Вот небольшой (возможно, 74 строки, включая пробелы) модуль, который я построил примерно за полтора часа (плюс почти час до рефакторинга):
str_to_token = {'True':True,
'False':False,
'and':lambda left, right: left and right,
'or':lambda left, right: left or right,
'(':'(',
')':')'}
empty_res = True
def create_token_lst(s, str_to_token=str_to_token):
"""create token list:
'True or False' -> [True, lambda..., False]"""
s = s.replace('(', ' ( ')
s = s.replace(')', ' ) ')
return [str_to_token[it] for it in s.split()]
def find(lst, what, start=0):
return [i for i,it in enumerate(lst) if it == what and i >= start]
def parens(token_lst):
"""returns:
(bool)parens_exist, left_paren_pos, right_paren_pos
"""
left_lst = find(token_lst, '(')
if not left_lst:
return False, -1, -1
left = left_lst[-1]
#can not occur earlier, hence there are args and op.
right = find(token_lst, ')', left + 4)[0]
return True, left, right
def bool_eval(token_lst):
"""token_lst has length 3 and format: [left_arg, operator, right_arg]
operator(left_arg, right_arg) is returned"""
return token_lst[1](token_lst[0], token_lst[2])
def formatted_bool_eval(token_lst, empty_res=empty_res):
"""eval a formatted (i.e. of the form 'ToFa(ToF)') string"""
if not token_lst:
return empty_res
if len(token_lst) == 1:
return token_lst[0]
has_parens, l_paren, r_paren = parens(token_lst)
if not has_parens:
return bool_eval(token_lst)
token_lst[l_paren:r_paren + 1] = [bool_eval(token_lst[l_paren+1:r_paren])]
return formatted_bool_eval(token_lst, bool_eval)
def nested_bool_eval(s):
"""The actual 'eval' routine,
if 's' is empty, 'True' is returned,
otherwise 's' is evaluated according to parentheses nesting.
The format assumed:
[1] 'LEFT OPERATOR RIGHT',
where LEFT and RIGHT are either:
True or False or '(' [1] ')' (subexpression in parentheses)
"""
return formatted_bool_eval(create_token_lst(s))
Простойтесты дают:
>>> print nested_bool_eval('')
True
>>> print nested_bool_eval('False')
False
>>> print nested_bool_eval('True or False')
True
>>> print nested_bool_eval('True and False')
False
>>> print nested_bool_eval('(True or False) and (True or False)')
True
>>> print nested_bool_eval('(True or False) and (True and False)')
False
>>> print nested_bool_eval('(True or False) or (True and False)')
True
>>> print nested_bool_eval('(True and False) or (True and False)')
False
>>> print nested_bool_eval('(True and False) or (True and (True or False))')
True
[Возможно, частично не по теме]
Обратите внимание, что вы можете легко настроить токены (как операнды, так и операторы), которые вы используете ссредство для внедрения зависимостей бедного человека (token_to_char=token_to_char и друзья), позволяющее одновременно иметь несколько различных оценщиков (просто сброс глобалов «injected-by-default» оставит вам одно поведение).
Например:
def fuzzy_bool_eval(s):
"""as normal, but:
- an argument 'Maybe' may be :)) present
- algebra is:
[one of 'True', 'False', 'Maybe'] [one of 'or', 'and'] 'Maybe' -> 'Maybe'
"""
Maybe = 'Maybe' # just an object with nice __str__
def or_op(left, right):
return (Maybe if Maybe in [left, right] else (left or right))
def and_op(left, right):
args = [left, right]
if Maybe in args:
if True in args:
return Maybe # Maybe and True -> Maybe
else:
return False # Maybe and False -> False
return left and right
str_to_token = {'True':True,
'False':False,
'Maybe':Maybe,
'and':and_op,
'or':or_op,
'(':'(',
')':')'}
token_lst = create_token_lst(s, str_to_token=str_to_token)
return formatted_bool_eval(token_lst)
дает:
>>> print fuzzy_bool_eval('')
True
>>> print fuzzy_bool_eval('Maybe')
Maybe
>>> print fuzzy_bool_eval('True or False')
True
>>> print fuzzy_bool_eval('True or Maybe')
Maybe
>>> print fuzzy_bool_eval('False or (False and Maybe)')
False