python selenium save image as base64

selenium_save_image.py

url = 'https://www.gravatar.com/avatar/ad45f075aa4d69cbd552b671a32bf3ed?s=32&d=identicon&r=PG&f=1'

def save_image_as_base64(browser, url):
    """
    
    """
    base = browser.execute_async_script("""
    var done = arguments[0];
    function toDataURL(url, callback) {
      var xhr = new XMLHttpRequest();
      xhr.onload = function() {
        var reader = new FileReader();
        reader.onloadend = function() {
          callback(reader.result);
        }
        reader.readAsDataURL(xhr.response);
      };
      xhr.open('GET', url);
      xhr.responseType = 'blob';
      xhr.send();
    }

    toDataURL('%s', done)""" % url)
    return base

save_image_as_base64(browser, url)