Conditional based on whether or not a module is being used

[Pete Emerson]

In a module, how do I create a conditional that will do something
based on whether or not another module has been loaded?

Suppose I have the following:

import foo
import foobar

print foo()
print foobar()

########### foo.py
def foo:
    return 'foo'

########### foobar.py
def foobar:
    if foo.has_been_loaded(): # This is not right!
        return foo() + 'bar'      # This might need to be foo.foo() ?
    else:
        return 'bar'

If someone is using foo module, I want to take advantage of its
features and use it in foobar, otherwise, I want to do something else.
In other words, I don't want to create a dependency of foobar on foo.

My failed search for solving this makes me wonder if I'm approaching
this all wrong.

Thanks in advance,
Pete