画像ファイル内の数字を全て加算して答えを出力する

from PIL import Image

import pyocr
import pyocr.builders

def calc(filename):
    tools = pyocr.get_available_tools()
    tool = tools[0]
    img = Image.open(filename)
    txt = tool.image_to_string(img, lang="eng", builder=pyocr.builders.TextBuilder(tesseract_layout=6))
    txt_list = list(txt)
    for i, v in enumerate(txt_list):
        if str.isdecimal(v) == False:
            txt_list[i] = ' '
    txt_changed = ''.join(txt_list)
    a = ''
    sp = txt_changed.split()
    for v in sp:
        if str.isdecimal(v):
            a = a + v + ' + '
    print(a[:-2], ' = ', eval(a[:-2]))