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)
