str_exp = "aaabbbbccc"
 pos_n = str_exp.index("a", str_exp.index("a")+1)
 print(pos_n)

def find_n_sub_str(src, sub, pos, start):
 index = src.find(sub, start)
 if index != -1 and pos > 0:
  return find_n_sub_str(src, sub, pos - 1, index + 1)
 return index

str.index(self, sub, start=None, end=None)
str_tmp[start:end]

 str_exp = "aaabbbbccc"
 print(str_exp)
 print(str_exp[str_exp.index("a"):str_exp.rindex("c")])

aaabbbbccc
aaabbbbcc

 str_tmp = "aaaabbbbccccdddd"
 print(find_n_sub_str(str_tmp, "a", 5, 0))
 print(str_tmp.find("a", 7))
 print(str_tmp.index("a", 7))

-1
-1
Traceback (most recent call last):
 File "/usr/local/pycharm-community-5.0.4/helpers/pydev/pydevd.py", line 2411, in <module>
 globals = debugger.run(setup['file'], None, None, is_module)
 File "/usr/local/pycharm-community-5.0.4/helpers/pydev/pydevd.py", line 1802, in run
 launch(file, globals, locals) # execute the script
 File "/usr/local/pycharm-community-5.0.4/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
 exec(compile(contents+"\n", file, 'exec'), glob, loc) 
 File "/usr/local/workspace/pycharmPro/StockAI/StockAI/DbDealer.py", line 140, in <module>
 print(str_tmp.index("a", 7))
ValueError: substring not found