Call __exit__ on all members of a class

Enn Michael Source

Is there a Pythonic way to automatically __exit__ all members of a class?

class C:
    def __init__(self):
        self.a = open('foo')
        self.b = open('bar')

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        # Is it correct to just forward the parameters here?
        self.a.__exit__(self, exc_type, exc_value, traceback)
        self.b.__exit__(self, exc_type, exc_value, traceback)

Can I do this without manually calling __exit__ on a and b? Am I even calling __exit__ correctly?

Suppose that the resources I have aren't files like in the example and there isn't a method like close or destroy. Is it perhaps good practice to implement a method like this on top of the __enter__ and __exit__?

pythonpython-3.x

Answers

answered 6 months ago sciroccorics #1

__enter__ and __exit__ special functions are almost never called manually. There are respectively called when entering and leaving a with block statement. So if you use something like:

with C() as c:
    # stuff
# other stuff

you call these magic functions. In you case, I would call the file open functions in __enter__ and the corresponding close function in __exit__

For instance:

class C:
    def __enter__(self):
        self.a = open('foo')
        self.b = open('bar')
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.a.close()
        self.b.close()

    def readline_a(self):
      return self.a.readline()

    def readline_b(self):
      return self.b.readline()

with C() as c:
  print(c.readline_a())
  print(c.readline_b())

prints the first line of each file foo and bar

answered 6 months ago Take_Care_ #2

So as I mentioned in comment :

I think most helpfull here will be : contextlib.ExitStack

You can create this object as a member of Your own class in the __init__. And then add to it all your depending contextmanagers with the enter_context(cm) where cm is context_manager

def __init__(self):
    self.exit_stack = contextlib.ExitStack()
    self.exit_stack.__enter__()
    self.exit_stack.enter_context(open('foo')) 
    ....

to clear all the depending contexts just call in the __exit__ the exit of this stack.

Or better just subclass ExitStack and in init call the enter_context.

answered 6 months ago Dunes #3

Writing your own __enter__ and __exit__ functions tends to not be a great idea. You need to understand what should happen if an exception occurs during a child __exit__ or what it means if __exit__ returns a truthful value.

It's generally better just to write a generator-based context manager instead. eg.

from contextlib import contextmanager

class A:
    def __init__(self, filename0, filename1, file0, file1):
        self.filename0 = filename0
        self.filename1 = filename1
        self.file0 = file0
        self.file1 = file1

    @classmethod
    @contextmanager
    def create(cls, filename0, filename1):
        with open(filename0) as file0, \
                open(filename1) as file1:
            yield cls(filename0, filename1, file0, file1)

with A.create('file0.txt', 'file1.txt') as a:
    a.do_something()

This will open the child context managers in defined order, and automatically close them in a defined order, propagating exceptions and return values correctly.

answered 6 months ago Rick Teachey #4

To provide context manager functionality for instance members, one might do something like this:

class MemberManager:
    managed_member_names = ('a', 'b', 'c')
    def __init__(self, a, b, c):
        self.a, self.b, self.c = a, b, c
    def __enter__(self):
        # yield statement means this enter method returns a generator
        for i in (getattr(self,n) for n self.managed_member_names):
            with open(i, mode="w") as x:
                # yield prevents the context manager from exiting
                yield x
    def __exit__(self, exc_type, exc_value, traceback):
        # all items will be closed by __enter__ context manager; nothing needed here
        pass

mm = MemberManager(fname1, fname2, fname3)
with mm as open_members:
    # open_members is a generator/iterator
    for member in open_members:
        member.write("foo")

However, note that you cannot do this:

with mm as open_members:
    open_member_list = list(open_members)
    open_member_list[0].write("foo") # ValueError: I/O operation on closed file.

The open_members iterator must remain unexhausted for the current file to remain open.

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