TypeError AxesSubplot object does not support indexing

Question

I have a question about an error I receive when looping to plot multiple subplots from a data frame.

My data frame has many columns, of which I loop over to have a subplot of each column.

This is my code

 def plot(df):
    channels=[]
    for i in df:
        channels.append(i)

    fig, ax = plt.subplots(len(channels), sharex=True, figsize=(50,100))

    plot=0    
    for j in df: 

        ax[plot].plot(df["%s" % j])
        ax[plot].set_xlabel('%s' % j)
        plot=plot+1

    plt.tight_layout()
    plt.show() 

I get the plot produced fine, but also an empty frame and the error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\AClayton\WinPython-64bit-2.7.5.3\python-2.7.5.amd64\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 538, in runfile
    execfile(filename, namespace)
  File "C:/Users/AClayton/Desktop/Data/TS.py", line 67, in <module>
    plot(all_data)
  File "C:/Users/AClayton/Desktop/Data/TS.py", line 49, in plot
    ax[plot].plot(reader["%s" % j])
TypeError: 'AxesSubplot' object does not support indexing

I can't see where this error comes from if the first plot is produced fine, or why the second figure is produced?

Answer 1

Hello ,

• If you plot multiple subplots, the plt.subplots() returns the axes in an array, that array allows indexing like you do with ax[plot]. When only 1 subplot is created, by default it returns the axes itself, not the axes within an array.
• So your error occurs when len(channels) equals 1. You can suppress this behavior by setting squeeze=False in the .subplots() command. This forces it to always return a 'Rows x Cols' sized array with the axes, even if its a single one.

So:

 def plot(df):
    channels=[]
    for i in df:
        channels.append(i)

    fig, ax = plt.subplots(len(channels),1, sharex=True, figsize=(50,100), squeeze=False)

    plot=0    
    for j in df: 

        ax[plot,0].plot(df["%s" % j])
        ax[plot,0].set_xlabel('%s' % j)
        plot=plot+1

    plt.tight_layout()
    plt.show() 

By adding the squeeze keyword you always get a 2D array in return, so the indexing for a subplot changes to ax[plot,0]. I have also specifically added the amount of columns .

Thank You!