how to turn string into nested list with elements separated with commas

Question

I have a string which looks like this:

'(a (b (c d e f)) g)'

I want to turn it into such a nested list:

['a', ['b', ['c', 'd', 'e', 'f']], 'g']

I used this function:

def tree_to_list(text, left=r'[(]', right=r'[)]', sep=r','):
    pat = r'({}|{}|{})'.format(left, right, sep)
    tokens = re.split(pat, text)    
    stack = [[]]
    for x in tokens:
        if not x or re.match(sep, x): continue
        if re.match(left, x):
            stack[-1].append([])
            stack.append(stack[-1][-1])
        elif re.match(right, x):
            stack.pop()
            if not stack:
                raise ValueError('error: opening bracket is missing')
        else:
            stack[-1].append(x)
    if len(stack) > 1:
        print(stack)
        raise ValueError('error: closing bracket is missing')
    return stack.pop()

But result is not what i expected. There are no commas among strings:

['a', ['b', ['c' 'd' 'e' 'f']], 'g']

Could you please help me with that

Answer 1

You can use recursion with a generator:

import re
data = '(a (b (c d e f)) g)'
def group(d):
    a = next(d, ')')
    if a != ')':
        yield list(group(d)) if a == '(' else a
        yield from group(d)
print(next(group(iter(re.findall(r'\w+|[()]', data)))))

Output:

['a', ['b', ['c', 'd', 'e', 'f']], 'g']

Answer 2

Using string replacements to turn the input into the string with the desired Python value, and literal_eval to turn it into the value itself:

>>> import ast, re
>>> data = '(a (b (c d e f)) g)'
>>> s = re.sub(r'(\w+)', r'"\1"', data)         # quote words
>>> s = re.sub(r'\s+', ',', s)                  # whitespace to comma
>>> s = s.replace('(', '[').replace(')', ']')   # () -> []
>>> ast.literal_eval(s)
['a', ['b', ['c', 'd', 'e', 'f']], 'g']