matplotlib SpanSelector widget: how to use inside a function?

Question

I'm trying to use a matplotlib SpanSelector inside a function. I modified the original example (http://matplotlib.org/examples/widgets/span_selector.html) but it didn't work. Here is the code:

#!/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()

I'm using Python 2.7, matplotlib 1.1.1rc on Ubuntu 12.04.

Why is this not working?

Joe Kington · Accepted Answer · 2013-06-15 21:58:20Z

1

You just need to hold onto a reference to the SpanSelector instance. Otherwise, it's garbage collected.

Basically, you need to do this:

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()

Note that this has nothing to do with it being inside a function. If you just do:

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

it won't work either.

This should be clearer in the documentation, though. I'm not sure offhand if this is by design or accident.

edited Jun 15, 2013 at 21:58

answered Jun 15, 2013 at 21:48

Joe Kington

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1 Comment

Gaël Écorchard Over a year ago

Thanks for the answer. There is maybe something about it in http://matplotlib.org/users/event_handling.html.

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