How to use kwargs in matplotlib.axes.Axes.arrow python 2.7

Question

I'm working from arrow_simple_demo.py here which I have already modified to be:

import matplotlib.pyplot as plt

ax = plt.axes()
ax.arrow(0, 0, 0.5, 0.5, head_width=0.05, head_length=0.1)

however, I want to change the line style of the arrow to dashed using a kwarg. The arrow docs suggest it is possible. arrow docs

So I tried to give arrow the **kwargs argument to import:

kwargs = {linestyle:'--'}

Now my code looks like this:

import matplotlib.pyplot as plt

ax = plt.axes()
kwargs={linestyle:'--'}
ax.arrow(0, 0, 0.5, 0.5, head_width=0.05, head_length=0.1, **kwargs)

But the result is:

NameError: name 'linestyle' is not defined

I'm wondering if anyone can tell me if I am using the kwargs correctly, and whether I need to import Patch from matplotlib's patches class to make this work. The statement "Other valid kwargs (inherited from :class:Patch)" which is in the arrow docs above the listing of kwargs makes me think it might be necessary. I have also been looking at the patches docs to figure that question out. here

EDIT:

Code finished when I passed linestyle key as a string, but I am not getting the dashed arrow line that I was hoping for.

import matplotlib.pyplot as plt
ax = plt.axes()
kwargs={'linestyle':'--'}
ax.arrow(0, 0, 0.5, 0.5, head_width=0.05, head_length=0.1, **kwargs)

see picture:

arrow plot solid line

Diziet Asahi · Accepted Answer · 2017-12-27 20:35:04Z

8

The key of your kwargs dictionary should be a string. In you code, python looks for an object called linestyle which does not exist.

kwargs = {'linestyle':'--'}

unfortunately, doing is not enough to produce the desired effect. The line is dashed, but the problem is that the arrow is drawn with a closed path and the dashes are overlaid on top of one another, which cancels the effect. You can see the dashed line by plotting a thicker arrow.

ax = plt.axes()
kwargs={'linestyle':'--', 'lw':2, 'width':0.05}
ax.arrow(0, 0, 0.5, 0.5, head_width=0.05, head_length=0.1, **kwargs)

If you want a simple dashed arrow, you have to use a simpler arrow, using annotate

ax = plt.axes()
kwargs={'linestyle':'--', 'lw':2}
ax.annotate("", xy=(0.5, 0.5), xytext=(0, 0),
    arrowprops=dict(arrowstyle="->", **kwargs))

edited Dec 27, 2017 at 20:35

answered Dec 27, 2017 at 19:53

Diziet Asahi

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3 Comments

AF2k15 Over a year ago

Oh such a simple mistake! The code finishes now but still a solid line for some reason?

AF2k15 Over a year ago

See my edit to the question that reflects your advice. Can you confirm that the arrow line is still solid on your system? It still shows solid on mine.

AF2k15 Over a year ago

Thanks for the great explanation. I will have to use annotate after all.

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