ValueError:以10为基数的int()无效文字.如何固定字母数字值

我正在使用一个代码，它给了我X,Y坐标和地图中的plot数字。例如:20号、21号、22号地块等。但是如果映射有像20-A,20A或20A这样的字母数字值，我就会卡住，因为当我输入像20-A这样的值时，我会得到这个错误"ValueError:无效的int()以10为基数"。所以请告诉我如何处理字母数字值。

这是我的代码。

import matplotlib.pyplot as plt
from PIL import Image
import numpy as np
import Tkinter as tk
import tkSimpleDialog
#Add path of map file here. The output will be saved in the same path with name map_file_name.extension.csv
path = "maps/21.jpg"

#Set this to true for verbose output in the console
debug = False

#Window for pop ups
root = tk.Tk()
root.withdraw()
root.lift()
root.attributes('-topmost',True)
root.after_idle(root.attributes,'-topmost',False)

#Global and state variables
global_state = 1
xcord_1, ycord_1, xcord_2, ycord_2 = -1,-1,-1,-1
edge1, edge2 = -1,-1
#Defining Plot
img = Image.open(path)
img = img.convert('RGB')
img = np.array(img)
if(debug):
    print "Image Loaded; dimensions = "
    print img.shape
#This let us bind the mouse function the plot
ax = plt.gca()
ax.axes.get_xaxis().set_visible(False)
ax.axes.get_yaxis().set_visible(False)
fig = plt.gcf()
#Selecting tight layout
fig.tight_layout()
#Plotting Image
imgplot = ax.imshow(img)
#Event listener + State changer
def onclick(event):
    global xcord_1,xcord_2,ycord_1,ycord_2,edge1,edge2, imgplot,fig, ax, path
    if(debug):
        print "Single Click Detected"
        print "State = " + str(global_state)
    if event.dblclick:
        if(debug):
            print "Double Click Detection"
        global global_state
        if(global_state==0):

            xcord_1 = event.xdata
            ycord_1 = event.ydata
            edge1 = (tkSimpleDialog.askstring("2nd", "No of 2nd Selected Plot"))
            #Draw here
            if edge1 is None: #Incase user cancels the pop up. Go to initial state
                global_state = 1
                pass
            else:
                edge1 = int(edge1)
                global_state = 1
                difference = edge2-edge1
                dif_state = 1;
                #So difference is always positive. Dif_state keeps track of plot at which side has the larger number
                if difference <0:
                    dif_state = -1;
                    difference *= -1
                #Corner Case; labelling a single plot
                if(difference == 0):
                    import csv
                    fields = [int(xcord_1),int(ycord_1),edge1]
                    plt.scatter(int(xcord_1),int(ycord_1),marker='$' + str(edge1) + '$', s=150)
                    with open(path+'.csv', 'a') as f:
                        writer = csv.writer(f)
                        writer.writerow(fields)
                else:
                    if(debug):
                        print "P1 : (" + str(xcord_1) + ", " + str(ycord_1) + " )"
                        print "P2 : (" + str(xcord_2) + ", " + str(ycord_2) + " )"
                    for a in range(0,difference+1):
                        #Plotting the labels
                        plt.scatter(int(xcord_1+(a*(float(xcord_2-xcord_1)/difference))),int(ycord_1+a*((float(ycord_2-ycord_1)/difference))),marker='$'+str(edge1+dif_state*a)+'$',s=150)
                        #Saving in CSV
                        import csv
                        fields = [int(xcord_1+(a*(float(xcord_2-xcord_1)/difference))),int(ycord_1+a*((float(ycord_2-ycord_1)/difference))),str(edge1+dif_state*a)]
                        with open(path+'.csv', 'a') as f:
                            writer = csv.writer(f)
                            writer.writerow(fields)
                        if debug:
                            print (int(xcord_1+(a*(float(xcord_2-xcord_1)/difference))),int(ycord_1+a*((float(ycord_2-ycord_1)/difference))))
                plt.show()

        elif(global_state == 1):
            xcord_2 = event.xdata
            ycord_2 = event.ydata
            print "Recorded"
            edge2 = (tkSimpleDialog.askstring("1st", "No of Selected Plot"))
            print type(edge2)
            if edge2 is None:
                root.withdraw()
                pass
            else:
                edge2 = int(edge2)
                global_state = 0

cid = fig.canvas.mpl_connect('button_press_event', onclick)
plt.show()