Exercise 3 solution

    import re

    LINE = re.compile(r"""
        ^\s*
        (?:
            \[(?P<section>[^\]]+)\]                  # [section]
          | (?P<key>\w+)\s*=\s*(?P<value>[^#\n]*?)   # key = value
        )?
        \s*(?:\#.*)?$                                # optional comment
    """, re.VERBOSE)

    def parse_config(text):
        result, current = {}, None
        for number, line in enumerate(text.splitlines(), 1):
            m = LINE.match(line)
            if m is None:
                raise ValueError(f"line {number}: cannot read {line!r}")
            if m["section"]:
                current = result.setdefault(m["section"].strip(), {})
            elif m["key"]:
                if current is None:
                    raise ValueError(f"line {number}: key outside a section")
                current[m["key"]] = m["value"]
        return result

    config = """
    # demo
    [server]
    host = example.org   # main host
    port = 8080

    [paths]
    root = /var/data
    """
    print(parse_config(config))
    for bad in ["[a]\nthis is not valid", "x = 1"]:
        try:
            parse_config(bad)
        except ValueError as e:
            print(e)
    # {'server': {'host': 'example.org', 'port': '8080'}, 'paths': {'root': '/var/data'}}
    # line 2: cannot read 'this is not valid'
    # line 1: key outside a section

(All lines were run and printed exactly what is shown.)

Explanation: a VERBOSE pattern with three named parts reads like a small grammar. Each line either is a section header, a key = value pair, or is blank or a comment. Anything else fails to match, which is how bad lines are caught.