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

Answer 1

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))