I'm having problems with the ax parameters of the rlocus function

I was trying today to plot the root locus of a transfer function, but wanted to pass a specific axes object to it, so I tried to use the ax parameter of the rlocus function:

from control import *
import matplotlib.pyplot as plt

s = tf([1, 0], 1)
G = 1/(s*(s+1))

fig, ax = plt.subplots(dpi=200)
rlocus(G, ax=ax)

But when doing this, the following error was raised:

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-1-80498c4b1a54> in <module>
      6 
      7 fig, ax = plt.subplots(dpi=200)
----> 8 rlocus(G, ax=ax)

~\anaconda3\lib\site-packages\control\rlocus.py in root_locus(sys, kvect, xlim, ylim, plotstr, plot, print_gain, grid, ax, **kwargs)
    173 
    174         if print_gain and not sisotool:
--> 175             fig.canvas.mpl_connect(
    176                 'button_release_event',
    177                 partial(_RLClickDispatcher, sys=sys, fig=fig,

UnboundLocalError: local variable 'fig' referenced before assignment

Control version: 0.9.0

Looks like you caught a bug, thanks! I think when we implemented this we had what you intended in mind but didn’t get around to testing it.

It's not too difficult to fix. I think we just need to unindent lines 171 and 172 by one tab:

python-control/control/rlocus.py

Lines 169 to 172 in 283f5e7

    
           if ax is None: 
        
               ax = plt.gca() 
        
               fig = ax.figure 
        
               ax.set_title('Root Locus')

I'm happy to implement this immediate fix (and #632 at the same time) and maybe add some unit tests, including for other plot functions where applicable. If I recall correctly, I was trying to do this a while back and the problem is that some plot funcs need to generate their own axis (e.g. radial plots) and some generate a figure containing more than one axis.

If you manage to also fix #634 while you are at it, it would be a blast. Not sure if it is worth to be done before/without implementing the new plotting paradigm, proposed by you in #65 (comment)

I'll have a look at #634. But I'm waiting for further input on the new plotting paradigm. That will require more thinking and discussion and probably take a while to figure out a way forward. There might be better ways to do it. I'm happy to try fixing the immediate bugs on the existing functions when I next get the chance.

fix for github python-control#623

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I'm having problems with the `ax` parameters of the `rlocus` function #623

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	if ax is None:
	ax = plt.gca()
	fig = ax.figure
	ax.set_title('Root Locus')

I'm having problems with the ax parameters of the rlocus function #623

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sawyerbfuller commented on May 22, 2021

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billtubbs commented on Aug 9, 2021

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I'm having problems with the `ax` parameters of the `rlocus` function #623