Context manager demo in Python 3.7

context_manager_demo.py

# Using this bash function (for color):
#     color()(set -o pipefail;"$@" 2>&1>&3|sed $'s,.*,\e[31m&\e[m,'>&2)3>&1
# repeat each section by pasting after this:
#     color python; echo $?
#
# Notice how ctx1 never does the correct cleanup by printing "exiting".


# Defining a context manager class.
class CTX:
    def __init__(self, val):
        self.val = val
    def __enter__(self):
        return self.val
    def __exit__(self, *args):
        print(args)
        print('exiting', self.val)

with CTX(50) as v:
    print('using', v)
    exit(f'finished {v}')



# Using contextlib the wrong way.
from contextlib import contextmanager
@contextmanager
def ctx1(var):
    yield var
    print('exiting', var)

with ctx1(50) as v:
    print('using', v)
    exit(f'finished {v}')



# Using contextlib the right way.
from contextlib import contextmanager
@contextmanager
def ctx2(var):
    try:
        yield var
    finally:
        print('exiting', var)

with ctx2(50) as v:
    print('using', v)
    raise SystemExit(f'finished {v}')