Downgrade f-strings (thanks https://github.com/JohannesBuchner/fstring-downgrade for inspiration and original code)

defstring.py

#!/usr/bin/env python 

import sys
import re

inp = sys.stdin
out = sys.stdout

linestart = 0
while True:
    if linestart == 0:
        line = next(inp, None)
        if line is None:
            break
    else:
        line = line[linestart:]
    while re.search(r'''(?:'|")\s*$''', line):
        nextl = next(inp, None)
        if nextl is None: 
            break
        line += nextl
    m = re.search(r'''(?i)(?:(?P<r>rf|fr)|f)(?P<q>'|")(?:(?(r)\\|\\\\)(?P=q)|(?P=q)\s*f?(?P=q)|(?!(?P=q)).)*(?P=q)''', line)
    if not m:
        out.write(line)
        linestart = 0
        continue
    try:
        out.write(line[:m.start(0)])
        start = line[m.start(0):m.end('q')]
        part = line[m.start('q'):m.end(0)]
        q = m.group('q')
        linestart = m.end(0)
        keys = [m.group(1) for m in re.finditer(r'{([^}:]*)(:[^}]*)?}', part)]
        n = [0]
        def replacer(m):
	        r = '{%d%s}' % (n[0], m.group(2))
	        n[0] += 1
	        return r
        part = re.sub(r'{([^}:]*)((:[^}]*)?)}', replacer, part)
        part = re.sub(q + r'(\s*)f' + q, q + r'\1' + q, part)
        out.write(start.replace('f', ''))
        out.write(part[1:])
        if len(keys) > 0:
	        out.write('.format(' + ', '.join(['%s' % (key) for key in keys]) + ')')
    except Exception as e:
        print(e)
        print("skipping problematic line: '%s'" % line.rstrip())
        out.write(line)
        linestart = 0

stdin->stdout filter generates numbered '...{n}...' format items from f-strings for 2.6 compat.

Issues:

• won't handle literal f-string concatenation with different quoting
• only handles shortstrings ('...', "...", not '''...''', """...""")
• also confused by ! formatting tweaks