*Docstrings* and *doctests* are nice ways to get started with the topics of documentation and testing. The idea is to write the string that shows up when you do `help(some_function)` and in particular to include examples of how to call your function. The beautiful thing is that we can check that those examples work as advertised, a very useful …

docstrings_and_doctests.py

def has_illegal_chars(string: str, illegal: str = ',;"!+=') -> bool:
    """
    Detect the presence of illegal characters in a string.
    By default, illegal characters are: `,;"!+=`
    
    Args:
        string: A string of text of any length.
        illegal: A sequence of characters that are not allowed.
        
    Returns:
        True if any of the illegals appear at least once, and
            False otherwise.
            
    Examples:
    >>> has_illegal_chars('This string is okay.')
    False
    >>> has_illegal_chars('This string is not okay!')
    True
    >>> has_illegal_chars('Is this okay?', illegal='?')  # This test fails, can you fix it?
    False
    """
    return any(c in string for c in illegal)

# Check that the examples work as we promised:
import doctest

doctest.testmod()