Matplotlib 中经常使用的一个不寻常的 Python 语法元素 [英] An unusual Python syntax element frequently used in Matplotlib

问题描述

一个附带条件:我问题核心的语法元素是 Python 语言；但是，该元素经常出现在Matplotlib库中，这是我所见过的唯一上下文.因此，我不确定是一般的Python语法问题还是库特定的问题.我所知道的是，无论是在 Python语言参考还是在Matplotlib文档中，我都找不到合适的东西.

One proviso: The syntax element at the heart of my Question is in the Python language; however, this element appears frequently in the Matplotlib library, which is the only context i have seen it. So whether it's a general Python syntax question or a library-specific one, i am not sure. What i do know is that i could not find anything on point--either in the Python Language Reference or in the Matplotlib docs.

那些使用和/或使用优秀的 Python 绘图库开发的人，Matplotlib 将识别下面的语法模式.(

Those who use and/or develop with the excellent Python plotting library, Matplotlib will recognize the syntax pattern below. (

from matplotlib import pyplot as MPL

>>> l, = MPL.plot(s, t)      # s & t are ordinary NumPy 1D arrays

此表达式左侧的结构是什么? 而且，

What is the construction on the left-hand side of this expression? And,

使用目的是什么?

what is the purpose for using it?

我熟悉Python的赋值解包，例如

I am familiar with Python's assignment unpacking, e.g.,

>>> a, b = [100, 200]

我也知道在 Python 中单项 元组有时表示为 t，

I'm also aware that in Python one-item tuples are sometimes represented as t,

可能是上述第一个问题的答案；如果是这样，那么我还不明白为什么这里只需要从 plot 调用返回的值的第一个元素.

And either could be the answer to the first question above; if so then i don't yet understand the reason why only the first element of the value returned from the call to plot is needed here.

(注意:"l"是小写的"ell"；我使用此字母是因为ls是此处最常用的字母，可能是因为它绑定到以相同字母开头的对象，请参见下文).

(note: "l" is a lower case "ell"; i am using this letter because ls is the letter most often used here, probably because it is bound to an object that begins with the same letter-see below).

一些额外的上下文:

对 plot 的调用返回 line2D 实例列表:

The call to plot returns a list of line2D instances:

>>> type(l)
  <class 'matplotlib.lines.Line2D'>

所以l是line2D类型的对象.

绑定到lines2D对象后，通常会在Matplotlib代码中看到此变量"，如下所示:

Once bound to the lines2D object, this "variable" is usually seen in Matplotlib code like so:

>>> l.set_color("orange")

此表达式更改表示绘图窗口内数据值的线的颜色(绘图线")

This expression changes the color of the line that represents the data values inside the plot window (the "plot line")

下面是另一个示例；它显示了这种变量逗号"结构的常见场景，即在 Matplotlib 绘图窗口中嵌入小型工具包/图形后端独立小部件，例如，通过复选框打开/关闭，绘图窗口中出现多个数据系列

Below is one more example; it shows a common scenario for this "variable-comma" construction, which is embedding small toolkit/graphics-backend-independent widgets in a Matplotlib plot window, e.g., to toggle on/off by checkbox, multiple data series appearing in the plot window.

在下面的代码中，创建了一个简单的 Matplotlib 图和一个简单的小部件，该小部件由两个复选框组成，每个复选框对应一个数据系列.

In the code below, a simple Matplotlib plot and a simple widget comprised of two checkboxes one for each data series are created.

l0 和 l1 再次绑定到对 plot 的调用；当在调用 *on_click* 时传入的自定义函数中调用 get_visible 和 set_visible 方法时，两者都会出现几个留置权.

l0 and l1 are again bound to calls to plot; both appear a couple of liens later when their get_visible and set_visible methods are called within a custom function passed in when *on_click* is called.

from matplotlib.widgets import CheckButtons

ax = plt.subplot(111)
l0, = ax.plot(s, t, visible=False, lw=2)
l1, = ax.plot(t, s1, lw=2)

rax = plt.axes( [.05, .4, .1, .15] )
check = CheckButtons(rax, ('raw', 'transformed'), (False, True))

def fnx(checkbox_label):
    if checkbox_label == 'raw': 
        l0.set_visible(not l0.get_visible())
    elif checkbox_label == 'transformed': 
        l1.set_visible(not l1.get_visible())

check.on_clicked(fnx)

plt.show()

推荐答案

l, = v

[几乎]与

[l] = v

示例

>>> l=[3]
>>> [v] = l
>>> v
3
>>>

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