How to use contextmanagers for instance variables

Question

How would I use a Contextmanager for instance variables? E.g. Let's assume I've got some Connection class, that must be closed on destruction. If I were to implement it as a ContextManager I could do.

with Connection() as c:
    c.write('FOO')
    c.ask('BAR?')

and it would get automatically closed on destruction. But what if I wanted to use it in a __init__ of another class, e.g. like the following example?

class Device(object):
    def __init__(self):
        self.connection = Connection()  # Must be closed on destruction.

I dont want it to be closed on exit of the constructor, it should die when the object get's destroyed. I could use __del__ but this has it's downsides. Being used to RAII in C++ it baffles me.

So what is the best way to do it in this case?

Answer 1

You should invoke self.connection.close in your Device.close() method, and then arrange for that to be invoked properly in your program, perhaps with a context manager.

__del__ is never worth it.

Answer 2

from contextlib import contextmanager

@contextmanager
def connection():
    conn = Conn()
    try:
        yield conn
    finally:
        conn.close()

class Conn(object):
    def close(self):
        print('Closing')

class Device(object):
    def __init__(self, conn):
        self.conn = Conn()

with connection() as conn:
    d = Device(conn)