Transaction example

output

python3 trans.py 
begin
initial_preparations
HO HO HO
main: i = 0
finishing_touches
commit
and a bottle of rum 0
begin
initial_preparations
HO HO HO
main: i = 1
commit
and a bottle of rum 1

# With "return" instead of "raise StopIteration" at lines 31-32
python3 trans.py 
begin
initial_preparations
HO HO HO
main: i = 0
finishing_touches
commit
and a bottle of rum 0
begin
initial_preparations
HO HO HO
main: i = 1
finishing_touches
commit

# (Note how this doesn't print "and a bottle of rum 1")

trans.py

import contextlib

def begin(): print('begin')
def rollback(): print('rollback')
def commit(): print('commit')
def initial_preparations(): print('initial_preparations')
def finishing_touches(): print('finishing_touches')

@contextlib.contextmanager
def transaction():
    begin()
    try:
        yield from do_it()
    except:
        rollback()
        raise
    else:
        commit()

def do_it():
    initial_preparations()
    yield  # Body of with-statement is executed here
    finishing_touches()

def gene():
    for i in range(2):
        with transaction():
            print('HO HO HO')
            yield i
            if i == 1:
                # return
                raise StopIteration  # This is wrong
        print('and a bottle of rum', i)

def main():
    for i in gene():
        print('main: i =', i)

main()