Question

For example, i have this code:

with MyClass() as x:
    print 'I have only {0}'.format(x)
    with MyClass() as y:
        print 'I have {0} and {1}'.format(x, y)
    print 'Again only {0}'.format(x)

x and y both should be de-initialized after exit of corresponding with blocks. Also x and y aren’t instances of MyClass.

__exit__ has only three arguments and each argument is None (if no exception supplied).

How can i determine at __exit__ which block is just exited and what value was returned by __enter__?

(N.B. code should be thread-safe).

---

Example:

class MyClass(object):
    def __enter__(self):
        if moon_phase > 0:
            return 123
        else:
            return 456
    def __exit__(self):
        number = what_was_returned_by_enter()
        print 'End of block with', number


 with MyClass() as x:
    print x  # 123
    with MyClass() as y:
        print x, 'and', y  # 123 and 456
    # printed "End of block with 456"
    print x  # 123
 # printed "End of block with 123"

Answer 1

Since you have a custom class handling your context, self will be the context manager instance.

You’ll need to examine its state (stored at __init__() creation time or when __enter__() was called) to determine which one you just exited.

The following example stores the return value of __enter__ on the instance so you can retrieve it again when __exit__ is being called:

class MyClass(object):
    def __enter__(self):
        if moon_phase > 0:
            self.returnval = 123
        else:
            self.returnval = 456
        return self.returnval

    def __exit__(self, *args):
        number = self.returnval
        print 'End of block with', number