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

user_12259375 0
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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

python-3.x list function python string
3 Answers

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']
user_73267380

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']
user_10441170

People have suggested their own solutions, but the problem with the code you are using is that sep is set to the regex r',', which matches a single comma. Like you say, you don't use commas to separate text, you use whitespace. If you replace the default value of sep with r'\s', or call the function like tree_to_list'(a (b (c d e f)) g)', sep=r'\s'), then it works for me.

user_54998710
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