问如何知道字符串是否属于python中字符串列表的组合？

EN

import collections

def flatten(l):
    for el in l:
        if isinstance(el, collections.Iterable) and not isinstance(el, (str, bytes)):
            yield from flatten(el)
        else:
            yield el

flatten_L = list(flatten(L))

if 'B' in flatten_L:
    print(True)
else:
    print(False)

def is_present(string_to_check, nested_list):
    output = False
    for element in nested_list:
         if isinstance(element, list):
             output = output or is_present(string_to_check, element)
         else:
             output = output or string_to_check in element
    return output

def exists(d, t):
  return any(i == t if not isinstance(i,list) else exists(i, t) for i in d)


L = [['A'],[['B'],['C']]]
print(exists(L, 'B'))

True