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Thursday, July 3, 2014

How do I Read Python Error Messages?

One of the complaints I've heard about Python from Matlab developers is that the error messages can be cryptic. 

Here's an example of an error I recently got while running some Python code (see Error Message below). When compared with a Matlab-style error, this can be a bit overwelming. Here's a few tips:
  1. Start at the bottom. That will be where the actual error message is. In this case it's "ValueError: to_rgba: Invalid rgba arg "['#eeeeee']" need more than 1 value to unpack". Notice that the argument in question is inside of square brackets - that means it's a list with 1 item in it. Perhaps that's telling.
  2. Find where the code you wrote is breaking. Often the stack of errors is deep inside some other module or code, but what you wrote is actually passing in bad data. The 2nd block of errors starting with "<ipython-input-2-2a6f1bb6961e>" is code that we wrote. Looks like the offending line is "color = colors_bmh[ y[ix].astype(int) ]". Somehow we're setting the array of "color" to some bad values.
  3. Look at the specific error message, "ValueError: ... need more than 1 value to unpack". This means that the code was expecting 2 or more values to be returned but only 1 was. 
It actually took me a while to debug the code. In this case, the point I made under Step 1 above about the square brackets (indicating a list) around the argument was the key. The colors array was defined this way:

color = colors_bmh[ y[ix].astype(int) ]

The problem with that statement is that if you look at the array shape, (np.shape(color)) it's 2-dimensional (100,1) whereas the function was expecting a list, or a 1-Dimensional array. Changing the code to this fixed it:

color = colors_bmh[ y[ix].astype(int) ].ravel()

The ravel() method removes singleton dimensions. It's similar to Matlab's squeeze() function. Hope that helps!

Error Message:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-808bb77074e6> in <module>()
      3 p = np.random.randn(len(y))
      4 
----> 5 separation_plot(p,y)

<ipython-input-2-2a6f1bb6961e> in separation_plot(p, y, **kwargs)
     29         bars = ax.bar( np.arange(n), np.ones(n), width=1., 
     30                 color = colors_bmh[ y[ix].astype(int) ],
---> 31                 edgecolor = 'none')
     32         ax.plot( np.arange(n), p[ix,i], "k", 
     33                 linewidth = 3.,drawstyle="steps-post" )

/Users/william/anaconda/lib/python2.7/site-packages/matplotlib/axes.pyc in bar(self, left, height, width, bottom, **kwargs)
   4976             color = [None] * nbars
   4977         else:
-> 4978             color = list(mcolors.colorConverter.to_rgba_array(color))
   4979             if len(color) == 0:  # until to_rgba_array is changed
   4980                 color = [[0, 0, 0, 0]]

/Users/william/anaconda/lib/python2.7/site-packages/matplotlib/colors.pyc in to_rgba_array(self, c, alpha)
    409             result = np.zeros((nc, 4), dtype=np.float)
    410             for i, cc in enumerate(c):
--> 411                 result[i] = self.to_rgba(cc, alpha)
    412             return result
    413 

/Users/william/anaconda/lib/python2.7/site-packages/matplotlib/colors.pyc in to_rgba(self, arg, alpha)
    363         except (TypeError, ValueError) as exc:
    364             raise ValueError(
--> 365                 'to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
    366 
    367     def to_rgba_array(self, c, alpha=None):

ValueError: to_rgba: Invalid rgba arg "['#eeeeee']"
need more than 1 value to unpack

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