Can I retrieve the return value of __exit__ from a python context manager?

Question

I use context manager in python. In want to get some logs back from my __exit__ method. So my code logs something like this:

class MyContextManager:
    def __init__(self, value1, value2)
        self.value1 = value1
        self.value2 = value2

    def __enter__(self)
        # Do some other stuff
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # Do some tear down action, process some data that is 
        # created in __enter__ and log those results
        return my_results

with MyContextManager(value1=my_value1, value2=my_value2) as manager:
     # Do some stuff

So how could I access my_results which is returned from __exit__ after (or at the end) of my with block. Is this even legit to return something other the True in the __exit__ method?

Answer 1

Is this even legit to return something other the True in the __exit__ method?

No, not really, but Python will just test for the truth value, so you can get away with it. In other words, if you return a truthy object here, any exceptions will be suppressed. If there was no exception, returning a truthy value is just a no-op.

how could I access my_results which is returned from __exit__ after (or at the end) of my with block.

You can't. The with expression machinery consumed it.

You should make it available in some other way; set it as an attribute on the context manager object itself:

class MyContextManager:
    def __init__(self, value1, value2)
        self.value1 = value1
        self.value2 = value2

    def __enter__(self)
        # Do some other stuff
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # Do some tear down action, process some data that is 
        # created in __enter__ and log those results
        self.my_results = my_results
        # returning None, we don't want to suppress exceptions
        return None

with MyContextManager(value1=my_value1, value2=my_value2) as manager:
     # Do some stuff

results = manager.my_results

The manager name is available after the with block has completed.

This is how the unittest.TestCase.assertRaises() context manager shares the captured exception, for example.