Simple contextlib.contextmanager implementation

context-manager.py

class ContextManager:
    def __init__(self, func):
        self.func = func
    
    def __call__(self, *args, **kwargs):
        self.gen = self.func(*args, **kwargs)
        return self
        
    def __enter__(self):
        return next(self.gen)
        
    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is None:
            pass
        else:
            self.gen.throw(exc_type, exc_val, exc_tb)

def contextmanager(f):
    return ContextManager(f)
    
@contextmanager
def my_func(tot):
    a_list = []
    try:
        yield a_list
    finally:
        print(tot, a_list)

with my_func(15) as l:
    l.append(1)
    l.append(2)

golfed-contextmanager-1.py

c=lambda f:(lambda *a,**k:type('H',(),{'__init__': lambda s,f,*a,**k:setattr(s,'g',f(*a, **k)),'__enter__':lambda s:next(s.g),'__exit__':lambda s, t, v, b:t if t is None else s.g.throw(t,v,b)})(f, *a, **k))
@c
def my_func(tot):
    a_list = []
    try:
        yield a_list
    finally:
        print(tot, a_list)
        
with my_func(15) as l:
    l.append(1)
    l.append(2)

golfed-contextmanager-2.py

exec("c=%sf:(%s`:type('H',(),{'__init__':%ss,f,`:setattr(s,'g',f(`)),'__enter__':%ss:next(s.g),'__exit__':%ss,t,v,b:t if t is None else s.g.throw(t,v,b)})(f,`))".replace('`','*a,**k')%(("lambda ",)*5))


@c
def managed_file(name):
    try:
        f = open(name, 'w')
        yield f
    finally:
        f.close()
        
with managed_file('naber.txt') as f:
  f.write('naber')

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is not None:
            self.gen.throw(exc_type, exc_val, exc_tb)