Python: 'Incorrect padding' error

Question

1. I am trying to base64 encode matplotlib generated plots and write them to a html page on GAE.

2. Later, digitize the plots

3. Finally, converting the html page to pdf using xhtml2pdf library.

The code worked well if plots are created by jqplot. However once switched to matplotlib, I am having an error called Incorrect padding, which I do not know how to solve. Thanks for any suggestions.

Step 1

Python CODE:

import matplotlib
import matplotlib.pyplot as plt
import StringIO
import urllib, base64
plt.figure(1)
plt.hist([2,3,4,5,6,7], bins=3)
fig = plt.gcf()
imgdata = StringIO.StringIO()
fig.savefig(imgdata, format='png')
imgdata.seek(0) # rewind the data
uri_1 = 'data:image/png;base64,'
uri_2 = urllib.quote(base64.b64encode(imgdata.buf))
uri_2 += "=" * ((4 - len(uri_2 ) % 4) % 4)
uri_3 = uri_1 + uri_2
uri_4 = '<img id="chart1" src = "%s"/>' % (uri_3)

Step 2

Javascript

var n_plot = $('img[id^="chart"]').size();
i=1;
var imgData = [];
while(i <= n_plot){
 //sometimes the plots are generated under jqplot
 try{
 imgData.push($('#chart'+i).jqplotToImageStr({}));
 i=i+1 
 }
 // This case is generated by matplotlib
 catch(e){
 imgData.push($('#chart'+i).attr('src'));
 i=i+1 
 }
}
imgData_json = JSON.stringify(imgData)

Step 3

Python

pdf = pisa.CreatePDF(imgData_json, file(filename, "wb"))

Answer 1

That's the problem:

uri_2 += "=" * ((4 - len(uri_2 ) % 4) % 4)

Why do you think the url needs extra padding? The output of b64encode will already be padded if neccessary, adding extra padding will only confuse the decoder. Some may ignore the extra padding, most however will just produce an error.

Oh, and also for data urls it's unnecessary to quote the string - base64 doesn't contain characters that really need to be escaped. That's only needed if you need to use base64 in a regular url, as + and = can cause problems, but that's not the case for data urls.

• TTT: Thanks for your help. I think it is the urllib.quote caused the issue.