The number of occurrences of statistical characters

[RobertW]

who can help me to resolve the question,thanks 

# -*- coding:utf-8 -*-
# python 3.x
import re

patter = [chr(i) for i in range(33,126)]

with open("a.txt","r") as file:
    content = file.read()
    for i in patter:
        result = len(re.findall(r"[%s]" % i,content))
        if result != 0:
            print("%s:%d" % (i, result))

Error:
Traceback (most recent call last):
  File "D:\robot\desk\Script\login_Discuz.py", line 13, in <module>
    result = len(re.findall(r"[%s]" % i,content))
  File "D:\Python\Python36-32\lib\re.py", line 222, in findall
    return _compile(pattern, flags).findall(string)
  File "D:\Python\Python36-32\lib\re.py", line 301, in _compile
    p = sre_compile.compile(pattern, flags)
  File "D:\Python\Python36-32\lib\sre_compile.py", line 562, in compile
    p = sre_parse.parse(p, flags)
  File "D:\Python\Python36-32\lib\sre_parse.py", line 855, in parse
    p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
  File "D:\Python\Python36-32\lib\sre_parse.py", line 416, in _parse_sub
    not nested and not items))
  File "D:\Python\Python36-32\lib\sre_parse.py", line 523, in _parse
    source.tell() - here)
sre_constants.error: unterminated character set at position 0