Is there a Python parsing library that can parse a TOML-like format that specifies nested fields with [ParentHeader_ChildSection]?

Tools like ANTLR, PLY, pyparsing, or Lark will give you almost no help with this problem. configparser might help a little, but I suspect it’d be more bother than it’s worth.

The following code is close to what you want. You’ll need to tweak it based on what you discover about the input-format, and what you’d like for the output-structure.

import re, json

def main():
    obj = parse('input.txt')
    print(json.dumps(obj, indent=2))

def parse(filename):
    root_object = {}
    current_object = None
    for line in open(filename):
        # trim trailing whitespace:
        line = line.rstrip()

        if line == '':
            # blank line
            pass

        elif mo := re.fullmatch(r'[(w+)]', line):
            # header line
            # This identifies, via a 'path' from the root object,
            # the object that subsequent name-value lines are talking about.
            header_path = mo.group(1)
            header_pieces = header_path.split('_')
            current_object = get_nested_object(root_object, header_pieces)

        elif mo := re.fullmatch(r'([^=]+)=(.*)', line):
            # name-value line
            (name_part, value_str) = mo.groups()
            # The {name_part} identifies a field in {current_object}
            # or some object nested within {current_object}.
            # The {value_str} encodes the value to be assigned to that field.
            name_pieces = name_part.split('_')
            prefix_pieces = name_pieces[:-1]
            field_name_piece = name_pieces[-1]

            if prefix_pieces == ['m']:
                # This is an 'immediate' field of {current_object}
                obj_w_field = current_object
            else:
                # This is a field of some object nested within {current_object}
                obj_w_field = get_nested_object(current_object, prefix_pieces)

            mo = re.fullmatch(r'([a-z]+)([A-Z][a-zA-Z]*)', field_name_piece)
            (type_indicator, field_name_pc) = mo.groups()

            field_name = to_snake_case(field_name_pc)
            field_value = value_str

            obj_w_field[field_name] = field_value

        else:
            assert 0, line
    return root_object

def get_nested_object(base_object, header_pieces):
    if header_pieces == []:
        return base_object
    else:
        prefix_pieces = header_pieces[:-1]
        last_piece = header_pieces[-1]

        obj = get_nested_object(base_object, prefix_pieces)

        if mo := re.fullmatch(r'[A-Za-z]+', last_piece):
            # e.g. "Schedule"
            # This identifies a field/property/member of {obj}
            field_name = to_snake_case(last_piece)
            # That field might or might not exist already.
            if field_name not in obj:
                # It doesn't exist yet.
                # We assume that the value of the field is an object
                obj[field_name] = {}
            return obj[field_name]
            
        elif mo := re.fullmatch(r'([A-Za-z]+)(d+)', last_piece):
            # e.g., "Step122", "Limit0"
            # This identifies an element of an array that is a field of {obj}
            # e.g., "Step122" implies that {obj} has a field named "steps",
            # whose value is an array,
            # and this identifies the element at index 122 in that array.
            (array_field_name_pc, index_str) = mo.groups()

            array_field_name = to_snake_case(array_field_name_pc) + 's'
            index = int(index_str)

            if array_field_name not in obj:
                obj[array_field_name] = {}
                # In practice, you might want to make this a list.
            array = obj[array_field_name]

            if index not in array:
                array[index] = {}
            return array[index]

        else:
            assert 0, last_piece

        assert 0

# "_pc" suffix denotes a Pascal-cased name, e.g. "MaxCurrent"

def to_snake_case(name_pc):
    assert '_' not in name_pc
    def replfunc(mo):
        cap_letter = mo.group(0)
        low_letter = cap_letter.lower()
        if mo.start() == 0:
            return low_letter
        else:
            return '_' + low_letter
    return re.sub(r'[A-Z]', replfunc, name_pc)

main()

For the example input, it prints:

{
  "schedule": {
    "steps": {
      "122": {
        "max_current": "0",
        "add_in": "Relay OFF",
        "label": "06 - End Charge",
        "limit_num": "2",
        "limits": {
          "0": {
            "equations": {
              "0": {
                "compare_sign": ">=",
                "right": "F_05_Charge_Capacity",
                "left": "PV_CHAN_Charge_Capacity"
              }
            },
            "step_limit": "1",
            "goto_step": "End Test"
          },
          "1": {
            "equations": {
              "0": {
                "compare_sign": ">=",
                "left": "PV_CHAN_Voltage",
                "right": "3"
              }
            },
            "step_limit": "1",
            "goto_step": "End Test"
          }
        }
      }
    }
  }
}