输入字符串或读取文本后字符计数

from collections import Counter
str = input("输入字符串或txt地址:\n")
cnt = Counter("")
if str[-4:] == ".txt":
try:
with open(str,'r') as f:
            a = f.readlines()
            cnt = Counter(list("".join(a)))
except FileNotFoundError as e:
print(e,"：找不到该文件")
else:
    a = str
    cnt = Counter(list(a))
b = []

for k,v in cnt.items():
#print(k,"--",v)
b.append((k,v))
b.sort(key=lambda x:x[1],reverse=True)
if len(b) > 4:
for i in range(3):
print(b[i])
else:
for i in range(len(b)):
print(b[i])

这是一个学生作业吧，敲了顺便把代码留一下