""backported"" string-interpolation for Python 2/3.5- (What do you mean, 'why'? Why not?)

f_strings.py

import inspect

try:
    unichr
except NameError:
    unichr = chr

def f(s):
    # find all {}'s
    stack = []
    ranges = []
    if isinstance(s, bytes):
        s = s.decode('utf-8')

    iterator = iter(enumerate(s))
    for i, c in iterator:
        # escape
        if s[i:i+2] == u'}}' or s[i:i+2] == u'{{':
            iterator.next()
            continue
        elif c == u'{':
            stack.insert(0, i)
        elif c == u'}':
            if not stack:
                raise SyntaxError("f-string: single '}' is not allowed")
            ranges.append((stack.pop(0)+1, i))

    if stack:
        raise SyntaxError("f-string: expecting '}'")

    expressions = {}
    for i, (start, stop) in enumerate(ranges):
        interp = s[start:stop]
        expr = interp
        if u':' in interp:
            expr = interp[:interp.index(u':')]
        if u'!' in interp:
            expr = interp[:interp.index(u'!')]
        key = unichr(i+1).ljust(len(expr))
        expressions[key] = interp[:len(expr)]
        interp = key + interp[len(expr):]
        s = s[:start] + interp + s[stop:]
    
    values = {}

    frame = inspect.getouterframes(inspect.currentframe())[1][0]
    for key, expr in expressions.items():
        # evalueate expresions in original stack frame
        #print('eval ', repr(expr))
        val = eval(expr, frame.f_globals, frame.f_locals)
        values[key] = val

    return s.format(**values)

if __name__ == "__main__":
    a = 'world'
    import math, os
    print(f("hello {a}! i'm {os.environ['USER']}. 2+2 is {2+2}, and the square root of pi is {math.sqrt(math.pi)}. \n{{expressions}} in f-strings work!"))