ascii_letters生成大小写字母
digits生成数字
第一种方式(字典统计)：

import string
from random import choices
s = ''.join(choices(string.ascii_letters + string.digits, k=1000))
print("string: ", s)
char_count = dict();
for i in s:
    char_count[i] = char_count.get(i, 0) + 1
print(char_count)

第二种方式(使用Counter)：

from random import choices
from string import ascii_letters
from collections import Counter
import string
s = choices(ascii_letters + string.digits, k=1000)
cnt=Counter()
print("list: ", s)
for i in s:
    cnt[i] += 1
print(cnt)