python_modules/dagster/dagster/core/execution.py
def user_code_context_manager(user_fn, error_cls, msg):
    '''Wraps the output of a user provided function that may yield or return a value and
    returns a generator that asserts it only yields a single value.
    '''
    check.callable_param(user_fn, 'user_fn')
    check.subclass_param(error_cls, 'error_cls', DagsterUserCodeExecutionError)

    with user_code_error_boundary(error_cls, msg):
        thing_or_gen = user_fn()
        gen = _ensure_gen(thing_or_gen)

        try:
            thing = next(gen)
        except StopIteration:
            check.failed('Must yield one item. You did not yield anything.')

        yield thing

        stopped = False

        try:
            next(gen)
        except StopIteration:
            stopped = True

        check.invariant(stopped, 'Must yield one item. Yielded more than one item')
def user_code_context_manager(user_fn, error_cls, msg):
    '''Wraps the output of a user provided function that may yield or return a value and
    returns a generator that asserts it only yields a single value.
    '''
    check.callable_param(user_fn, 'user_fn')
    check.subclass_param(error_cls, 'error_cls', DagsterUserCodeExecutionError)

    with user_code_error_boundary(error_cls, msg):
        thing_or_gen = user_fn()
        gen = _ensure_gen(thing_or_gen)

        try:
            thing = next(gen)
        except StopIteration:
            check.failed('Must yield one item. You did not yield anything.')

        yield thing

        stopped = False

        try:
            next(gen)
        except StopIteration:
            stopped = True

        check.invariant(stopped, 'Must yield one item. Yielded more than one item')
Wraps the output of a user provided function that may yield or return a value and
    returns a generator that asserts it only yields a single value.