Go-like defer statements

defer.py

import sys
import inspect
from contextlib import contextmanager
from collections import defaultdict


deferred_lines = defaultdict(list)


def defer_trace(frame, event, arg):
    if event != "line":
        return defer_trace
    with open(inspect.getsourcefile(frame)) as f:
        lines = f.readlines()
    line = lines[frame.f_lineno - 1].strip()
    if line.startswith("defer; "):
        deferred_lines[frame].append(line.strip()[len("defer; "):])
        frame.f_lineno += 1
    else:
        pass
    return defer_trace


@contextmanager
def deferable():
    frame = sys._getframe().f_back.f_back
    sys.settrace(lambda *a:None)
    frame.f_trace = defer_trace
    try:
        yield
    finally:
        for line in reversed(deferred_lines[frame]):
            exec(line, frame.f_globals, frame.f_locals)
        deferred_lines.pop(frame)
        sys.settrace(None)


with deferable():
    print("Hello")
    defer; print("Bye")
    print("Middle")