python - matplotlib SpanSelector 小部件 : how to use inside a function?

Question

我正在尝试在函数内使用 matplotlib SpanSelector 。我修改了原始示例( http://matplotlib.org/examples/widgets/span_selector.html )，但它不起作用。这是代码:

#!/usr/bin/env python
"""
The SpanSelector is a mouse widget to select a xmin/xmax range and plot the
detail view of the selected region in the lower axes
"""
import numpy as npy
from pylab import figure, show
from matplotlib.widgets import SpanSelector

fig = figure(figsize=(8,6))
ax = fig.add_subplot(211, axisbg='#FFFFCC')

x = npy.arange(0.0, 5.0, 0.01)
y = npy.sin(2*npy.pi*x) + 0.5*npy.random.randn(len(x))

ax.plot(x, y, '-')
ax.set_ylim(-2,2)
ax.set_title('Press left mouse button and drag to test')

ax2 = fig.add_subplot(212, axisbg='#FFFFCC')
line2, = ax2.plot(x, y, '-')


def onselect(xmin, xmax):
    indmin, indmax = npy.searchsorted(x, (xmin, xmax))
    indmax = min(len(x)-1, indmax)

    thisx = x[indmin:indmax]
    thisy = y[indmin:indmax]
    line2.set_data(thisx, thisy)
    ax2.set_xlim(thisx[0], thisx[-1])
    ax2.set_ylim(thisy.min(), thisy.max())
    fig.canvas.draw()

## set useblit True on gtkagg for enhanced performance
# SpanSelector works this way
#span = SpanSelector(ax, onselect, 'horizontal', useblit=True,
#                    rectprops=dict(alpha=0.5, facecolor='red') )

def fun_with_spanselector_inside():
    # set useblit True on gtkagg for enhanced performance
    span = SpanSelector(ax, onselect, 'horizontal', useblit=True,
                        rectprops=dict(alpha=0.5, facecolor='red') )

# SpanSelector does not work this way
fun_with_spanselector_inside()

show()

我在 Ubuntu 12.04 上使用 Python 2.7、matplotlib 1.1.1rc。

为什么这不起作用？

Answer 1

您只需保留对 SpanSelector 实例的引用即可。否则，它将被垃圾收集。

基本上，您需要这样做:

def fun_with_spanselector_inside():
    # set useblit True on gtkagg for enhanced performance
    span = SpanSelector(ax, onselect, 'horizontal', useblit=True,
                        rectprops=dict(alpha=0.5, facecolor='red') )
    return span

span = fun_with_spanselector_inside()

请注意，这与它在函数内部无关。如果你这样做:

SpanSelector(ax, onselect, 'horizontal', useblit=True,
             rectprops=dict(alpha=0.5, facecolor='red'))

这也行不通。

不过，这在文档中应该更清楚。我不确定这是有意为之还是意外。