I think you probably mean something other than what you actually write. It doesn't really make sense for "any expression" as far as I can tell. What would it possibly mean to write:

with (2+2) as foo:
print(foo)

I have occasionally thought it would be nice to do something like this (and I could, but I haven't, so I guess I don't think it that strongly):

>>> @contextmanager
... def bind(val):
... yield val
...
>>> with bind(2+2) as four:
... print(four)
...
4

Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons. Intellectual property is
to the 21st century what the slave trade was to the 16th.