107 lines
4.3 KiB
Markdown
107 lines
4.3 KiB
Markdown
|
[#]: 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
|