思路：用列表存储大小写字母、数字和下划线，用random随机产生一个下标对应一个字符然后画出图形。

import random
a=['0','1','2','3','4','5','6','7','8','9','10','_']
b=[]
for i in range(26):
    i+=65
    r = chr(i)
    a.append(r)
for i in range(26):
    i+=97
    r = chr(i)
    a.append(r)
for i in range(7):
    s = random.randint(0, 63)
    b.append(a[s])
    print(a[s],end=' ')
print()
for i in range(5):
    if i!=2:
        for j in range(7):
            if j==0 or j ==6or j==3:
                s = random.randint(0, 63)
                b.append(a[s])
                print(a[s], end=' ')
            else:
                print(" ", end=' ')
        print()
    else:
        for i in range(7):
            s = random.randint(0, 63)
            b.append(a[s])
            print(a[s], end=' ')
        print()
for i in range(7):
    s = random.randint(0, 63)
    b.append(a[s])
    print(a[s], end=' ')
print()
dic={}
for i in b:
    dic[i]=b.count(i)
print(dic)