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

blhsing · Accepted Answer · 2019-10-23 01:47:55Z

5

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']

edited Oct 23, 2019 at 1:47

blhsing

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answered Oct 22, 2019 at 20:26

Ajax1234

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4 Comments

wim Over a year ago

This is a good approach, but I would recommend just to iterate the string directly and add handling for whitespace, rather than using regex. +1 anyway.

Edgar Zakharyan Over a year ago

this function removes dots. for example if i have words instead of letters('07.45' instead of 'a') it will turn '07.45' into '07' '45'

Ajax1234 Over a year ago

@EdgarZakharyan Simply adjust the regex: re.findall(r'[\w\.]+|[()]', data).

Homunculus Reticulli Over a year ago

Nice, clean concise code. Care to add some comments to explain how the function works a bit more clearly?

fferri · Accepted Answer · 2019-10-22 20:56:25Z

3

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']

edited Oct 22, 2019 at 20:56

answered Oct 22, 2019 at 20:35

fferri

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Comments

Izaak Weiss · Accepted Answer · 2019-10-22 20:45:30Z

1

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.

answered Oct 22, 2019 at 20:45

Izaak Weiss

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