fstring_parser.py

import re


def fstring(s):
    """Parse f-string expressions and return the formatted version.
    
    This lets you replace f'...' with fstring('...') and have it work on Python versions older than 3.6.
    Note that this is just a fun project rather than a serious one, and may not be perfect, but feel free to use it.
    
    Example:
    >>> name = 'John'
    >>> fstring('Hello, {name}!')
    'Hello, John!
    """
    fields = re.findall(r'(?<!\{)\{([^}]+)\}(?!\})', s)
    for field in fields:
        field = field.split(':')[0]
        if field.endswith('='):
            s = s.format(**{field: '{0}{1!r}'.format(field, eval(field[:-1]))})
        else:
            s = s.format(**{field: eval(field)})
    s = re.sub(r'\{\{([^}]+)\}\}', r'{\1}', s)
    return s